Storm Forecast
Valid: Sat 29 Apr 2017 06:00 to Sun 30 Apr 2017 06:00 UTC
Issued: Sat 29 Apr 2017 00:47
Forecaster: TASZAREK

A level 1 was issued for a corridor stretching from parts of W Ukraine, through CNTRL Romania, up to NW Bulgaria mainly for the large hail, severe wind gusts and excessive precipitation.

SYNOPSIS

Two broad mid-level long-waves (850 hPa temperature ~ -5°C) with surface troughs over British Isles and Baltic Countries cover W, CNTRL and PRTLY E Europe. A blocking ridge stretches from N Scandinavia to W Russia. Sharp thermal gradient extends along quasi-stationary frontal boundary from Baltic Countries, through W Ukraine, W Romania and Albania. Jet streak extends from S Italy, up to Estonia. Low-topped weak thunderstorm activity is possible on the SE flanks of the "sinking" long-waves over Spain, NE Germany, S Sweden and Poland. Severe thunderstorms are possible in moist and unstable warm section along eastern flank of stationary frontal boundary and over parts of Ukraine, Romania and Bulgaria

DISCUSSION

... parts of W Ukraine, CNTRL Romania, NW Bulgaria...

Unstable and moist warm sector developing in the wake of the trough over Baltic Countries consists of 8-9 g/kg MIXR and ~ 6.75 K/km lapse rates. Thanks to the support of the diurnal heating and moisture convergence, CAPE is building up to 1000-1500 J/kg and is PRTLY covered by around 15 m/s DLS. Convective initiation is most likely after the passage of the shallow PVA lobe. Due to the mean wind vector being parallel to the initiation axis, convective cells are expected to cluster into multicells and move northerly in the training mode. In such configuration and PW exceeding 30mm, heavy rain with local excessive convective precipitation is possible. Given moderate CAPE and lapse rates locally reaching 7 K/km, large hail cannot be ruled out. Few local severe wind gusts may also develop within better organized multicell clusters.

Limitation of this forecast is associated with uncertain low-level moisture prediction which is not consistent given various NWP models. It is possible that GFS model scenario overestimates dew point by 2-3°C and instability in reality may be lower than predicted, thus decreasing severe weather potential.