Storm Forecast
Valid: Sun 18 May 2014 06:00 to Mon 19 May 2014 06:00 UTC
Issued: Sun 18 May 2014 05:38
Forecaster: TUSCHY

A level 2 was issued for parts of the Ukraine, Belarus Latvia, Lithuania and S-Estonia mainly for severe to damaging wind gusts, a few tornadoes (a strong event possible) and large to very large hail.

A level 1 surrounds the level 2 area mainly for a similar risk but with less confidence in extreme events.

A level 1 was issued for NE Morocco and NW Algeria mainly for a few large hail events and excessive rain.


SYNOPSIS

The lengthy and spacious upper low over SE Europe, which wreaked havoc in numerous countries now lifts towards Denmark and opens up into an upper trough, which runs from the North Sea all the way to the Black Sea. Downstream ridge over W-Russia strengthens and evolves into a blocking anticyclone. In the meantime, another major trough west of Ireland builds south and approaches the far NW Iberian Peninsula during the end of the forecast. This trough becomes the next feature which ensures a continuance of the blocking pattern over most of Europe.

The most interesting boundary, convective-wise, will be a wavy quasi-stationary baroclinic zone, which runs from the CNTRL Baltic Sea to the Ukraine. Atlantic fronts approach the Bay of Biscay and Portugal/Spain but most likely will not yet have any influence on our thunderstorm forecast.

DISCUSSION

...NW Poland and NE Germany until 12 Z...

The dissolving and slowly NW-ward drifting vortex still continues to wrap an occlusion around its center, which ensures an ongoing advection of moist and unstable air to NE Germany. The front becomes quasi-stationary over NE Germany and dissolves. During the start of the forecast, a cluster of showers/thunderstorms moves along that tongue of unstable air towards NE Germany. This cluster is accompanied by heavy rain and gusty winds. A temporal upswing of lightning activity is forecast during the forenoon hours, before probabilities decrease around noon as CAPE wanes due to an extensive area with stratiform rain. Embedded convection can still cause heavy rainfall amounts with slow moving convective elements and effective PWs in excess of 20 mm.
Over NW Poland, the occlusion gradually lifts north and probably moves offshore around noon. Until then, scattered thunderstorms continue and grow upscale into smaller clusters with heavy rain and marginal/isolated large hail. However, as some models and synop data continue to show a closed cyclonic circulation center at the surface, LL shear may remain somewhat enhanced over extreme NW Poland to support an isolated tornado event before noon (SRH-1 around 100 m^2/s^2 and LCLs below 600 m). This risk ends around noon. The overall risk seems to be too marginal for a level 1.

...Ukraine, Belarus, Lithuania, Latvia, the CNTRL Baltic Sea and Estonia...

An outbreak of long-lived and damaging thunderstorms is once again forecast!

An extensive N-S aligned baroclinic zone remains in place with no significant west/east motion expected. BL moisture peaks along the front with surface dewpoints in the mid tens. A plume of steep mid-level lapse rates (in excess of 8K/km) covers the front and regions to its east with substantial weaker lapse rates to the west. Therefore, abundant MCLAPE once again evolves along/east of the front with 1500-2000 J/kg forecast. CAPE rapidly drops to 1000 J/kg and less just west of the front. Due to the strengthening anticyclone over W-Russia, the gradient flow intensifies especially in the lowest 3 km, so expect 20 m/s front-parallel 0-3 km shear. Forcing once again remains a bit nebulous. A short-wave probably becomes the main focus for CI, which crosses the Ukraine until noon from S to N, affecting Belarus until the evening hours and approaches the CNTRL Baltic Sea during the overnight hours.

We expect isolated to scattered thunderstorms to evolve over the CNTRL Ukraine, which grow upscale rapidly into one or more organized clusters. These clusters then race north over Belarus towards Latvia and Lithuania. Models differ significantly with timing of those clusters, but all models show at least one organized cluster to cross the level 2 during the forecast. Any thunderstorm cluster will produce long swaths of severe to damaging wind gusts and large to very large hail. A few tornado events are well possible and a significant event cant be ruled out with SRH-1 in excess of 200 m^2/s^2 and aforementioned CAPE. Aside from those clusters, discrete storms may initiate along outflow boundaries of the previous day or due to the orography. Shear/CAPE both favour long-lived supercells with a large to very large hail threat (hail diameter in excess of 5 cm), severe wind gusts and an isolated but strong tornado event. Strong cap adds uncertainties if and where those storms evolve, but this risk extends all the way into Estonia.

During the night, we do not trust the models rapid decay of ongoing thunderstorm clusters and therefore we expanded the level 1 far to the north. Despite decreasing CAPE and tendencies of convection to become more elevated, severe winds and large hail continue to pose a risk next to heavy rain.

...NE Morocco and NW Algeria...

A broad trough covers the area of interest and causes enhanced thunderstorm chances. Overlap of 400-800 J/kg MLCAPE and 15 m/s DLS assist in organized thunderstorms with a large hail threat. Storms cluster betimes, move offshore and weaken. Thunderstorms continue all night long over NW Algeria.

...Lightning areas...

Scattered diurnal driven thunderstorms are forecast with widespread activity over the Alps and from Romania to Bulgaria. Slow moving storms pose an heavy rainfall risk and strongest storms could contain large hail. Weak shear and marginal CAPE preclude a level 1, although flash flooding could become an augmented problem for areas with ongoing flood problems.
EZ and GFS both agree in initiation over NE-UK and SE-Scotland around sunset and during the night as upper divergence of a 300 hPa's entrance region crosses the area. A cluster of storms could form, which moves to the northeast. Despite augmented shear, marginal CAPE lowers any severe risk....although a marginal hail risk can't be excluded.