Storm Forecast
Valid: Wed 20 Jun 2012 06:00 to Thu 21 Jun 2012 06:00 UTC
Issued: Wed 20 Jun 2012 05:22
Forecaster: PUCIK

A level 1 was issued for Southwestern France mainly for large haila nd severe wind gusts.

A level 1 was issued for Southeastern France, Northwestern Italy, Switzerland mainly for excessive precipitation and marginally large hail.

A level 1 was issued for Southeastern Germany, Czech Republic and Western Poland mainly for excessive precipitation, large hail and to the lesser degree for severe wind gusts.

A level 1 was issued for Central to Eastern Poland and Western Ukraine mainly for large hail and marginally severe wind gusts.

SYNOPSIS

An active weather pattern is forecast for much of Western and Central Europe which will be placed under southwesterly mid to upper level flow, between the upper level trough over the Atlantic and a ridge, stretching from the Central Mediterranean towards Russia. More subtle features are forecast in the southwesterly flow, with at least one insignificant short-wave trough that will likely affect the situation over Central Europe during the afternoon to evening hours. A wavy frontal boundary is forecast to stretch from Iberia towards France, Germany and Poland. Exact location of the subtle short-wave troughs in the southwesterly flow, shallow low pressure centers along the wavy front is uncertain and handled differently by NWP. Ahead of the frontal boundary, moderately unstable airmass is forecast with scattered to widespread thunderstorm activity over Central Europe.

DISCUSSION

...Southeastern France, Northwestern Italy through Switzerland, Southeastern Germany, Czech Republic and Poland/Western Ukraine...

A complicated forecast for this region does not allow to pinpoint very precisely the locations with the greatest threats. Nevertheless, slightly to moderately unstable airmass (MLCAPE values around 1000 J/kg) will likely be in place, as agreed by most of NWP and soundings from the previous forecast period. Weak to moderate vertical wind shear with bulk values of 10-15 m/s in the 0-6 km layer might be enhanced by the backing flow nearby the surface low centers, yielding over 15 m/s of DLS and enhanced SREH. Stronger mid-level flow might also be observed in the vicinity of the subtle short-wave trough towards the evening hours.

Location of the low centers and of the boundary will likely play the most important role in the evolution of the tomorrow´s scenario, along with the timing of the short-wave trough. As of 5 UTC, there is already a large multicell cluster along the warm front. Initiation of DMC today will be tied to the:

A/ frontal boundaries - e.g. lifting warm front over Poland or close to the wavy cold front over Switzerland and Germany

B/ orography - Alpine range and northern part of Carpathians.

C/ short-wave trough in the late afternoon and evening hours.

Widespread coverage of thunderstorms is most likely around and to the north of the Alpine Range especially when the forward flank of the short-wave approaches and also along the advancing warm front over Poland. Another MCS is very likely by the evening hours and a bulls-eye in QPF is discernible in several NWP consecutive runs over Bavaria. Moderate instability and shear might at first result in a well organised, strong multicells with isolated large hail or marginally severe wind gusts. Brief supercells are not ruled out especially in the spots with backed surface flow. Towards the late afternoon or evening, clustering will yield one or more MCS travelling towards NE along wavy cold front, with the most significant threat of excessive precipitation, especially along or on the cool side of the boundary, where very moist airmass and low LCLs might foster high precip. efficiency in DMC. Excessive precipitation might also very well occur with the morning MCS lifting along the warm front over Poland. Severe wind gusts might occur in MCS in the late afternoon, in case that it is rooted in the boundary layer, but the threat will likely not be very high.

... Southwestern France ...

There are some discrepancies regarding the degree of latent instability and CI for this region with GFS being quite agressive with MLCAPEs over 1000 J/kg while ECMWF simulates only spotty and marginal instability for this region. Bordeaux 00 UTC sounding shows very weak lapse rates, which might change however, as steeper lapse rates are advected from Pyrennees area during the day. Moderate wind shear is forecast, strenghtening towards evening/night hours as stronger, diffluent mid-level flow approaches the region. Towards this evening period, diffluent flow might also promote some forcing for weakening the CIN. Albeit confidence in the overall threat remains quite low, a Level 1 is issued for large hail and marginally severe wind gusts. These might occur especially towards evening/early night hours with stronger and well organised multicells and/or brief supercells.