Storm Forecast
Valid: Fri 28 Jun 2013 06:00 to Sat 29 Jun 2013 06:00 UTC
Issued: Thu 27 Jun 2013 21:39
Forecaster: TUSCHY

A level 2 was issued for parts of Moldova, parts of W-Ukraine and S-Belarus mainly for large hail (a few extreme events possible), severe downbursts and heavy rainfall amounts.

A level 1 surrounds the level 2 mainly for similar risks but with less coverage and intensity.

A level 1was issued for W-Turkey mainly for a few damaging downburst events.

SYNOPSIS

Despite a weakening trend of the cyclonic vortex over C-Europe, no change in the pattern is forecast for the following 24 hours. Numerous impulses circle the main vortex and influence CI and coverage of DMC. Ridging over the Iberian Peninsula keeps conditions stable.

The quasi-stationary frontal boundary, which runs from Finland all the way to the W-Black Sea, transforms into a leisurely westward moving warm front from Belarus to the south which will be the focus for another round of severe thunderstorm development. Otherwise Atlantic fronts enter the scene from the NW and affect NW/C Europe but their role is more a decrease in DMC probabilities due to a stabilization of the low/mid-troposphere.

DISCUSSION

... Romania, Moldova to W-Ukraine into SW-Belarus ...

Models agree well in abundant MLCAPE build-up along and east of the westward surging warm front. The magnitude exceeds 1500 J/kg even in the more reluctant models and we can't argue against event higher values. Placement of the front and attendant CAPE plume remains a bit uncertain as convection of the night before likely causes some regional modification. Hence we kept the level 2 area pretty tight towards Moldova and expanded the level to the north.

It is likely that convection from the night before continues during the first hours of my forecast before clusters start a temporal decreasing trend in coverage and intensity. Between 12-15Z we expect new DMC with exceedance of the convective temperature and due to mesoscale forcing along residual outflow boundaries but also due to forced lift along the orography. Also, the right entrance of a modest mid/upper-tropospheric jet induces a supportive background environment for CI. DLS in the range of 15-20 m/s will be more than adequate for supercells with very large hail (an isolated extreme event possible) and severe downbursts. Wind-driven large hail with the potential for extensive damage is a concern. LL shear increases somewhat during the evening hours which might induce a low-end tornado chance from N-Moldova into C-Ukraine and S-Belarus.
During the overnight hours, thunderstorms grow upscale into numerous intense storm clusters with large hail, severe wind gusts and heavy rain. Favorable synoptic pattern might even result in an MCC event over W-Ukraine into Belarus.

Over Romania, CAPE increases from W to E, conform to the westward moving warm front. 0-6 km shear of 15-20 m/s supports well organized multicells/isolated supercells with a large hail and strong to severe wind gust risk. Initiation of storms will be early, probably before noon and storms last well into the overnight hours. We extended the level 1 all the way to E-Romania, where sea-breeze activity (on an isolated scale) might produe large hail/severe wind gusts and an isolated tornado report. There, impressive looking dewpoints in the lower twenties likely support surface based thunderstorms well into the night.

East of the front, e.g. W-Russia, numerous intense thunderstorms likely evolve during the afternoon and evening hours. Weak forcing and DLS below 5-10 m/s keep confidence in organized storms low, but MLCAPE in excess of 1000 J/kg points to a few severe pulsating storms. Large hail and strong to severe downbursts are possible. Thunderstorms grow upscale, but lack of forcing should keep those clusters less organized compared to those over Belarus and the Ukraine.

... S-C Europe and W-coast of Norway ...

Daytime driven convection beneath the upper trough results in scattered thunderstorms between noon and 21Z. 10 m/s DLS and modest CAPE produce pulsating thunderstorms which could temporarily acquire severe status (especially south of the Alps) with locally large hail and strong to severe wind gusts. However the meager looking shear/CAPE fields don't support any level area.
Enhanced LL CAPE and some offshore convection might cause a few waterspout reports mainly over the N-Adriatic Sea. Morning convection in the northern part of the 50-% lightning area (between Venice and Trieste and south to Pula) may see the best environment for waterspout activity before probabilities decline and eventually increase again during the overnight hours in the southern part of the 50-% lightning area.

... W-Turkey ...

This area was added into the broad level 1 for a few afternoon/evening storms, which will be high based. Inverted-V profiles support a few damaging downburst and isoalted large hail events with any developing thunderstorm. The activity rapidly diminishes after sunset.