Storm Forecast
Valid: Wed 25 May 2011 06:00 to Thu 26 May 2011 06:00 UTC
Issued: Wed 25 May 2011 06:30
Forecaster: PUCIK

A level 1 was issued for Extreme eastern Belarus, Northern Ukraine, Smolensk region mainly for marginally severe hail and excessive precipitation.

SYNOPSIS

Two prominent short-wave troughs at mid-levels are expected to translate eastwards during the day, on the northern flank of the ridge stretching from Iberia into Northern Italy. The first short-wave trough will affect British Isles during the day while the trough will start a slight southern amplification. The second short-wave, affecting Baltic States and Western Russia, will also amplify towards southeast, becoming negatively tilted and providing strong lift with DCVA on its forward flank. A strong cold front is associated with this trough and a surface low observed over Northern Scandinavia. Cold front is expected to cross Baltic States early in the day and continue across Belarus, Western Russia and Northern Ukraine with cold airmass spreading southeastwards. A surface high pressure system is moving across Central Europe eastwards, in the wake of the cold-front. Another interesting feature will be a very shallow mid-level trough/cut-off low, filled with slightly cool airmass, stretching across Southeastern Europe and Southern Italy.

Abundant diurnally driven deep-moist convection is forecast over Southern half of continent, namely Northern Iberia, Italy, South-Eastern Europe and Ukraine/Belarus/Western Russia. Most of the areas will not be covered in the separate discussion paragraph, as conditions will prove rather unfavorable for severe weather, given very weak flow across most of troposphere. Moreover, GFS constantly overforecasting dew-points and comparing to the ECMWF output, degree of instability might elsewhere prove much less than advocated by the model output. Nevertheless, with very weak flow, almost stationary storms might be observed and especially over hilly/mountainous terrains, highly localised excessive precipitation might lead to some flash flooding. Also very isolated marginally severe hail or wind gusts might be observed across parts of Ukraine or Northern Iberia.

DISCUSSION

... Extreme Southeastern Belarus, Northern Ukraine, Smolensk region ...

A low-end Level 1 is issued for this region as the approach of the mid and upper level trough and frontal system is expected to enhance wind shear, albeit the overlap of 15 m/s DLS and moderate instability will be rather small. Actually, it is well possible that the wind fields will increase most significantly after the front passes, leaving most of the prefrontal storms in rather weak shear. Still, it is probable that storms forming close to the front will profit from slightly better shear profiles and some organised multicellular convection might occur. Main threats should be marginally severe hail or excessive precipitation in the northern part of Level 1, where the formation of linear storm system parallel to the mid/upper level flow with some potential for back-building and quite slow advective component motion of the cells in the system.