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.
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