Extended Forecast
Valid: Mon 23 Jun 2008 06:00 to Tue 24 Jun 2008 06:00 UTC
Issued: Sat 21 Jun 2008 22:50
Forecaster: TUSCHY

SYNOPSIS

An active severe weather day is in store for most parts of Europe. A strong SW flow has established over central Europe with a strong trough entering the forecast area over Portugal and Spain from the SW. A very hot airmass ( 850 hPa temperatures just north of Algeria between 25 and 30 °C ) advances north / northeastwards, resulting in stable but hot conditions over most parts of the western / central Mediterranean. At the surface a west - east aligned frontal boundary, running from central France over S - Germany to W - Belarus, serves as a focus for thunderstorm initiation. The front will be nearly stationary during the forecast period, despite some local fluctuations An intense depression affects parts of N - Europe with strong CAA on its upstream side.

DISCUSSION

... Central France ...

A southward ploughing cold front slows down during the morning hours, becomes stationary somewhere over central France and reconverts into a warm front during the afternoon and evening hours as an intense upper trough over Spain shifts northeastwards. This front will be the focus for convection during the forecast period. Despite the fact that model guidance was quite consistent regarding placement of the front we think that convective activity of the day before could alter the final position of the front so some uncertainties remain. Another missing ingredient will be the forcing although GFS now has a weak short - wave coming out of Spain, crossing the area during the afternoon and night hours from the WSW. Progged dewpoints of global models are in the range of 20 - 22°C just along the boundary and we see no reason to drop those values as upper tens / lower twenties are already in place over S - France. Modified EML in place so mid - level lapse rates are and stay very steep and combined with abundant BL moisture, MLCAPE values above 1000 J/kg are likely, however those values are confined to the immediate boundary. Both, wind shear and veering ( DLS of 25 m/s and SRH 3 values of 200 - 400 J/kg ) are very favorable for organized thunderstorms with a significant hail risk and extreme hail events above 5cm will be possible. Another risk will be a tornado risk, more defined to the northern parts of the boundary, where LCLs stay low. LL SRH increases during the evening and night hours just north of the front and coupled with plenty of shear, organized thunderstorms with a tornado risk likely evolve along the boundary. Thunderstorms will tend to move NE - wards along the northward moving warm front, so we decided to expand the level area well northwards. Uncertainties regarding storm coverage and exact placement of the front preclude a level - 2 area but an upgrade may be issued later - on.

... Spain ...

Another area where conditions become favorable for intense thunderstorm evolution will be central / north Spain. A negative tilted and rapidly northeastward racing upper trough will cross Portugal and Spain from the SW during the evening and night hours. Attendant forcing is compact and very intense so thunderstorm initiation should finally take place around sunset despite very warm low levels / intense capping as 850 hPa temperatures top out at 20 - 25°C over E / SE Spain. Mid - levels start a constant cool down, resulting in rapidly steepening lapse rates during the midday and afternoon hours. It is very difficult to say if thunderstorms will manage to develop already well ahead of the forcing over augmented topography but DLS of 30 m/s, some veering and steep lapse rates ( up to 1000 J/kg MLCAPE release ) would result in a significant hail threat with each thunderstorm. Another area of concern will be an eastward moving, diffuse cold front, crossing Spain between 18 Z and 06 Z from the SW, where scattered thunderstorms will likely evolve as cap weakens and forcing moves in. Shear is still more than abundant for severe hailstorms. Finally the third area where scattered thunderstorms will evolve will be under the base of the cold upper trough as LL stay moist. As DLS weakens and atmosphere becomes uncapped, a cluster of thunderstorms likely evolves somewhere over N - Portugal / W - Spain, moving north / northeastwards during the night hours with an excessive rainfall risk. Marginal hail and strong wind gusts will accompany stronger thunderstorms. Again, a broad level - 2 area due to the hail threat may be issued later - on if confidence regarding storm coverage along and ahead of the front becomes higher.


... N - Switzerland, S - Germany and Austria ...

There will be two areas to look at. One will be the stationary frontal boundary over S - Germany. Rich BL moisture coupled with steep lapse rates will lead to 700 - 1000 J/kg MLCAPE and frontal forcing / complex topography of the Black Forest or the Bavarian Forest should support scattered thunderstorms. DLS of 20 - 25m/s is strong enough for organized thunderstorms with a large hail risk and an isolated significant hail report will be possible. Thunderstorms will also pose an excessive rainfall and strong wind gust threat and they will last well into the night hours.

The other area will be the Alpine region, where diurnal driven thunderstorm activity can be expected although widespread nature of storms and attendant clustering could result in thunderstorm activity, lasting well into the night hours. DLS is somewhat weaker but still strong enough for large hail / strong wind gusts and excessive rain.

... Estonia, Latvia, Lithuania, parts of Belarus, parts of Poland and the western Ukraine ....

A strong cold front shifts rapidly eastwards. A strong jet points into the rear of the surface front over Estonia, Latvia, Lithuania, E - Poland and western Belarus resulting in a widespread severe to damaging wind gust risk as wind field is very intense with DLS of 25 - 30 m/s and up to 25m/s at 700 hPa. Widespread thunderstorms will go on / develop along the front already during the morning hours and those storms will continue during the day, before weakening as they move off the instability axis and into a more stable environment over extreme western Russia. An extensive MCS with severe wind gusts and an isolated tornado threat will be the result.

Further towards the south, over S - Poland and western Ukraine, the background flow is aligned more parallel to the surface front and DLS drops below 20m/s. Scattered thunderstorms will go on / develop and multicells / isolated supercells will likely result in a large hail and strong wind gusts risk. Latest model guidance indicates an organized cluster of thunderstorms, moving into western Ukraine after 18Z and this would enhance the flash flood risk. Favorable environment will help thunderstorms to keep going through the night.

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