The Weather Blog

The Return of El Nino

It's official: El Nino is back. The warming taking place in the equatorial Pacific Ocean, which brought an end to La Nina earlier this year, has reached the degree that it now qualifies as an El Nino. The Climate Prediction Center issued their first advisory on the new El Nino on July 9.

The image at left shows the sea surface temperature (SST) anomalies across the Pacific basin, as of July 12 (click to open a full-size version in a new window). Most of the equatorial Pacific between Indonesia and the west coast of South America is painted in the reds indicating SSTs that are warmer than normal by 0.5-1.5 C. The SST anomalies near the South American coast are quite strong; the yellow shades indicate waters that are on the order of 2-3 C warmer than normal.

The SST anomalies, when averaged over a set of geographic regions within the equatorial Pacific, yield the standard temperature anomaly indices that are used to gauge the oceanic component of ENSO (El Nino-Southern Oscillation). Typically, the rule of thumb is that when these indices reach +0.5 C (or more), we are in El Nino. If the anomalies are -0.5 C (or less), we are in La Nina. In between is ENSO-neutral. Most of the SST indices are now in the range of +0.6 C to +0.9 C, so the situation now officially qualifies as El Nino.

This nascent El Nino may have some legs to it. Subsurface waters (down to about 1000 feet deep) across the equatorial Pacific are also warmer than normal, which tells us that there actually is a mass of anomalously warm water extending to some depth, and not just a thin skin of unusually warm waters at the surface. In addition, the forecast plumes from IRI also suggest this El Nino could be more than a passing fad. Most of the forecasts agree that the El Nino will continue into the fall season, and it could grow stronger. It isn't out of the question that the El Nino could grow to moderate strength.

The ramifications of El Nino on the weather in the U.S. are many. However, El Nino's impacts during summer are rather muted and it won't be until the fall and winter that the weather patterns over the U.S. begin to show a more definitive El Nino influence. What can be seen during the summer is the impact that El Nino has on the degree of tropical activity in the Atlantic basin. Typically, El Nino generates stronger winds than usual at high levels in the atmosphere. These stronger winds in turn lead to greater wind shear, and wind shear is the enemy of many a tropical cyclone. The development of El Nino was anticipated to a certain degree in many of the seasonal outlooks for hurricanes and tropical storms in the Atlantic, and is the primary reason why most of the outlooks call for near or below normal activity.

In our next entry, we'll take a look at some recently published research documenting a variant of El Nino, known colloquially as "modoki El Nino." In this form, the warm anomalies in the equatorial Pacific are displaced to the central Pacific as opposed to the eastern Pacific, which could affect the degree of suppression of tropical activity in the Atlantic.

Active Weather Pattern in Europe

A weather pattern more reminiscent of January than June is affecting our neighbors across the pond. Unseasonably cold air has spilled down into western Europe from Scandinavia and the Arctic, leading to below and much below normal temperatures and a plethora of unusual weather.

Snow was reported in northern England on Friday, June 5. A light dusting fell in the relatively higher elevations (approximately 1500-2000 feet) near the Scotland border, which was the first significant snow that had fallen in England during the month of June since 1975. The snow came on the heels of a very hot spell for the UK at the end of May.

Severe thunderstorms also broke out over the UK and other parts of western and central Europe during the past several days. Following the snow, a band of thunderstorms brought heavy rain to southern England on June 6 and 7, including funnel clouds and tornadoes in the southwestern parts of the country. Tornadoes were also reported in Italy and Russia, and severe thunderstorms also affected France, the Benelux, and Germany with severe wind and hail.

From the heat at the end of May to snow and severe thunderstorms at the start of June? What a radical pattern shift! However, the pattern shift over Europe is a somewhat common phenomenon, falling within the purview of the North Atlantic Oscillation, or NAO. The NAO is an approximate measure of the pressure differences between Iceland and the Azores. Typically the semipermanent Icelandic Low occupies the North Atlantic, with the semipermanent Bermuda-Azores High covering the Atlantic subtropics and mid-latitudes. In this situation, the NAO index has positive values. On occasion though, ridges of high pressure can build north into the higher latitudes, often associated with blocking patterns. Low pressure is shunted south, and the NAO index reverses and becomes negative.

Negative NAOs have been associated with cold outbreaks and stormy conditions during winters in Europe. With the blocking ridge over the North Atlantic, the flow of milder air off the ocean is displaced away. A further complication can ensue when the blocking ridge retrogrades, or moves west, contrary to the typical west-to-east flow. When the blocking ridge over the North Atlantic retrogrades away from the continent, cold upper lows are free to slide down from Scandinavia and northern Russia, bringing outbreaks of cold air and unsettled weather.

The change from unseasonable warmth at the end of May to the cold and unsettled pattern currently over Europe was precisely due to the retrogression of the blocking ridge over the North Atlantic. A cold upper level low dropped down the North Sea from Scandinavia. The cold air aloft with the upper low was enough to produce instability and snow showers in the north of England, and also contributed to the production of the severe thunderstorms.

The satellite image shown above shows the current weather situation over Europe on the morning of June 9. The cold upper low we mentioned is centered over the North Sea, marked by a well-formed spiral of clouds north of The Netherlands. The cyclonic (counterclockwise) flow around this low extends all the way into the south of France. Over the British Isles, the flow is generally out of the north, bringing relatively cold polar air over the UK.

Cold upper lows like this over the North Sea often exhibit some interesting "substructure." In this case, using the visible satellite image at left, we can pick out a few "swirls within the swirl". The most evident is the bright one over the North Sea that we mentioned earlier. A second counterclockwise swirl can be seen over the English Channel, with some of the cloud bands over northern France spiraling into it. Finally, a third swirl can be seen to the north of Scotland and west of Norway. The three smaller circulations are pinwheeling about a common center, the center of circulation of the larger parent upper low.

Incidentally, these small features can often be polar lows, small and often subtle circulations embedded within the cold polar air streams, not associated with any fronts. Considered by some meteorologists to be distant cousins of tropical storms and hurricanes, polar lows are difficult to anticipate, and their behavior is not easy to predict. They can produce squalls of rain or snow, sometimes accompanied by thunder with gale or storm force winds over a small area. Sometimes they can spin up rapidly, or decay unexpectedly. It is very unusual, though not unheard of, to see polar lows in June. They are much more frequent during the winter months when the overlying polar air masses are strongest.

The May 8, 2009 Derecho - Part Three

After blasting across southeastern Kansas with widespread damaging straight line winds of 80 to 100 mph, the derecho roared into southwestern Missouri early in the morning of May 8, 2009. The dangerous system continued to produce severe wind gusts, but as it pressed east towards Springfield, the character of the system began to change. As the broad scale rotation in its comma head strengthened, low level wind shear was enhanced, making all modes of severe weather - hail as well as tornadoes to go along with the ongoing straight line wind - a very serious threat.

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A large scale radar look at the derecho as it approached Springfield at approximately 7:30 AM local time is shown at left (click to open a full-size version in a new window). The bowing line of thunderstorms producing the widespread wind damage takes up just a small part of the total area encompassed by the system. The shield of precipitation extended north all the way to Kansas City, and then as far northeast as St. Louis. The line of strong to severe thunderstorms trailed back into northeastern Oklahoma and northern Arkansas.

From a quick glance at the radar reflectivity image, the overall appearance of the system was becoming more reminiscent of a tropical cyclone. A number of spiral banding features can be seen wrapping into and around the center of circulation, which was located northwest of Springfield. The most obvious of the bands was the bowing line of intense thunderstorms producing the wind damage of the derecho, while another secondary band (highlighted by yellows) can be seen to the north of the center, wrapping into it from the west. Radar animations clearly showed the rotation about the comma head, unfortunately we cannot host animations in our blog.

With the rotation in the system strengthening, the derecho became more than just a straight line wind threat. At about this time, reports of large hail began to become interspersed with the wind damage reports. Joplin and Carthage, which had received winds of up to 93 mph, also reported golfball sized hail. Ahead of the main line and circulation center, thunderstorms developed across central and eastern Missouri that exhibited supercell characteristics. Baseball sized hail fell from one such storm in Washington County, situated to the southwest of St. Louis and well northeast of the center. It was also at about this time, 8:00 AM, that the first reports of tornadoes began to be received.

The radar image at left is a high-resolution plot of Doppler velocities close in to the Springfield site. The city of Springfield is located near the far left edge of the image. Reds indicate outbounds, and greens are inbound winds. Note the two areas, at center and bottom center, where the greens penetrate slightly into the solid wall of red returns. In both cases, enhanced outbound velocities (pinks and oranges) were also measured nearby. These signatures of small scale rotations were tornadic circulations forming within the line of thunderstorms. Numerous such features dotted the main line as it passed through and east of Springfield. Tornadoes from a line (or quasi-line) of thunderstorms are not unheard of, but it is not at all a textbook scenario. Tornadoes spawned by lines of thunderstorms are very difficult to anticipate with very much lead time - they often spin up quickly, have short paths, and then dissipate not long after.

In all, the derecho produced 27 tornadoes in southwestern and south-central Missouri. The strongest was rated an EF3 with winds estimated at 165 mph. A long-tracked EF2 tornado also occurred, its 21-mile long path very unusual for a tornado spawned by a line. The National Weather Service office in Springfield performed admirably during the event. In a situation that was not a classic tornado environment (a line of storms, in the early morning hours no less), adequate warnings were provided for 18 of the tornadoes, giving residents an average of 20 minutes of lead time. Nevertheless, 1 death and 3 injuries were attributed to the tornadoes in the Springfield area.

The radar grab at above left shows a wider look at the system as it pushed east of Springfield at about 8:45 AM. North of Springfield (which is marked by the dot labeled "KSGF"), there is a large area of blues and greens in a roughly spiral shape; this is a signature of the strengthening and enlarging circulation of the complex. The increased size of the comma head circulation can at this point be more accurately called a mesoscale convective vortex, or MCV. The active line of thunderstorms is marked by the boundary between reds and greens that is situated east of Springfield. Note the little kinks and indentations within the line indicative of potential tornadic circulations. It's also interesting to point out the existence of a few velocity couplets (bright reds or pinks immediately next to greens) near the eastern edge of the radar's range. These correspond to supercell thunderstorms to the east of the main line of the derecho which were producing severe hail. By this point, it's probably apt to drop the term "derecho" for the complex, as it had added the impressive MCV as well as the threats for tornadoes and large hail to its arsenal.

Leaving the Springfield area behind, the complex continued eastward across southern Missouri as the morning went on. By 10:45 AM, the derecho appeared as shown in the image at left, which is a radar reflectivity image from St. Louis. St. Louis is located near and just above the center of the image, marked by "KLSX" and "TSTL". At this point, the spiral character of the convection was readily apparent. Two or perhaps three bands of strong thunderstorms were located in the complex's eastern quadrant, spiraling in toward the center. The region of steady, stratiform rain remained very large in areal coverage to the north and northeast of the center. Note also that an area of weak reflectivity, almost like the eye of a tropical cyclone, had formed in the vicinity of the center of circulation itself.

The remarkable part of the evolution of the MCV is shown at left. In the first image, taken from St. Louis at 12:08 PM local time, the system had taken on the structure of a comma head, but at a much larger scale. A large region devoid of echoes had punched in behind the leading edge of thunderstorms and partially wrapped around the MCV. In the second image, taken from St. Louis at 12:47 PM, note the MCV itself. An eye-like feature had developed in the middle of the ball of convection, giving the system a stunning appearance overall and an uncanny resemblance to a tropical cyclone.

Another interesting evolution within the MCV took place a short while later, and is captured in the two images at left taken by the Doppler radar in Paducah, Kentucky. The image at left shows the MCV with its eye-like feature at 1:05 PM. Note also the area of weak reflectivity (depicted by shades of blue) trailing to the west of the eye-like feature. In the next image, taken at 1:15 PM, the trailing area of weak reflectivity had become the new eye-like feature, with the old "eye" moving eastward and filling in with light echoes (greens). It is difficult to say what is going on during the two images as a new eye-like feature forms and becomes dominant, or what the physical processes are that drive such changes in structure, but it is remarkable nonetheless. There is some temptation to draw parallels to Tropical Storm Erin of 2007, which re-intensified over Oklahoma well after landfall in Texas a few days earlier, and exhibited a similar structure on radar complete with an eye-like feature.

The complex with its compelling MCV continued east, working gradually up the Ohio Valley. The radar grabs at left, taken from the Doppler radar in Evansville, Indiana at 1:59 PM reveal some further structural changes in the complex. First, the main line of thunderstorms, pushing east across Kentucky, had become well displaced away from the MCV, trailing behind over southern Illinois. Within the MCV itself, the eye-like feature had begun to expand, and was quite a bit larger than about an hour earlier when it was nearly a pinpoint. The severe threat associated with the complex was decreasing at this time. In the overnight hours, a separate complex of thunderstorms had affected Kentucky and Tennessee, and had since advanced east into the Carolinas. Although the storms themselves were gone, they had used up most of the atmospheric instability in the area, leaving behind a region of rain-cooled and relatively stable air.

Robbed of its fuel source, the complex weakened as it continued east across Kentucky. At left is a radar image from Evansville at just before 3:00 PM. Note how the convection associated with the MCV had shrunk dramatically. The circulation of the MCV was still apparent, but the structure had decayed, looking much more ragged and far less organized - almost skeletal. Meanwhile, the active thunderstorm line was also weakening as it encountered the more stable air. The line had become more broken, more ragged, and less intense. Although some reports of severe weather were received across western and central Kentucky and Tennessee, they were neither as significant nor as widespread as what occurred hours earlier over Kansas and Missouri.

The decreasing trend over western Kentucky brought on by the stable air left behind by previous convection was only temporary. The air mass over eastern Kentucky was less "worked-over" by earlier thunderstorms, and provided a modest amount of instability to help the thunderstorms in the complex reintensify in the afternoon hours. This was to be the "last gasp" of the long-lived thunderstorm complex, and it is characterized well by the radar images above from Louisville. In the reflectivity image at above left, we see the decaying MCV to the west of Louisville (which is marked by the "KLVX" and "TSDF" identifiers). The line of thunderstorms along the complex's leading edge remained diffuse and more broken in character. As in earlier phases of the complex's lifetime, the storms out ahead of the line were taking on supercell characteristics. This can be seen in the velocity image (the right image from the two above; click to open a full-size version in a new window). In the velocity image, the MCV circulation can still be readily seen to the west of the radar site, but notice to the south and southeast of the radar, near the Kentucky-Tennessee border, the velocity couplets typical of rotating supercell thunderstorms. There are five (perhaps six?) such couplets where reds and greens are in close proximity. These supercells produced 8 tornadoes in eastern Kentucky and Tennessee, along with a swath of both wind and hail damage reports.

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All told, the derecho of May 8, 2009 was a remarkable weather event. It produced a swath of damage from Kansas to Kentucky - in the form of widespread damaging winds primarily, but also in tornadoes and large hail as well. The complex spawned 44 tornadoes, a large number for a quasi-linear system, including an EF3 and several with long tracks. In addition, there were over 200 reports of damaging winds (including 19 instances of winds of hurricane force), and approximately 50 reports of severe hail. As of this writing, there are 6 fatalities and 13 injuries from the storm. The large amount of real estate affected by the storm, as well as its rapid progression (it went from genesis in northwestern Kansas to its eventual decline in eastern Kentucky in approximately 18 hours) and the numerous unusual and exemplary phenomena observed in association with it, will make this derecho one that both the residents it affected and the meteorologists who observed it will remember for a long time to come.

The May 8, 2009 Derecho - Part Two

This is the second in a series of posts chronicling the derecho that affected a swath of the U.S. from Kansas to Kentucky on May 8, 2009. In the first series, we followed the derecho from its genesis in northwestern Kansas through its "adolescence" across the state as it produced severe hail and damaging straight line winds of up to 80 mph across a large part of western and central Kansas, including Dodge City and Great Bend, and was about to do the same in Wichita. In this post, we follow the mature derecho in its most destructive phase across southeastern Kansas and into southern Missouri.

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At about 4:30 AM local time, the mature bow echo, having left a swath of hail and wind damage across Kansas, approached the Wichita metro area. The images at left are radar captures from the Doppler radar at Wichita (marked by the blue dot labeled "KICT" on the map). The left is reflectivity, and the right panel is velocity (click on either image to open a full-size version in a new window). Note the bright greens and blues to the north of the radar site in the velocity image. These denote strong inbound velocities and are the radar signature of the strong straight line winds. Also worth noting are two subtle features to the west of the radar site, located along the leading edge of the high reflectivity associated with the thunderstorms. On the reflectivity map, these features appear to look like little "stair-steps," where the yellows and reds jut southward from the main line. On velocity, the features appear as small regions of gray and red entangled with the predominant greens of the line. These features are small-scale circulations, possibly indicative of supercell-type thunderstorms, embedded within the larger line. Radar signatures such as this can be associated with small or brief tornadoes occurring at the leading edge of the line, and they only add to the concern operational meteorologists experienced as this event unfolded. No tornadoes were reported in the area around Wichita, however.

On the northeast side of Wichita, the derecho took the next step in its development, with a new downburst forming and pushing out to the east at just before 5:00 AM. The radar grabs at left (reflectivity, left; higher resolution velocity (thanks to the storm's close proximity to the radar site) at right) show this process and its immediate results. On reflectivity, the feature of interest is the crescent-shaped arc of reds extending approximately southeast of Newton, then curving back southwest toward Wichita. The apex of this short bowing structure, where straight line winds are often at their most fierce, is pointed directly at the town of El Dorado, located to the northeast of Wichita along the Kansas Turnpike. On the velocity image, it's easy to see that El Dorado was in trouble. The intense reds, pinks, and oranges signify outbound velocities of at least 75 mph, and not long after this image was taken, wind gusts of up to 81 mph were measured in the El Dorado area. One motorist was injured when the strong winds flipped his vehicle on the Kansas Turnpike.

The bow continued toward the east, with winds gusting to at least 80 mph, picking up speed along the way. One fatality occurred in Wilson County when a woman was killed in her mobile home, which was no match for the powerful straight line winds. The radar grabs at left (reflectivity on the left, velocity on the right) show the progress the storm had made by approximately 5:45 AM. Reaching a forward speed of almost 70 mph, the bow was racing east across southeastern Kansas. Note also the storm cluster that had formed out ahead of the main bow echo. If you look closely at the velocity image, you'll notice within this cluster a couplet where inbounds (green) are located very close to outbounds (bright reds). This thunderstorm out ahead of the main line was exhibiting some of the characteristics of a supercell thunderstorm, and was a threat for large hail and possibly a tornado for a brief time before it was caught and absorbed by the main line.

Note also in the reflectivity image how the portion of the storm complex to the south of Wichita was not faring well as it pressed further south toward Oklahoma. Compare with the first image of reflectivity shown above (or one of the reflectivity images from Part 1), and it's apparent that the weak yellows and oranges of the storms as they moved toward the border are not a match for the strong reds seen earlier in their lifetime. As the storms attempted to press farther to the south, they were fighting an increasingly uphill battle. The most conducive thermodynamic conditions for the derecho existed over Kansas. Over Oklahoma, the atmosphere was much less favorable. In fact, a strong capping inversion present over Oklahoma had prevented thunderstorms from developing there during the previous afternoon despite the presence of strong instability. As the southern extent of the line of storms neared Oklahoma, it began moving into this less favorable thermodynamic environment, and it had to "work uphill" against the inhibition presented by the cap. Eventually the cap would win out, and the storms were never able to make much progress to the south. This would become a theme to the mature stage of the derecho. Where earlier it surged south from near Goodland to Dodge City and Wichita in its adolescent phase, the increasing inhibition to the south over Oklahoma blocked its continued southward progress, and instead it propagated quickly eastward along the periphery of the stronger inhibition.

Further evidence of the derecho's struggles in southward propagation comes from the reflectivity image at left. Taken from the Doppler radar in Tulsa, Oklahoma, at approximately 6:50 AM, the image shows that the derecho has raced east; the apex of the bow echo was approaching the Kansas-Missouri border. The thunderstorms had been able to make some progress into northeastern Oklahoma, but notice the thin blue line running parallel to but out ahead of the storms near the center of the image. This line marks the gust front, the leading edge of the strong outflow winds. In mature derechos, the inflow and the outflow are balanced, which means that the leading edge of the outflow winds is collocated with the leading edge of the strongest reflectivity. When the outflow becomes dominant, the gust front will push out ahead of the main line, which is usually a sign that the storms are weakening slightly. The greater convective inhibition over Oklahoma continued to thwart the derecho's attempts to propagate south.

However, in Kansas the derecho raged eastward unhindered. Widespread wind gusts of 80 mph were reported all over southeastern Kansas, including near Buxton in Elk County, around the town of Altoona, and also the area around Parsons. The most severe winds in the damage swath were reported near the town of Cherryvale where considerable structural damage took place under winds gusting as high as 100 mph.

After Kansas, the next target of the derecho was southwestern Missouri. The radar grabs at left show the derecho as it approached the Kansas-Missouri border, as seen from Springfield at approximately 7:00 AM. The familiar bowed-out appearance of the line remained evident on the reflectivity image, and on velocity the rear inflow jet (discussed in Part 1) was also visible as the region of strong inbound velocities (bright greens and blues) appearing over eastern Kansas. Notice the couplet of inbounds (greens) next to outbounds (reds) approaching the Kansas-Missouri border. This circulation is the signature of what is called a bookend vortex. The bookend vortices form at either end of a bow echo as the line bulges out to form the bow. On the northern flank of the bow, a counterclockwise circulation forms, and a clockwise circulation forms at the southern side of the bow. The Springfield radar was able to see the northern bookend vortex with its cyclonic circulation (colloquially known as the "comma head") as the system approached. This feature would take on an increasing importance later on in the derecho's life as it moved across southern Missouri.

Despite its evolving character with the development of the pronounced cyclonic bookend vortex, the main impact from the derecho on southwestern Missouri was the same as in southeastern Kansas: destructive straight line winds. Wind gusts of 60-70 mph were common across the Joplin area (and also in the far northeastern corner of Oklahoma where some wind damage occurred in Ottawa County). In addition, hail up to the size of golfballs pelted the area, including the city of Joplin and neighboring Carthage. Carthage also reported a 93 mph wind gust as the bow arrived. Farther to the east, an additional swath of 60-80 mph wind gusts occurred, including Springfield with 70 mph wind gusts, and Branson with 60 mph gusts.

About the time the line approached Springfield, an additional region of strong winds began to affect southwestern Missouri near Joplin, well behind the active thunderstorms. These winds, which have been seen often on the back edge of thunderstorm complexes in the Plains this year, are caused in part by a phenomenon called the wake low. Often times a small area of low pressure will form quickly behind a thunderstorm complex - which may be influenced in part by the rear inflow jet above. Winds associated with the wake low can sometimes reach severe criteria (58 mph) on their own. The image at above left is an analysis of the surface pressure tendency in the area around southwestern Missouri. Oranges and reds indicate areas where the surface pressure was rising, and the blues and purples show falling pressures. Joplin is marked by the black dot labeled "JLN", and Springfield by the black dot labeled "SGF". This analysis comes from a large-scale forecast model, so it does not completely capture what was going on in the vicinity of the derecho (a smaller scale phenomenon by comparison), but it can illustrate some salient details. First off, at the time of this analysis (approximately 8:00 AM), the leading edge of the bow echo was pushing through Springfield. Joplin, well behind the active thunderstorms, was clearing out but also experiencing an area of strong pressure rises. These rises, in association with the developing wake low (a feature too small to be picked up by the model), created a strong pressure gradient over a small region, and the resulting accelerations led to the severe - but non-thunderstorm - winds behind the derecho. Winds gusted as high as 85 mph in Joplin in association with the wake low. In the surrounding areas, the winds may have peaked at nearly 90 mph. More remarkably, the severe winds in excess of 60 mph persisted for as much as 20-30 minutes in any given location.

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The derecho of May 8, 2009 was mature by the time it blasted through southeastern Kansas and southwestern Missouri. However, it was not done yet. As its structure began to change and the "comma head" bookend vortex strengthened, the character of the damage changed from primarily a straight line wind threat to a combined threat of severe hail, straight line winds, and tornadoes. In our next post, we'll examine the tornadoes spawned by this system as it continued east across southern Missouri and take a close look at the phenomenal structural changes of the derecho and its comma head as it continued east across the Mississippi River.

The May 8, 2009 Derecho - Part One

This is the beginning of a multi-part series on the complex of severe thunderstorms known as a derecho that left a swath of damage from Kansas to Kentucky on May 8.

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During the late evening of May 7, a rather innocent looking cluster of thunderstorms developed over northwestern Kansas. Few suspected that from those initial storms, a powerful phenomenon known as a derecho would strafe across southeastern Kansas and the entirety of southern Missouri in the overnight and early morning hours, would cross the Mississippi River and cause damage in Illinois and Kentucky in the afternoon while undergoing a spectacular metamorphosis, and continue on into the Appalachians by nightfall.

A derecho is a relatively rare form of severe weather. From the American Meteorological Society's Glossary of Meteorology, a derecho is defined as "a widespread convectively induced straight-line windstorm." Derechoes are often produced by a line of thunderstorms, or alternatively, by a squall line that is bowing out - the aptly named "bow echo" described by Fujita back in the 1970s. Often times both stages - the line and the bow - are observed during the evolution of a derecho. The end result is a huge swath of straight line wind damage that can vary in width but is usually hundreds of miles long. Some of the most intense straight line thunderstorm winds occur in derechoes, with extreme gusts occasionally reaching over 130 mph. The Storm Prediction Center maintains a very informative primer on derechoes, including the technical definition of what a derecho is, where and when they are observed to form, and also numerous case studies of past derechoes in the U.S. and Canada.

The derecho of May 8 had its genesis from a pair of thunderstorms in northwestern Kansas. At far left is the radar reflectivity image taken at approximately 11:55 PM Central time on May 7 from Goodland, Kansas, showing the key features of these storms. The town of Goodland is marked by the dot labeled "KGLD," and I-70 is the red line running from left to right. From this image alone, the only clue we can gather about the impending event is given by the pronounced gust front arcing to the south of these two storms. You can see it most noticeably to the south of the larger storm (the one with the more intense red and pink echoes) as a thin curved band. The Doppler velocity image is the image to the right, with its familiar greens and reds. Note in the velocities how most of the returns to the west of the radar are red, while to the east and south, the radar returns are green. The green regions signify air that is moving toward the radar site, so in this case we can assume that winds over much of western Kansas were approximately blowing from the southeast.

The thermodynamic environment across Kansas was quite conducive for downbursts. There was an ample supply of warm and humid air moving up from Oklahoma, while at the mid-levels of the atmosphere there was a layer of very dry air. When it gets entrained into a thunderstorm, dry air will cool via evaporation and contribute to the thunderstorm's downdraft. The situation was such that with the amount of very dry air present, that thunderstorm downdrafts could become quite strong. This was noted by the Storm Prediction Center around midnight when they made a bold move. By being aware of the environment over Kansas and the potential opportunities it provided, and by observing with what ease the initial thunderstorms had developed gust fronts from their strengthening downdrafts, the forecasters issued a severe thunderstorm watch for Kansas and a portion of northern Oklahoma with enhanced wording - the rarely seen "Particularly Dangerous Situation" severe thunderstorm watch - which highlighted the potential for widespread and destructive thunderstorm winds.

It's worth lingering on this for a moment. Here is the watch that the SPC issued, both the graphical form delineating the threatened area, as well as the actual text of the watch. Note the enhanced wording: "This is a particularly dangerous situation", "extremely damaging thunderstorm wind gusts to 80 mph". It is common for many of us (even me, a professional meteorologist who should know better) to at times ignore the watches and warnings that are issued for severe thunderstorms. Most of us who live east of the Rockies have been through countless severe thunderstorms. Occasionally we get the storm that produces some large hail that dents the car or damages the roof, or produces some gusty winds that bring down weakened tree branches - but it seems to me that the severe thunderstorms that actually do noticeable damage are the exception and not the rule. We pay attention to tornado watches and hit the deck during tornado warnings, but are not as mindful of severe thunderstorms. By using the "PDS" wording in this particular watch, the SPC forecasters conveyed the higher-end danger of this developing system - that this was not going to be an average event.

One final item before we return to the derecho itself, and that is to notice the extent of the area outlined by the watch, and then examine the initial radar image. The watch included huge tracts of land - the bulk of Kansas and a slice of northern Oklahoma as well. On the initial radar image from this PDS watch, we do not see a mature, raging derecho - we see two ordinary looking thunderstorms off in the far northwest corner of the box! Anticipating the development of the massive convective complex from those two lonely thunderstorms is a tremendous forecast achievement.

Moving ahead during the overnight hours, the two lonely thunderstorms quickly grew upscale. While the initial storms raced off to the east, the gust front they left behind moved south, plowing into the warm and unstable air flowing northward from Oklahoma, and initiating new thunderstorms. The image at far left is how the nascent complex of storms looked an hour after the PDS severe thunderstorm watch was issued, at approximately 1:00 AM local time. Notice how there are new storms oriented in a west-to-east band to the south of I-70.

At far left is a view of the storms at about 2:30 AM local time, as seen from the radar in Dodge City in southwestern Kansas. The left image is the wide view of reflectivity, showing the impressive storms advancing southward. At about this time, reports of damage started trickling in to the National Weather Service. The storms had robust updrafts, enabling them to produce hail of generally nickel size. The downdrafts were also strengthening at this time also. The image to the right is the Doppler velocity, zoomed in on the storms on the western edge of the formation (click on any of the images to open a full-size view in a new window). In the velocity image, note the regions of bright green located to the northwest and northeast of Garden City, and also in the vicinity of Jetmore. These bright greens represent strong inbound velocities detected by the radar. At about the time of this image, 60-70 mph winds were occurring in association with the leading edge of these storms, stretching from Garden City to Great Bend. About an hour later, winds gusted as high as 80 mph near Dodge City.

The final set of images in our examination of the early stages of the derecho comes from Wichita. As the intensifying storms advanced both toward the south and the east, the large metropolitan area of Wichita came under the gun. These two radar images (reflectivity at far left, velocity at left) were taken at approximately 3:30 AM. The reflectivity image shows a very dangerous curved band of severe thunderstorms. At this time, the storms were still generating large hail, which would continue as they advanced into Wichita about an hour later with hail as large as about half-dollars (1.25" in diameter) being reported. The velocity image continued to show the bright greens on the leading edge of the storms that were indicative of the damaging straight line winds, but the more telling feature that the radar picked up lay behind the leading edge.

Notice the area of bright greens and light blues to the northwest of Wichita. In this area, the Doppler was actually "pegged" in terms of the inbound velocities it was measuring - it was maxed out at approximately 70 knots inbound. At this distance from the radar though, these strong inbound returns were actually being measured at a considerable height from the ground, approximately 13,000 feet up in fact. These strong inbound velocities aloft, behind the leading edge, is a feature known as a rear inflow jet, which is a hallmark of mature severe thunderstorm complexes. Propelled by the rear inflow jet, the thunderstorm complex can then accelerate forward and "bow out" as the line begins to bulge at the apex of the jet. The region at the apex of the bow echo is susceptible to the strongest straight line winds at the surface.

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At this point, the thunderstorm complex was maturing as it bore down on Wichita. It would barrel through the city at about 4:30 AM, dropping large hail and raking the region with winds of up to 80 mph. But the worst was still to come as the line would bow out and accelerate east-southeastward across the southeastern part of Kansas.

Drought Status Update

After all the talk of severe weather and flooding rains over the past few weeks, now is a good time to re-examine the drought situation across the country. As the growing season really cranks up across much of the central and eastern U.S., we find that conditions are looking pretty good - if anything, some areas have been a little too wet so far this Spring.

We begin with the latest edition of the U.S. Drought Monitor, whose summary map appears at left (click to open a larger version in a new window). The Drought Monitor classifies drought severity on a five-point scale, ranging from D0 (abnormally dry), through increasingly serious drought categories that culminate with D4 (exceptional drought). From the summary map, the majority of the central and eastern U.S. is in good shape. There are patches of yellow in the East, signifying areas that are abnormally dry (D0), and an area of northern Wisconsin that is experiencing moderate (D1) to severe (D2) drought. Much of California is in drought once again after some welcome rains earlier this year. The most serious drought conditions exist in Texas and southern Florida.

The dry season has not been kind to southern Florida. Rainfall for the year to date is running less than 40% of normal for most of central and southern Florida:

City		YTD Precip	Normal

Orlando		4.24 in.	11.23 in.

Miami		3.41		10.68

Fort Myers	1.51		 9.04

Naples		1.38		 8.78

Including the months of November and December, the period from November-April has been the second driest on record for much of southern Florida. As a result, much of central and southern Florida is classified in severe drought, and the southern reaches of the peninsula are in extreme (D3) drought. According to estimates of areal coverage compiled by the Drought Monitor, about 47% of the land in Florida is in D2 or worse drought. The D3 area encompasses approximately 11% of the state, and this is a 2% increase compared to last week. The entire severe-extreme drought area has developed during the very dry spell of the past 3 months.

In California, drought conditions are not as grim as in other parts of the country, but the drought is affecting virtually the entirety of the intensely agricultural Central Valley. Based on estimates from the Drought Monitor, approximately 72% of California is experiencing moderate to severe drought. The area in severe drought totals about 35% of the state, but includes the Central Valley as well as the northern coastal valleys (such as the Napa wine region). The drought situation in California has actually been improving slightly of late. Welcome showers have helped reduce the D2 area by about 3.5% compared to last week, and by almost 20% compared to 3 months ago. However, 96% of California is still classified as at least abnormally dry (D0 and worse) - but this is down about 3% from 3 months ago.

The worst drought conditions in the U.S. currently are in Texas. The highest category of drought, D4, exists across southern Texas and covers about 10% of the area of the state. Almost 54% of the state is in at least moderate drought, with 40% in severe drought or worse. Conditions in southern Texas have not shown much improvement of late. While rains have been abundant in the eastern part of the state, the western and southern parts of Texas have largely missed out. As a result, the most severe D3 and D4 areas are holding steady, although the periphery of these areas most impacted has seen some retreat of the D1 and D2 conditions.

The Drought Monitor change maps do not show much change either way (improvement or deterioration) in the drought areas over the past week, so we'll skip to the 1-month (far left) and 6-month (left) change maps. On the change maps, brown (and red) areas are where drought has either developed or worsened, green (and blue) areas are drought areas that have seen some improvement, and yellow areas are drought areas where conditions are generally unchanged. Over the past month, the news has generally been good. There are green regions across parts of Texas (such as the Panhandle) where the most serious drought conditions have relaxed slightly. In addition, drought conditions across Georgia and northern Florida improved so much that they are no longer in drought anymore. However, areas of California, southern Texas, and southern Florida witnessed an intensification of the drought.

The 6-month change map is not one we present often, but in this case it is very striking. Note the expansive area of blue and green from the Ohio Valley to the Southeast. This region is one we talked about often in these updates. Over the past 6 months, the drought has been pretty much eliminated in the Southeast and the Ohio Valley. In southern Texas and southern Florida, we also see the brown and red areas that indicate the development of the current serious drought conditions over the past 6 months. So, we see that over the course of the winter and early spring months, that drought has been relieved in one part of the country while redeveloping elsewhere. Indeed, it seems that there is always drought somewhere in the country.

Hot off the presses, we have the newest drought outlook from the Climate Prediction Center, and it appears at left. The areas shaded in brown are forecast to see their ongoing drought persist or worsen. Areas in green are drought areas that are expected to see improvement of at least one Drought Monitor category over the next three months, while the areas in brown with green hatching are drought areas that are forecast to see less substantial improvement, but some improvement nonetheless. The news on this map is good, as one might expect heading into the warm season. The onset of the rainy season in Florida is approaching (it typically begins in late May), and once the almost daily showers are established drought conditions should ease. Improvement is also expected in parts of Texas, but the most seriously afflicted area in southern Texas may not see much in the way of improvement heading into July. It may take a landfalling tropical system to bring the kind of rains needed to relieve the drought there. In California, the news is not as optimistic as the wet season is drawing to a close and the dry season looms ahead.

We usually don't do this in our drought status updates, but we also need to highlight the areas of the country that have been excessively wet recently. At left is the most recent map of the Palmer Drought Severity Index (PDSI) for the country. Not surprisingly, the drought regions of Florida, California, and Texas appear as dry (yellows and oranges) on the map. But notice the large area of the central U.S. shaded in green. Encompassing the Plains, much of the Corn Belt, and extending into the Great Lakes, this green region reflects where conditions have been extremely moist lately. Not surprisingly, these are the areas that have been experiencing flooding lately due to the recent excessive rains, and these same regions will remain vulnerable to additional episodes of flash flooding and/or river flooding until a substantial dry period can occur.

Microburst Strikes Dallas Cowboys Practice Facility

A round of severe thunderstorms packed a serious wallop as it moved through the Dallas-Fort Worth metro area on Saturday afternoon. Numerous reports of severe hail were received across the region, along with straight line winds. Funnel clouds and possible tornadoes occurred to the northeast of Dallas. The most notable damage occurred just after 3:00 PM local time on Saturday afternoon when a microburst moved over the city of Irving, home of Texas Stadium and the Dallas Cowboys football team. The severe straight line winds of the microburst collapsed the roof at the team's practice facility in Irving, resulting in several serious injuries as well as significant damage to the building itself.

A microburst is a phenomenon associated with severe thunderstorms that was elucidated by Ted Fujita (he of the famous Fujita Scale for tornadoes). A microburst is a very small-scale version of a downburst, which occurs when a mass of rain-cooled air held aloft in a mature thunderstorm suddenly comes crashing down to Earth. When the rapidly descending air reaches the ground, it spreads out radially in all directions from the point of contact, often with great ferocity. In extreme cases, the straight line winds from a microburst have reached 150 mph.

The Dallas-Fort Worth area is no stranger to microbursts. On August 2, 1985, Delta Air Lines Flight 191 was attempting to land at the DFW Airport when it encountered a microburst. The extreme vertical motions and wind shear caused the plane to crash before reaching the runway, killing a total of 135 people on the plane and on the ground.

At left is a radar image from the Doppler radar serving the Dallas-Fort Worth area, taken as the storms were exiting Fort Worth (on the left side of the image) and heading toward the Dallas side of the metroplex at 3:01 PM local (click to open a larger version in a new window). This image shows what is a fairly typical configuration for severe thunderstorms, with the most intense parts of the storms lined up from Fort Worth off to the northeast. The area of interest, which would go on to produce the microburst, is the segment of the line just east of Fort Worth, straddling I-20 and I-30 (the two roads shown on this map running east-west between Dallas and Fort Worth).

The series of three images at left shows the Doppler velocities at the time of the microburst. The first image was captured at 2:53 PM, the middle image at 3:01 PM, and the third image at 3:14 PM when the strongest winds were affecting Irving. For orientation, Fort Worth is on the left, Dallas is on the upper right. Irving is approximately located by the small red dot (which is most easily located from the third image). In these images, the radar is located in the lower left, marked by the blue dot labeled "KFWS." These maps of Doppler velocities show us how the winds are blowing in relation to the radar. The green areas have winds blowing toward the radar, and brighter greens indicate higher inbound velocities. Red areas are where the winds are blowing away from the radar. Stronger outbound velocities are indicated by brighter reds and pinks.

Note the area of brighter reds that first appears to the east of the radar site in the leftmost image. These are the strong winds generated by the severe thunderstorms. The outbound velocities in the first image show where the Doppler radar was able to measure winds of 50 mph to nearly 65 mph. By the time of the middle image, taken just eight minutes later, the area of strong winds has rocketed quickly northeast and is affecting the cities of Arlington and Euless, approaching the DFW Airport. In the third image, the thunderstorm winds slam into Irving. The Doppler radar at this time was able to measure winds of approximately 65 mph.

One of the limitations of Doppler radar is that it only senses winds in the radial sense; that is, it can only measure that part of the wind that is oriented directly toward or directly away from the radar site. If the wind we want to measure is blowing perpendicular to the radar's beam, the Doppler cannot sense it. This is likely why the reported wind speeds on the ground (estimated in the 70 mph range) were greater than what the Doppler was sensing - because there was a component of the wind perpendicular to the radar beam that the Doppler could not detect.

In any event, the microburst in Irving will be another unfortunate chapter in the phenomenon's history in the region. Although there were no deaths reported from Saturday's microburst, the impact on the region's beloved Cowboys was severe. Several injuries were reported among the players and staff who were at the facility at the time of the microburst. The same cluster of severe thunderstorms would go on to leave a swath of wind damage across northeastern Texas after departing from the Dallas-Fort Worth area. Emergency management in Anderson County (in the Palestine vicinity) reported widespread significant tree damage all around the county, with "more trees down than with Hurricane Ike."

Much more information on this weather event is available through the mainstream news media. For another meteorological analysis, the CIMSS Satellite Blog has posted an excellent summary from a satellite's perspective.

A Soggy Forecast for the Central U.S.

The region of the country from the southern Plains to the western Corn Belt has seen a very soggy time lately. A stalled frontal boundary is serving as a focusing mechanism for abundant moisture streaming north from the Gulf of Mexico, generating rounds of heavy showers and thunderstorms. This regime has been in place since this past weekend, and soils across much of the area are nearing saturation. Excessive runoff is also contributing to rises on creeks and rivers. In addition to the areal flood threat, the intensity of the rainfall has led to some exceptional rainfall totals, and certain localized areas have been especially victimized by the rains.

One of the first locales to see flash flooding was the Houston metro area. A deluge of rain occurred from April 24-25. Thunderstorms with very heavy rainfall developed repeatedly and then trained across the same areas. Rain was falling at an intensity of 2-3 inches per hour at times, which is enough to overwhelm even Houston's drainage and flood control. Creeks and bayous through the city spilled out of their banks, flooding basements and first floors of structures nearby and leaving motorists stranded on impassable roads. The map at left shows the estimated precipitation which fell during this episode (click to open a full size version in a new window). The red shading indicates at least 3 inches of rain, while the magenta shades show at least 6 inches of rain.

Another heavy rain event hit the Houston metro area on April 27-28 (rainfall map pictured at left; click to open a full size version in a new window). Thankfully, the areas hit hard a few days earlier were spared the worst of this round of rainfall. Still, much of the metro area received 1-2 inches of rain, with the western sides of the metro getting 3 inches or more, with isolated amounts approaching 9 inches. It has been a very wet time across metro Houston in April. The Harris County Office of Homeland Security and Emergency Management has a slick interface for displaying rainfall reports across the region. The 30-day totals are amazing.

Much of Oklahoma has also been experiencing heavy rain lately. Intense rainfall occurred in northwestern and north-central Oklahoma over the weekend in association with some severe weather with tornadoes reported near the city of Enid. Some locations recorded over 7 inches of rain during this episode. More recently, thunderstorms with heavy rain passed repeatedly from west to east along the Red River valley, producing multiple episodes of torrential rain. The town of Burneyville, located on the Red River and about an hour's drive north of Fort Worth, received an incredible 12 inches of rain on April 29. The map at left shows the 7-day rainfall totals, as of April 30. Note the purple area in far southern Oklahoma; this is the area around Burneyville which received over 10 inches of rain. Red shades indicate over 5 inches of rain during the 7-day period. The area of heavy rain extends north into Kansas and Missouri. Creeks and streams in the Kansas City metro area are swelling from the excessive rainfall.

You can also see from the map at left how the heavy rain continues northeastward along the stalled frontal boundary through the western Corn Belt and toward the Great Lakes. This map shows the precipitation anomalies over the 7-day period ending on April 30. Shades of green, blue, and purple indicate above normal rainfall. The light blue indicates a surplus of 2 inches during the period, while the dark blues and purples indicate excesses of 3 to as much as 8 inches of rain.

Because of the widespread above normal rainfall that has occurred over much of the central U.S. this past week, flooding is becoming a concern. The map at left shows the watches, warnings, and advisories from the National Weather Service on the afternoon of April 30. The various counties across the region that are shaded in green are under some sort of flood threat. The brighter green which extends from Illinois through central Missouri and into eastern Kansas indicates flood warnings, where flooding is already occurring, mainly either lowland flooding or river flooding. Note also the flood warnings on the James River in South Dakota and the Red River of the North in North Dakota and Minnesota. The slightly darker shade of green which covers much of Oklahoma as well as southern Kansas and Missouri, indicates flash flood watches. In these areas, the ground is saturated or nearly so, and it will not take much additional rain to produce rapid runoff, quick rises, and flash flooding.

The outlook for the upcoming week does not provide much in the way of good news for the areas of the central U.S. that are already growing weary of the rain. At left is our forecast precipitation anomaly for the next 7 days, which is current as of the afternoon of April 30. The forecast calls for continued opportunities for rain, which may be heavy at times, from the central and southern Plains up the Ohio Valley. Most of these regions are shaded in the darkest blue, corresponding to forecast rainfall surpluses of at least 1 inch, and likely quite a bit more.

To view rainfall maps in near-real time, we recommend the National Weather Service's Advanced Hydrologic Prediction Service page, available at http://water.weather.gov/ .

Eye Candy for Weather Geeks

Every now and then, we see something in the course of our daily work that captures our attention and brings out the inner "weather geek". We enjoy seeing cool things that make us go "Ooooh... what is that?" and take a closer look. Today we found three such things; two are interesting looking even to casual weather observers, while the third is a sight for sore eyes in parts of California where the air conditioners have been working hard lately.

We begin with this item, seen on visible satellite imagery at left moving across northern Missouri (click to open a full-size version of the image in a new window). Astute observers will notice that the swirl of clouds bears some resemblance to a developing tropical storm, with the central mass of convection but most notably with the spiral band of clouds on the northern semicircle, extending from northwestern Missouri, up through Iowa, and then from there southeastward across northern Illinois.

What we're seeing here is not a tropical cyclone, even though it might look a little like one. This is an ordinary complex of thunderstorms, called a mesoscale convective system (MCS). What makes this MCS so special is its well developed swirl (which is clear in satellite and radar animations, which, sadly, we cannot reproduce here). The small scale circulation within an MCS is known as a mesoscale convective vortex, or MCV. Not all MCS's can generate MCV's, but once the MCV is present it can help sustain the thunderstorms as the complex moves along. Occasionally the MCV can help spawn tornadoes, but this is not common. This particular MCV is very pronounced.

The other feature of note in this thunderstorm complex is the high level outflow. This is manifested in the spiral band of high clouds on its northern side that we touched on earlier. Well developed MCS's will sometimes develop the MCV, the counterclockwise circulation at the low and mid levels, but at higher levels the circulation reverses and becomes clockwise. Satellite animations make this clear; the thunderstorms within the complex exhibit some counterclockwise turning, but the high level outflow exits the system in a clockwise fashion. This is thermodynamically very similar to a tropical storm or hurricane, which have the usual counterclockwise flow at low levels, with clockwise outflow aloft. This similarity has led some meteorologists to casually refer to MCS's like this as "landphoons."

The next item on the list may require some readers to turn their monitors upside-down, since it comes to us from the Southern Hemisphere. It is Autumn for our friends down under, and the tropical season is still ongoing although the end is approaching. The item of interest (you'll probably need to open the full-size version of the satellite image at left in a new window) is near the right edge of the screen. We're looking at the swirl of clouds and showers located between the east coast of Australia and New Zealand.

This system began about a week ago when a common storm system stalled over the Tasman Sea. The center of the storm became cutoff as its upper support moved on, leaving behind what meteorologists refer to as a "stacked" area of low pressure: a low at the surface, with an upper level low right above. In most cases, vertically stacked lows tend to weaken. However, if the stacked low is stalled, and it is sitting over an area of moderately warm ocean waters, and the upper level low is strong enough and cold enough, the atmosphere can gradually destabilize and thunderstorms can form. The vertically stacked low provides an environment of reasonably low wind shear, and low wind shear plus thunderstorms plus an established cyclonic (clockwise in this case since we're in the Southern Hemisphere) circulation is a recipe for an uncommon type of tropical development: the subtropical or "hybrid" storm.

We've seen cases like this before (last season, Tropical Storm Laura in the Atlantic had subtropical origins; the "perfect storm" of 1991 in the North Atlantic is an extreme example). However, the scenario that seems to be most like this current case is the 2004 tropical cyclone which formed off the coast of Brazil in the South Atlantic. In both cases, a stacked low pressure stalled over marginally warm ocean waters, and in time developed convection and found new life as a tropical entity.

In any event, the cyclone (Tropical cyclone? Subtropical cyclone? Hard to say for sure.) in the Tasman Sea has not been officially designated as a tropical or subtropical system by the responsible parties (the Australian Bureau of Meteorology office in Brisbane, or the New Zealand Weather Service in Wellington), although there are gale and storm warnings for mariners in the area. The storm, which has been slowly making a clockwise loop over the past week, is expected to drift south and then southeast, where it should get picked up by the main belt of westerlies in the Southern Hemisphere. As it gains latitude, it will lose whatever tropical characteristics it has managed to cobble together. It may bring gusty winds to New Zealand's South Island this weekend, but mainly this system is just a meteorological curiosity and not a significant threat.

The last item of meteorological eye candy is far less dramatic than the "landphoon" or the hybrid/subtropical storm, but for residents of California it is the welcome return of an old friend. Earlier this week, numerous record highs were set in California as a strong upper level ridge built in over the region. The combination of warm temperatures aloft and offshore flow spelled heat for the coastal regions. Temperatures in the valleys surrounding Los Angeles reached over 100. San Diego checked in with 98 degrees and even San Francisco got up into the 90s during the hot days. Now that the upper ridge has moved on, a more typical regime has returned to California, and you can see it in the satellite image we have at left (click to open a full-size version in a new window).

The marine layer off the West Coast is the layer of air in contact with the cool Pacific waters. It is usually cool and somewhat damp. When the marine layer is present, it usually sweeps inland at night to blanket the coastal areas with cool air, low clouds, and sometimes fog. During the day it retreats and breaks up. This diurnal cycle of the marine layer leads to the typical Southern California forecast in May and June: late night and early morning low clouds and fog, clearing by afternoon. With the upper ridge gone, the marine layer has returned. It can be seen in satellite images by the blanket of low clouds that hugs the California coast and has pushed inland several miles.

As seen in our image, the marine layer has pushed exceptionally far inland over Southern California. Almost the entire Los Angeles Basin is shrouded by the low clouds and fog, including several valleys well inland from the coast. The cool maritime air, with temperatures only in the 60s across much of the Basin, provides welcome relief and stands in sharp contrast to the temperatures of a few days ago which in some cases were higher by almost 40 degrees.

Deadly Tornadoes in the Southeast

An outbreak of severe thunderstorms struck the south-central and southeastern U.S. last week. An unseasonably strong and dynamic upper level storm developed over western Oklahoma and pushed east-northeast across the lower Mississippi Valley and on toward the Tennessee and Ohio Valleys. Severe thunderstorms produced abundant reports of hail, damaging straight line winds, and tornadoes. Some of the tornadoes that affected the region on April 9 and 10 proved to be deadly.

Severe thunderstorms initially erupted in eastern Oklahoma on the evening of April 9. The first tornado was reported in the northeastern corner of the state, but the more significant and long-tracked tornadoes developed in the southeastern part of Oklahoma as well as adjacent extreme northeastern Texas, before trucking along into Arkansas and Louisiana. The most damaging storm of the day struck the mountain town of Mena in far western Arkansas. Rated EF3 on the Enhanced Fujita Scale, this tornado did substantial damage to the city center and has resulted in 8 fatalities thus far. Another EF3 tornado struck farther to the south near the town of Dierks, and a third EF3 affected Miller County. The Shreveport-Bossier City metro area in northwestern Louisiana was also affected by tornadoes during the event. In all, a total of 19 tornadoes from April 9 have been confirmed by National Weather Service survey teams. Of these, 5 were rated as EF2 (strong), to go along with the 3 tornadoes rated EF3 (severe). In addition, there were also 103 reports of damaging straight line winds (58 mph or greater), and 151 reports of severe hail (0.75 inches in diameter or larger).

As the storm system advanced east on Friday, April 10, the afternoon heating generated enough atmospheric instability to spark another round of severe thunderstorms and tornadoes. These storms affected areas of Kentucky, Tennessee, Mississippi, Alabama, Georgia, South Carolina, and North Carolina. The most significant storm of the day laid waste to a part of Murfreesboro, Tennessee with an EF4 tornado that claimed two lives and caused an additional 42 injuries. Five more EF3 tornadoes raked the region, to go along with two EF2 tornadoes. A total of 32 tornadoes have been confirmed across the seven states so far from April 10.

The preliminary totals from this two-day outbreak of severe weather:
51 total tornadoes
- 7 EF2 (strong)
- 8 EF3 (severe)
- 1 EF4 (devastating)

214 straight line wind reports
- 2 significant (greater than 74 mph)

419 severe hail reports
- 29 significant (greater than 2" in diameter)

A potentially greater threat is evolving in the southern Georgia and northern Florida. Rivers are already running very high thanks to a widespread heavy rain event last week. Additional rains fell over the weekend during the severe weather episode, and now today another heavy rain event is taking place. Rivers such as the Suwannee and Withlacoochee are in major flood and record flooding is becoming a very real threat with the continued rains. US Highway 90 is already closed in the area due to flooding, and there is a risk that Interstate 10 may be forced to close as well.