Storm Forecast
Valid: Mon 30 Apr 2018 06:00 to Tue 01 May 2018 06:00 UTC
Issued: Sun 29 Apr 2018 23:19
Forecaster: VAN DER VELDE

A level 1 was issued N Poland, Lithuania, N Belarus and E Czech Republic mainly for isolated severe convective wind gusts, large hail and heavy rainfall.

A level 1 was issued across N Algeria mainly for large hail and severe wind gusts.

SYNOPSIS

A low pressure system centered over The Netherlands is pushing cool airmass into western and central Europe, replacing a considerably warmer unstable airmass stretched out between Poland and Serbia. This warm air with MLCAPE up to 1000-1500 J/kg advances to the NNE behind a warm front stretching from eastern Denmark to northern Belarus and western Russia. The occlusion point moves from NE Germany to Estonia during the 06Z-06Z period. The Iberian Peninsula and parts of France also develop modest CAPE under the cold upper trough. This trough also sends a frontal wave and jet streak into N Algeria.

DISCUSSION

...Poland, Lithuania, Belarus...

Several models indicate moderate MLCAPE along the warm front across N Poland, Lithuania and eastward. They are more conservative towards the south, wih GFS and ICON being the more optimistic about triggering convection with weak capping. The cold-frontal trigger is not backed by strong dynamics. SREH and forecast hodographs show good low-level shear in the level 1 area close to the warm front. Over the Baltic Sea the lowest levels are cold, and effective shear above is weak. Deep layer shear is weak thanks to very modest shear in the mid levels. This rather benefits precipitation efficiency, while the moderate to strong low-level shear can help form bow echoes, also helped by rather high LCL heights of about 2000 m. While neither storm motion nor Delta-Theta-E is alarming, severe wind gust producing storm clusters are therefore quite likely. Locally excessive rain may also occur.
Over eastern Czech Republic, models forecast enhanced low-level shear/helicity and convective triggering on the cold front. This may have supercell characteristics with chances of large hail and perhaps a tornado.

...Algeria...

A strong convergence line or front defines a narrow zone where convective development can take place. While only 500 J/kg MLCAPE may be present, extreme SREH over 500 m2/s2 and deep lapse rates suggest potential for supercell storms capable of producing large hail and severe wind gusts.