Storm Forecast
Valid: Fri 08 Jun 2012 06:00 to Sat 09 Jun 2012 06:00 UTC
Issued: Fri 08 Jun 2012 02:54
Forecaster: PUCIK

A level 1 was issued for NE France, SE Belgium, W to NE Germany mainly for marginally large hail and tornadoes

A level 1 was issued for SE France, N Italy, Central and E Austria mainly for large hail and severe wind gusts.

A level 1 was issued for NE Austria, E Moravia, W Slovakia mainly for excessive precipitation and severe wind gusts.

A level 1 was issued for N Slovakia to SE Poland mainly for large hail and to the lesser degree for severe wind gusts and excessive precipitation.

SYNOPSIS

A mid and upper level cyclonic vortex is forecast to more or less reside over the region of British Isles with only slight northeastward movement. Two attendant short wave troughs are forecast across Europe, The most significant one, stretching across W Iberia during the beggining of forecast period will translate eastwards. Another short-wave will move over N Germany towards Scandinavia. At the forward flank of the low, enhanced SW-ly flow overlies most of Central Europe with 500 hPa windspeeds locally exceeding 30 m/s. A ridge will amplify over the Central Mediterranean, spreading towards SE Europe. To the northeast a trough over W Russia will turn into a separate cyclonic vortex.

Regarding low-levels, a deep occluded low pressure system will persist over British Isles with a more shallow, but broad region of low pressures covering most of Western to Central Mediterranean and Western to Central Europe. Separate cyclonic centers are likely along the wavy frontal boundary that will stretch from W Iberia into SE France, N Italy/Switzerland, Austria and further towards Poland, especially connected to the lee cyclogenesis around the Alpine range. Another frontal system with surface low will be slowly drifting eastwards across W Russia.

DISCUSSION

... E Spain ...

Models agree on surface E to NE flow advecting moister airmass over the coast, topped by pronounced EML, resulting in possible instability built-up. Models are relatively inconsistent regarding convective initiation, but any possible initiating cell might become well organised in DLS exceeding 20 m/s, posing a slight risk of severe hail and wind gusts. Questionable initiation precludes issuance of Level 1 attm.

... Southern UK ...

A belt of strong flow at low levels, reaching 20-25 m/s at 850 hPa level is simulated by the models thanks to the proximity to the deep surface low. Scattered, embedded convection within predominantly stratiform precip is forecast with very weak, if any, latent instability. Albeit marginally severe wind gusts can be expected in the downdrafts of DMC, contribution of convection to their strength should be small and limiting instability precludes issuance of Lvl 1.

... N France through SE Belgium towards NE Germany ...

Scattered showers and thunderstorms are expected to develop in a post-frontal airmass, characterised by steep lapse rates and rather marginal latent instability. Especially towards east of the highlighted region, strong DLS is forecast, while LLS should ramp up during the evening hours as boundary layer begins stabilising across the whole area. Initiation should not be an issue, albeit no strong concentrated forcing can be inferred from the models. Isolated well organised multicells or even a supercell might occur with attendant threats of marginally large hail and towards the evening also weak tornadoes are not ruled out (enhanced LLS and low LCLs). Tornado threat will be somewhat limited by mostly unidirectional vertical shear.

... SE France through N Italy, Austria towards SE Poland ...

A rather complicated scenario is unfolding for the region, especially regarding the timing and initiation of DMC as wavy frontal boundary affects the area topped by the band of moderately strong W to SW-ly flow. A plume of EML is advected towards NE across Italy (00 UTC Ajaccio sounding), the Adriatics and much of SE Europe. However, this EML is significantly capped from the boundary layer and therefore, CI is expected only in close vicinity to the frontal system and in the areas of enhanced convergence near the surface lows. Low to moderate instability is simulated with MLCAPE values exceeding locally 1000 J/kg most likely over N Italy and E Austria. Strong to moderate DLS is forecast (Nimes and Milano 00 UTC soundings both display strong veering in the low levels with around 30 m/s of DLS), with the highest values, in the range of 20-30 m/s for SE France to Austria and 15-20 m/s for Slovakia and SE Poland. Moreover, in the areas with backing surface flow thanks to the surface cyclogenesis (this concerns especially N Italy and Central to E Austria), enhanced SREH with significantly veering profiles might exceed 300 J/kg in the 0-3 km layer..

Therefore, especially over the southern half of the region, vertical wind profiles could easily favor long lived supercells and bowing segments within MCS, but the degree of organisation along the front itself might be hindered by unfavorable cell interaction. Moreover, there is no significant concentrated lift forecast, although enhanced IPV values are simulated by GFS above the frontal zone. Towards NE, an MCS might evolve with the surface low along the wavy boundary over Austria, moving towards S Poland during the evening and night hours.

Currently, the greatest threat from severe convection is expected in the afternoon and evening hours over N Italy and Central to E Austria with chance of significant and well organised storms producing large hail and severe wind gusts. Regarding N Italy, 15 to 20 m/s flow at 700 hPa flow might in well organised storms, especially bow echoes, support damaging wind gusts that might attain speeds of around 30 m/s. An update to Lvl 2 (regarding N Italy and E Austria) will be considered as the question of CI becomes better answered with the morning and noon data. Well organised multicells or marginal supercells over N Slovakia and SE Poland might pose a risk of localised large hail. Evening and night MCS is expected to affect NE Austria, E Moravia, W Slovakia and S Poland with threat of wind gusts on the eastern flank of the storm and excessive precipitation on its western flank.