Storm Forecast
Valid: Sat 25 May 2013 06:00 to Sun 26 May 2013 06:00 UTC
Issued: Sat 25 May 2013 02:42
Forecaster: TUSCHY

A level 1 was issued for Croatia all the way to Romania mainly for large hail and severe downbursts. A similar risk extends towards Bulgaria. E-Romania/Bulgaria may also see locally excessive rainfall.

A level 1 was issued for parts of W-Russia and adjacent regions mainly for large hail, severe wind gusts and locally excessive rainfall.

SYNOPSIS

Large upper trough over central Europe carves south as pocket of sub -30°C at 500 hPa slides south. Strong 40 m/s mid-level streak accompanies that pocket of cold air and results in a tightened gradient flow, affecting the N-Mediterranean during the forecast.
Other weak trough axes or mid-level cyclonic vortices either round that main steering low or evolve along its periphery, affecting e.g. E-Europe.
Retrograde moving surface low approaches Germany from the east with considerably gradient flow expected. Weak lee cyclogenesis evolves south of the Alps but remains ill defined during the forecast.

DISCUSSION

... Alps, surrounding areas and the N-Balkan States ...

Southbound moving mid/upper low causes cold mid-levels to spread southward, overlaying BL air mass which was worked over by previous cold intrusions. Latest synop data reveals bad/low surface dewpoints north of the Alps and with weak surface ridging building in from the west, confidence in more than isolated thunderstorms remains low. Nevertheless, some regional moisture pooling might occur N of the Alps, increasing BL moisture and therefore LL CAPE. Latest thoughts keep GFS output too bullish with very robust LL CAPE, but can't neglect overlap of cold 900-700 hPa temperatures atop 12-14°C surface temperatures. Modified forecast soundings show that not much BL moisture change is needed to ramp up LL CAPE. Hence we will mention the risk for a few non-supercell spouts extending from E-France to S-Germany all the way to E-Austria. No level will be issued due to spotty nature of that risk. E-NE Austria may see higher risk...please see next paragraph for more information.

Confidence increases in more robust BL moisture recovery over NE Italy, Slovenia, Croatia and parts of Hungary. Moisture may sneak as far north as E/NE Austria, still depending on strength and placement of lee cyclogenesis south of the Alps, which might increase aforementioned funnel/tornado risk somewhat. We modified the 50-% lightning area over E-Austria for that reason. Otherwise, highlighted areas see low shear and increasingly buoyant 0-3 km layers. Also, incoming 40-50 m/s mid-level speed maximum causes enhanced divergence along its exit region, affecting aforementioned areas. Depending on depth of BL moisture and insolation, some mixing might occur during the afternoon hours, reflected by LCLs at or above 1 km at some spots, but main LCL height remains confined to 800-1000 m over the aforementioned areas. Non-supercell tornadoes/funnels will be the main risk along any evolving mesoscale convergence zone. Storms tend to grow upscale pretty fast with messy outcome expected. Locally marginal hail and gusty winds accompany those storms.
Croatia south and southeast experience an increasingly more favorable shear environment for organized multicells and isolated supercells. Forecast soundings show DLS increasing to 20 m/s with strong veering and steep lapse rates in the 1000-700 hPa layer. Increasing BL mixing causes LCLs to climb to 1-2 km, so despite large hail, strong to severe wind gusts is likely, given healthy looking downburst profiles. This risk extends from Croatia to Serbia all the way to W-Romania, where a broad level 1 was added. Betimes storm clustering causes isolated flash flood risk.

... E-Romania and Bulgaria ...

High-level streamline analysis showes ongoing diffluent mid/high level streamline pattern over that part of Europe. A subtle wave at 850 hPa is embedded in this flow regime, pulling a plume of moist and unstable air from the NE Aegean Sea to the N ... herein E-Bulgaria and Romania. Rapid BL moisture recovery is expected and combined with gradually steepening mid-level lapse rates, SBCAPE of 1000 J/kg and more looks reasonable. Despite weak to moderate winds at all levels, strong veering and magnitude of CAPE may cause a few well organized multicells/isolated supercells with large hail and strong wind gusts. Heavy rain and flash flooding will be another hazard, given modest storm motions. Added that area to the level 1 further to the north.. Risk continues all night long as near surface based instability remains/spreads north. Would not be surprised to see big cluster of storms to spread N/NE, affecting the W-Black Sea areas during the night.

... E-Russia ...

Leisurely eastward moving cold front will spark scattered to widespread thunderstorms within the highlighted area. Weak shear but strong CAPE signals point to well organized multicells with large hail, strong wind gusts and heavy rain. A broad level 1 covers that risk.