Storm Forecast
Valid: Mon 20 May 2019 06:00 to Tue 21 May 2019 06:00 UTC
Issued: Sun 19 May 2019 18:55
Forecaster: TUSCHY

A level 2 was issued across SW-Germany mainly for excessive rain.

A level 2 was issued across E-Poland, SW-Belarus, W-Ukraine and N-CNTRL Romania mainly for excessive rain.

A level 1 surrounds both level 2 areas for isolated large hail and severe downburst events next to excessive rain.

A level 1 was issued across Bulgaria to Albania mainly for large hail and severe wind gusts.

SYNOPSIS

A weakening -2 sigma trough over CNTRL-Europe relocates from CNTRL Italy to the N with a reforming center just to the N of the Alps (over Bavaria and CNTRL-Austria). This vortex draws a moist and unstable air mass from E-Europe W which gets incorporated into the circulation of the Bavarian/Austrian vortex. Peripheral broad and healthy low-amplitude troughs circle the main vortex. One exits Poland/the Czech Republic W around noon while another one approaches the Balkan States beyond sunset from the W. A third and weaker one in-between crosses most areas of E-CNTRL Europe. A radial vector pattern of elongated irrotational wind vectors surrounding that vortex support the idea of an active day with widespread intense convection during this forecast period.
Along its eastern fringe (over E-CNTRL Europe), peripheral troughs interact with a long-lasting plume of moist/unstable air which extends from the Black Sea to NE-Poland. This moisture input points to an augmented rainfall risk with initiating convection.

DISCUSSION

... E-Poland to the W-Ukraine to Romania ...

Surface pressure gradient tightens somewhat during the daytime hours between the Bavarian vortex and an anticyclone over far W-Russia. Resulting strengthening LL wind field with 850 hPa winds in excess of 35 kn results in enlarged and looped hodographs up to 700 hpa with rapidly weakening shear atop. Early and widespread CI is forecast in the highlighted area ahead/along a NE-ward surging cold front and a passing mid-level wave. Rapid upscale growth into numerous slow moving clusters is forecast. Initiating storms may produce large hail with SBCAPE in the 1000-2000 J/kg range and a few severe (wet) downbursts. The dominant risk however will be excessive rain with effective warm rain production (given deep warm cloud layers) and PWs in excessof 30 mm. Effective moisture advection could result in temporal training convection with a flash flood risk.
The latest idea is that the most enhanced excessive rainfall risk exists over far E Poland, far W Belarus into W-Ukraine and N-CNTRL Romania. Conditions are favorable for training convection during CI and later on for slow moving clusters. We therefore upgraded to a level 2.

Although a general downward trend in convection is forecast during the night, ongoing PVA atop an unstable air mass keep CI alive all night long with excessive rain the dominant risk.

...Bulgaria to Albania ...

400-800 J/kg MLCAPE, elongated straight hodographs and strong forcing support scattered CI with organized multicells and a few splitting supercells. Large hail and severe wind gusts will be the main risk.

...N-Poland to Germany ...

A westward circling mid-level trough and available unstable air mass (800-1500 J/kg SBCAPE)result in a rapid flare up of convection around noon and onwards over NW Poland and NE Germany. Pre-convective soundings reveal skinny/elongated CAPE profiles, a deep moist troposphere and overall weak shear. MCS propagation vectors indicate sideward moving activity which remains progressive during the day. Hence, clustering and upscale growing convection is forecast to affect NW Poland/NE Germany during the day, probably growing into an elongated MCS. Isolated large hail and heavy rain will be the main hazard.

Over S-CNTRL-Germany, the concern exists for widespread slow moving DMC activiy in a diffuse/broad and elongated low tropospheric confluent flow regime. Early CI is forecast in 500-1000 J/kg SBCAPE with weak shear - somewhat enhanced sub-850 hPa flow highlights the chance for training convection. Again, effective warm rain process should support excessive rain/flash flooding.
Along the N-Alps, orographic lift increases rainfall rates with more than 100 l/qm/24h forecast by most numerical models. Embedded/elevated thunderstorm activity increases rainfall rates on a local scale. A level 2 upgrade for excessive rain was issued, where confidence is highest.

An overall severe risk minimum exists over N-CNTRL Poland with only isolated severe (hail/rain).

...Lightning areas ...

Neither shear and/or instability support organized thunderstorm activity. Graupel and gusty winds are possible. Hungary could see a temporal CAPE/shear space during the afternoon for an isolated large hail risk