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.
---
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.
---
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.
Comments