Storm Forecast
Valid: Tue 08 Aug 2017 06:00 to Wed 09 Aug 2017 06:00 UTC
Issued: Mon 07 Aug 2017 20:44
Forecaster: DAFIS

A level 2 was issued for parts of France, Switzerland and Germany mainly for severe wind gusts, large hail, tornadoes and excessive precipitation.

A level 1 was issued for NE Spain, France, Benelux, Switzerland, Germany, W Austria, N Italy and W Czech Republic mainly for severe wind gusts, large hail, tornadoes and excessive precipitation.

A level 1 was issued for E Ukraine mainly for large hail and severe wind gusts and to a lesser extent for excessive precipitation.

SYNOPSIS

A large low pressure system, with its center over NE France - Benelux, will be the main feature of today's outlook. The cold front of the cyclone creates a elongated line of low-level convergence extending from SE France towards Germany and Czech Republic. The mid-level vortex is accompanied by several waves which are expected to provide additional lift to the unstable air masses near the surface. Several other areas around the continent will experience convective activity such as Sweden, Russia and the Balkans where diurnal heating will be the main driver of these storms. A quasi-stationary front in Ukraine will lift some moist air masses and considerable DLS may assist storms to become organized and produce severe weather, this is why a level 1 was issued.

DISCUSSION

.... France, Benelux, Switzerland, Germany, W Austria, N Italy and W Czech Republic ....

As the mid-level wave crosses France during the early morning of Tuesday, a jet streak is found just above the convergence zone, creating a complex and dangerous environment for every storm inside the level 2 area. DLS fluctuates between 25-35 m/s among NWP models and MLCAPE may locally exceed 2000 J/kg in South Germany. Less CAPE in France and Switzerland but still high (about 1000 J/kg according to GFS and ECMWF) will also overlap with high DLS values as well as high 0-3km shear. Storms will quickly acquire rotation, able to produce any kind of severe weather. The main threat will be excessive precipitation but the most discrete cells will be able to produce large hail and severe wind gusts. Tornado threat is elevated in France during the morning and in S Germany during the late afternoon when LLS (speed and directional) is the strongest and NWP models show a drop of LCL heights.
For the largest part of France we expect pulsating storms which many of them may produce small hail or graupel reaching the surface. Ongoing convective activity from the previous night in southern parts of France may produce some heavy rain events in Massif Central and later on the Alps. In the afternoon, large hail is not ruled out in W Austria as steep lapse rates create fat CAPE profiles, DLS is strong and cloud bases are calculated above 1500m. For Benelux and W Germany, near the core of the low-pressure system, locally enhanced overlap of CAPE-DLS can also lead to large hail and severe wind gust events in the afternoon. Elevated convection will feed several storms in Germany during the night entering Czech Republic and in N Italy. Excessive precipitation threat will be high due to training activity.

.... Ukraine ....

A stationary front is found where warm and moist air masses from the south collide with cooler and more dense air masses from the north. Steep mid-level lapse rates and moist PBL create some hundreds of CAPE and DLS between 15-20 m/s may assist storms to cluster into MCS. A level 1 was issued for this probability, as large hail and severe wind gusts are expected. Slowly moving storms may also produce high amounts of rainfall in a short period of time as PW is about 35-40 mm.

.... Spain ....

A highly sheared environment is expected tomorrow in NE Spain, although different NWP model solutions disagree about any CI due to a strong cap. Strong upper-level divergence may be better calculated by the models. Any storm that managed to form inside the level 1 area will be able to produce large hail and severe wind gusts. Steep lapse rates and LL advection of moist marine air masses show high NCAPE and high delta-theta-e values as a result of mid-level dry conditions. Low-level lightning probability was issued due to uncertainties for CI, otherwise a solid level 2 would had been issued.