Monday, June 08, 2009

First major severe thunderstorm outbreak of the 2009 summer season.

A severe thunderstorm outbreak occurred over parts of western and central Europe on Monday, May 25th, and Tuesday, May 26th. The synoptic maps on both days featured favorable conditions for severe thunderstorms over an extensive area. An upper-level trough moved in from the Bay of Biscay on the 25th, lifted northwards on the subsequent day and was placed over the North Sea on the morning hours of the 27th. An warm and unusal humid airmass covered NE-France, Belgium, Luxembourg and the Netherlands on the 25th with surface dewpoints in the twenties over NE-France. Steep lapse rates, generated by an elevated mixed layer and a moist boundary layer below, were responsible for extreme amounts of convective available potential energy (CAPE) over these areas with modelled values in excess of 3000 J/kg. Such values are extremely rare in Europe, and occur only once in a few years at a given point.

Additionally, moderate to high amounts of wind shear were in place, prompting ESTOFEX to issue a broad level-2 in its convective forecast. Emphasis of the convective forecast were the possibilities of giant hail (given the extreme CAPE values) and a serious wind gust threat.

After a long-lived cluster of thunderstorms evolved over the Bay of Biscay during the evening hours of the 24th and entered the western English Channel at the 25th around midnight, little convective storms developed. Then, new storms intiated over northern and western parts of France and across Belgium in the early afternoon. A small cluster, including a massive supercell crossed Belgium moving into Germany. Further initiation took place over central France at roughly 5 p.m. CET and this activity organized rapidly into a northeastward moving MCS with numerous, discrete supercells ahead of it. Hail around 10 cm in diameter was reported with these supercells, that naturally caused widespread hail damage. Overnight, a strong squall-line developed that produced gusts in excess of 35 m/s across Northern France, Belgium and the Netherlands. Spectacular photography of those storms can be found in for example, a report by our French colleagues of Keraunos. A radar loop of the beleux countries can be found here.

Another severe thunderstorm outbreak occurred over parts of Switzerland and S/SE-Germany as a long tracked bow echo raced northeastwards on Tuesday, May 26th. For that day, ESTOFEX again issued a level-2. Thunderstorms developed over Switzerland between 11 and 12 UTC and a continuous line evolved, running from the Black Forest area all the way down to Ticino. The most organized part of the MCS was situated over N-Switzerland with a rapid motion towards the northeast. During the passage of the Lake of Constance, radar scans revealed further organization into a well structured bow echo with a sharp reflectivity gradient along the leading edge and a pronounced rear inflow jet. During the passage of Landsberg, doppler velocity radar data measured 35-40m/s winds at the lowest 500-2000m, which overlaps nicely with a 39m/s wind gusts from Landsberg. Intense rain amounts with flash flooding and 3-4cm hail were observed in this line, which produced a swath from NE-Sitzerland to SE Bavaria with wind gusts in excess of 25m/s. See The bow echo finally decayed over SE Bavaria during the evening hours with new initiation next to Salzburg.


Anonymous Anonymous said...

link to my video, made on march 26th 2009 in Z├╝rich: much hail and excessive rainfall in the city

Tuesday, July 28, 2009 11:33:00 AM  

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