Storm Forecast
Valid: Tue 12 May 2015 06:00 to Wed 13 May 2015 06:00 UTC
Issued: Tue 12 May 2015 04:06
Forecaster: VAN DER VELDE

A level 1 was issued for NE France to E Germany, Poland and Czech Republic for chances of large hail, tornadoes and severe convective wind gusts.

SYNOPSIS

A warm sector over western Europe belonging to a low pressure system centered northeast of Scotland gets pushed eastward over France, Germany and Poland by a cold front. This cold front is backed by dynamic support over northern and eastern Germany into Poland, moving east/southeastward, while it should move more slowly southward over northeastern France and south-central Germany. Models predict significant mid level lapse rates across the warm sector, while low-level moisture gets advected from France and evaporated over Germany into a band of some 200 km wide before the cold front. The setup holds potential for severe storms due to the combination of strong shear and modest CAPE.

DISCUSSION

Various models develop convective precipitation during the late afternoon and evening starting from central/eastern Germany and moving into Poland. The western 1/3rd of the level 1 will only have shallow lifting near the surface, while the upper trough affects the other 2/3rd with more certain initiation. Model MLCAPE predictions indicate mainly less than 500 J/kg, which makes any differences in near-surface moisture critical for succesful storm development. For 12Z and 18Z GFS and ECMWF agree in all parameters including 2m dewpoints, while at 15Z the GFS seems biased by a few degrees affecting CAPE values. The highest dewpoints over 16° should occur over northeastern France, which occurred also the previous day.

The presence of strong mid/upper level winds causes strong vertical shear, of 20-30 m/s 0-6 km and around 10 m/s in the 0-1 km layer. The mean wind blows parallel to the front, while supercells may develop which deviate to the right (eastward storm motion). The associated storm-relative helicity is in the order of 100-250 m²/s³. Large hail is the most likely threat, but in a good part of the region relatively low LCL under 1000m could limit updraft properties. Instead, tornado chances are favored by lower LCLs. The orientation of shear and mean wind vectors along the front may favor linear clustering rather than isolated supercells. This reduces the overall threat, together with relatively marginal available energy. For these reasons a large level 1 was chosen, with best overall potential for central-eastern Germany.

Over eastern Germany and Poland, lifting will be stronger and shear vectors cross the wam sector and triggering low level convergence line. This can favor an MCS with threat of severe wind gusts, but again this is relatively modest due to the lack of sufficient CAPE to support it. Embedded mesocyclones are possible which are foci for tornado and hail chances. Low level shear seems further enhanced along the warm front over Poland, but storms should occur only at the western edge of the warm sector.