Storm Forecast
Valid: Sun 24 May 2009 06:00 to Mon 25 May 2009 06:00 UTC
Issued: Sat 23 May 2009 23:32
Forecaster: PUCIK

A level 2 was issued for N Spain and SW France mainly for large hail, severe wind gusts, excessive precipitation and tornadoes.

A level 1 was issued for Spain mainly for large hail, severe wind gusts and excessive precipitation.

A level 1 was issued for Central France, Germany, Austria, Slovenia, N Croatia, W Hungary mainly for large hail and severe wind gusts.

A level 2 was issued for SE Germany, Austria and N Slovenia mainly for large hail, severe wind gusts and tornadoes.

SYNOPSIS

A composite situation is anticipated across Europe with several significant features at mid-levels. One of them is a trough, with its axis lying over E Russia and Ukraine by Sunday morning. This trough is predicted to deepen while shifting southeastwards and it will be surrounded by a strong flow, having more than 20 m/s at 500 hPa level. At the same time a ridge, extending from the Central Mediterranean into Italy, France will become more prominent and later on, will spread into Germany. Strong WAA regime has established in the wake of the ridge with temperatures at 850 level reaching 20°C across the Mediterranean Sea. Another feature will be a cut off low, with center over Eastern Spain with southerly jet at its forward flank.

At the surface, a deep low pressure system will stall over E Russia, while a ridge of high pressure stretching from Atlantic into Central Europe will develop into a separate high with center over Germany by Monday morning. Slight cyclogenesis is expected over Spain and probably a shallow low will develop, moving to France by Monday 06Z. A warm front will stretch from S Britain to Central Germany, slowly advancing northwards and a cold front will cross Spain during the forecast period.

DISCUSSION

...N Spain and SW France...

Favourable kinematic setup is expected in this region, being under a jet on the forward flank of the mid-level cut off low with more than 20 m/s ( locally even over 25 m/s) of DLS expected and shear in the lowest 3 km should stay around 15 m/s. At the same time a day time heating should lead to the realease of 500 - 1000 J/kg of MLCAPE ( though locally higher values are not ruled out). QG forcing along with the frontal uplift should aid in the storm initiation and well organised storms will form, including multi and supercells. A development of the surface low will lead to the backing surface flow and a belt of enhanced SREH, with the highest values over extreme N Spain / SW France, sustaining rotating updrafs.

Large hail can be expected, especially in case supercells manage to form. Moreover, severe wind gusts will be another threat, with drier air at lower levels as suggested by model soundings and at the same time an expected forward building of the potential MCS . Provided a squall line will develop with embedded bowing segments, wind gust threat might even become the most serious hazard of the day. Strong release of the instability at the lower levels, slightly enhanced low level shear ( over 10 m/s) are simulated by the models, so tornadoes are not ruled out either (although . A heavy rainfall event might occur as well, due to the fact, that storm propagation should be parallel to the boundary and a "training" effect might occur. At the moment, it looks like MCS will form in Spain and travel to the north across Bay of Biscay. High amounts of rainfall are simulated in the path of this MCS.


...Spain excluded from Level 2...

In the region surrouding Level 2, environment will be less conducive to severe thundestorms, especially concerning the instability. ECMWF in the Saturday 12Z run does exhibit only less than 500 J/kg of CAPE over the region (GFS has slightly higher values). Nevertheless, with storms developing in strong wind shear, isolated large hail might occur, as well as severe wind gust. Heavy convective rainfall is forecast across the eastern part of the region and a local flash flooding, especially in the mountaineous areas, can occur. The highest threat will exist between 15Z and 21Z, dimnishing later on.

... Central France to Germany, Slovenia, N Croatia...

Impressive values of MLCAPE are simulated both by GFS and ECMWF models with values well above 2000 J/kg, obviously resulting from the presence of a very humid airmass, with dew points over 20°C shown by models for the region of Central France. This is not far from the current observations so the large values of MLCAPE seem reasonable. Wind shear should be relatively weak, only between 10 - 15 m/s, but a belt of enhanced SREH is anticipated in wake of the warm front. The strenghtening ridge and at the same time a thick cirrus deck predicted by the models should limit the storm development with ECMWF being more in favour of isolated storm formation while GFS trends towards leaving most of the area free of activity. Large values of CAPE suggest that any well organised multicell will have a potential to produce large, or even very large hail ( on a very isolated manner, however). Values of Delta Theta-E should exceed 16 K over France and therefore, strong wind gusts might occur with wet microburst activity over this area.

This area will be closely monitored for a possible update, especially concerning the storm initiation.

... SE Germany, Austria, N Slovenia...

A moderate to strong instability release is anticipated with MLCAPE values having more than 1000 J/kg across most of the region both per GFS and ECMWF. Strong mid level flow will lead to the enhanced wind shear values, around 20 m/s in 0-6 km layer and between 15 - 20 m/s in 0-3 km layer. Furthermore, GFS shows that belt of enhanced SREH should nicely overlap with the strong instability and combined with the favourable wind shear profiles, well organised multicells or even supercells are expected. Thanks to the high CAPE and high level of storm organisation, large or even very large hail might occur. Severe wind gusts migh occur in association with supercell downdrafts. Strong release of instability in the lower levels and a belt of stronger low level shear ( approximately 10 m/s) suggest, that an isolated tornadic event can not be excluded.

Also this area will be under a close watch, as models keep on changing the expected storm coverage. If the storm initiation proves to be less widespread as expected a downgrade to Level 1 might become necessary later on.