Storm Forecast
Valid: Tue 05 Jun 2012 06:00 to Wed 06 Jun 2012 06:00 UTC
Issued: Tue 05 Jun 2012 05:45
Forecaster: TUSCHY

A level 2 was issued mainly for parts of the Ukraine and S-Belarus mainly for a few tornadoes, large hail, severe wind gusts and excessive rainfall.

A level 1 surrounds the level 2 and expands all the way to parts of Greece mainly for an isolated tornado, large hail, severe wind gusts and excessive rainfall amounts.

A level 1 was issued for NW Spain mainly for an isolated large hail event and strong wind gusts.

SYNOPSIS

A pattern transition is underway with a shift of a diffuse low index to an evolving high index pattern during the end of the forecast at least over the western part of Europe. With this transition underway, we still have to deal with abating troughs (regarding strength and amplitude) over C/E Europe. Responding to the more zonal pattern shift over W-Europe, geopotential height increase is noted over SW Europe in all forecast guidances. Hence, departing troughs over C Europe see strongest height gradients still along their SW fringes, which keeps mid/upper streaks along the upstream side, precluding any significant tilt of the eastward marching troughs. The first one exits SE Germany at 06Z , crosses the Czech Republic until sunset and enters S-Poland thereafter but has not much moisture to work with until late the night. The second one exits the Adriatic Sea until noon and enters the Aegean Sea during the night and proves to be the main forcing mechanism for scattered to widespread initiation over SE Europe.

At the surface, a wavy and quasi-stationary boundary extends from Bosnia and Herzegovina to Romania/Ukraine during the start of the forecast and serves as focus for a slowly deepening surface depression, fostered by an increasing divergent upper flow and intense diabatic heating during the daytime hours with mainly better dynamics after sunset. Hence, expect the boundary to enclose a broad warm sector over SE Europe with a sharpening warm front moving from the C Ukraine to the north. This warm sector serves as area with best DMC chances throughout the forecast period.


DISCUSSION

... SE Balkan States, Moldova, most of the Ukraine, Belarus and parts of far W-Russia...

Latest sounding data reveal decent lapse rates at mid and upper levels atop rich BL moisture with dewpoints in excess of 10 °C within the lowest 2 km AGL. Surface dewpoints run from lower to upper tens, so despite strong diurnal heating, no significant lowering of the BL mixing ratio is expected. Night time convection was pretty widespread and mainly focused along the wavy front over C-Romania to W/C Ukraine. In addition, surface obs indicate weak vortices on the meso-beta scale, which seem to travel along that boundary to the north and localy increase convergence (as seen over S-C Romania at 02 Z). Numerous, partially severe, clusters emerge from the night and will continue during the morning hours onwards. Not much diabatic heating is needed to rapidly increase MLCAPE over the area of interest to values in excess of 1 kJ/kg, locally exceeding 2 kJ/kg. We think that ongoing clusters rapidly gain strength while traveling to the N/NE along that boundary (despite the Romanian cluster, which may cross the warm front during the following hours, encountering a more hostile environment). Aforementioned boundary transforms into a leisurely eastward moving cold front over Romania/Bulgaria and a rapidly sharpening warm front over the Ukraine. Also, numerous ouflow boundaries from overnight's convection serve as foci for initiation during the daytime hours. Hence, additional initiation will be a mess with models probably having a hard time to resolve final hot spot of convection.

Current thinking is that best forcing exists over the Ukraine with the interplay of the deepening depression and the northward lifting warm front. Exiting storm clusters from the morning/noontime hours may leave a more stable air mass behind, but rapid air mass recovery is expected with MLCAPE well in excess of 1000 J/kg. Another round of widespread thunderstorms is forecast, evolving into extensive storm clusters and we even don't want to exclude an MCC feature, moving from N Ukraine into Belarus and parts of Russia during the overnight hours.

Further south, over Romania, Bulgaria and Greece, forcing becomes a bit weaker, but still expect a weakly capped air mass to be prime for rapid thunderstorm development (beside already ongoing clusters from the night). Again, numerous clusters are forecast with a slow movement to the N/NE.

Weak kinematics over most of the level 1 area are well offset by abundant CAPE with DLS increasing from 10 m/s over Greece/Bulgaria to abundant 15-20 m/s over the Ukraine and Belarus. LL shear is also maximized along the evolving surface low and eastward arching warm front over Ukraine and Belarus. Hence, the tornado risk may be augmented over that area (level 2), however 'mesoscale accidents" of clashing outflow boundaries may allow for an isolated tornado event over all of the highlighted areas. Large hail, strong to severe wind gusts (especially in case of strong cold pool development) and excessive rainfall are all expected with any storm. Extreme events may be suppressed by limited time of discrete storm structure and rapid upscale growth into clusters.

We agreed with a sharp decrease in thunderstorm activity towards the Black Sea due to unfavorable (dry airmass) trajectories emerging out of a weak anticyclone centered over the W-Black Sea. We expanded all lightning and risk areas far to the north to account for bad model handling of current (05 Z) convective precipitation and the expected long-lived convective complex events during the night. Also, despite the marginal set-up for extreme events, we upgraded parts of the level 1 area to a level 2, mainly due to the widespread nature of severe storms and the enhanced tornado risk. Despite overall weakening during the night, storms continue all night long mainly over the E/N part of our level areas.

... Parts of Poland ...

Added a 50 % thunderstorm area to account for moderate CAPE build-up ahead of the upper trough (no.1), which approaches from the west. Despite 15 m/s DLS, confidence in longer lived DMC is too low for a level 1. However, an isolated hail event, matching our criterion, may occur. Storms rapidly decay around sunset (also probably experience storm-induced subsidence by ongoing extensive clusters further east).

... NW-Spain ...

We monitor a NE ward sliding plume of subtropical air ahead of an approaching trough W of Ireland for abundant onshore moisture flow over NW Spain. In fact, 05 Z surface data already indicates dewpoints in the mid tens. However, representative of a subtropical air mass, deep WAA is forecast with rapidly weakening lapse rates at mid-levels. Nevertheless, adequate moistening over NW Spain is expected for initiation mainly over the Cordillera Cantabrica. 6 km and anvil layer shear will be adequate for storm organization and a few well organized multicells are possible, which move to the east/northeast. With expected air mass, large hail reports will be hard to receive. However, given shear/instability overlap, we won't rule out an isolated event. Also, strong wind gusts are possible and a low-end level 1 was added. We would not be surprised to see weakening overnight storms affecting SW France with weak MUCAPE signals spreading eastwards. We therefore expanded the 15-% thunderstorm area far to the east.

For areas further south, the BL features much lower moisture content with strong diurnal mixing forecast. Anyhow, eastward moving moisture at 700-500 hPa assists in some negative LIs, so given intense diabatic heating, an isolated storm over C Spain is possible (not highlighted). Another area to look at will be E-C Spain, where inland moving sea breeze fronts encounter the aforementioned air mass and may spark an isolated thunderstorm (highlighted due to higher confidence). Weak shear precludes any severe risk and storms rapidly decay after sunset.

... N-Morocco, Algeria and parts of Sweden and Norway ...

Modest CAPE build-up during the day within a weak shear environment is expected with loosely organized multicells forecast. Isolated large hail may accompany stronger storms, but coverage and intensity does not justify a level area. Up to 150 J/kg LL CAPE in the 50-% thunderstorm area over Norway and Sweden is forecast by HIRLAM/GFS, so will not exclude an isolated tornado event during the afternoon hours. Storms diminish after sunset.