Storm Forecast
Valid: Sat 08 Jun 2013 06:00 to Sun 09 Jun 2013 06:00 UTC
Issued: Fri 07 Jun 2013 23:40
Forecaster: TUSCHY

A level 1 was issued for parts of Romania and NW Moldova mainly for heavy rainfall amounts and isolated large hail.

A level 1 was issued for C/E Germany and W Poland mainly for heavy to isolated excessive rainfall amounts and isolated large hail.

A level 1 was issued for France and W-Switzerland mainly for heavy to isolated excessive rainfall amounts. NE Spain and the SW-coast of France could also see large hail and an isolated tornado event.

SYNOPSIS

A cut-off low over the Iberian Peninsula moves gradually to the NE and starts to join an amplifying trough over Norway and Sweden. Both features combined will result in an ill defined and positively tilted trough, which extends from Scandinavia all the way to Spain/Portugal. Another weak upper trough is placed over far SE Europe with no serious change in strength and only a low-end eastward motion forecast. Weak ridging affects the C-Mediterranean, UK/Ireland and Scotland.

Down at the surface, the main frontal boundaries will be a gradually strengthening warm front over C/E-France and an intensifying eastbound moving cold front over the W-Mediterranean. A weak occlusion/quasi-stationary boundary still extends from Finland to NE Poland and remains the 'player' regarding thunderstorm development. Over E/SE Europe, various outflow boundaries have evolved but their final placement remains unclear due to still ongoing DMC activity during writing.

DISCUSSION

... E/SE Europe ...

I could start to copy and paste previous outlooks with no serious change in CAPE/shear fields forecast. However, there are some indications that slightly stronger 1-8 km shear overspreads the quasi-stationary synoptic front from Poland to E-Estonia. This region will be discussed in the following paragraph.

From Slovakia to Hungary to Romania to the Ukraine to E Belarus, shear at all levels remains very low to non existent so once again we see daytime driven thunderstorm activity. Scattered to widespread thunderstorms with pulsating character evolve early in the day and grow upscale into numerous more or less organized clusters of showers and thunderstorms. Both, the regional CAPE/LL moisture dispersal but also the depth of the mixed sub cloud layer determine the strength of any cold pool, so degree of organization with those clusters will be highly variable. Would not be surprised to see a few concentrated swaths of strong to isolated severe wind gusts with cold pool driven DMC. Local flash flooding is certainly possible due to slow storm motions and training effects along any well placed outflow boundary with the highest risk probably being present over Romania, where PW values will be the highest (a marginal level 1 was added). Thunderstorms decrease in intensity and coverage during the night, but the atmosphere remains unstable and re-developing thunderstorms during the night are likely ... especially from Belarus to Hungary to Romania.

... The Czech Republic to Poland to W-Belarus to E Lithuania/Latvia and Estonia ...

Both, the quasi-stationary surface front and subtle impulses (emerging from the Baltic Sea and Germany to the east) support widespread CI over the area of interest. Initiation will be early during the day and storms continue well into the night. MLCAPE of 500-1000 J/kg once again exists but somewhat stronger shear spreads east and affects the frontal boundary during the afternoon hours. 10 m/s 0-6 km and 15 m/s 1-8 km shear are present in forecast soundings with complex reversed shear pattern still affecting most of the areas next to the boundary. Above the boundary, some slight veering with weak WAA is expected, so a marginal increase in directional shear is seen next to increasing upper ventilation (20-30 kt anvil layer westerlies). All that said, expect strong pulsating thunderstorms and with complex low tropospheric flow pattern, would not discount an isolated multicell event. Strongest storms may conatin marginal large hail and may produce a few strong downbursts. Clustering of thunderstorms and an increasing heavy rainfall risk is expected betimes.

However the highest severe risk is currently expected over W-Poland, where 40kt anvil layer flow and good veering with 1000 J/kg MLCAPE are forecast. This area was added to the German level 1 mainly for large hail and local flash flooding as PW values exceed 25 mm during the evening hours. Thunderstorms likely redevelop along the boundary during most of the night but with a decreasing severe risk.

...C/E-Germany ...

With a leisurely southward sagging cold front (entering a col and consequently starting a slow-down through the forecast period) and a northward pushing warm front converging over C-Germany, an E-W aligned plume of deepening moisture evolves along the sharpening frontal boundary. PW values increase into the upper twenties and reverse flow regime along the boundary weakens and gradually transforms into a deeply veered wind profile in response to the strengthening warm front character of that boundary. 30-40 kt anvil layer flow overspreads the area from the west, so obviously slow moving thunderstorms with some anvil-layer support (ventilation) are forecast. CI will be early during the day (both due to exceedance of the convective temperature, along the orography and due to lowering mid-tropospheric geopotential heights in response to a subtle impulse, which crosses the area from the SW to the ENE) and expect scatterd thunderstorms with a distinct heavy rainfall risk....even a few excessive rainfall events with amounts well in excess of 60mm within a few hours can't be ruled out. Isolated large hail and wet downburst events may occur with strongest storms during their initiation stages, but rapid clustering of storms/expanding cold pools should limit that risk. Thunderstorms gradually weaken during the overnight hours but again a few storms may continue well into the night.

... NE Spain, France to W-Switzerland ...

With the compact cut-off moving in from the SW during the day, a more dynamic thunderstorm pattern is expected. This deep vortex weakens a bit during the time, but the lower part of that vortex remains vivid until SUN 06Z. Hence a weak surface low pressure center over W-C France drifts a bit to the ENE.
Both the cold front (NE Spain and SW France during the start of the forecast moving to W-Switzerland/SE France during the end of the forecast) and the warm front (C-E France) will be the main foci for scattered to widespread CI all day long.

From the morning hours onwards, cool mid-levels and the left exit region of a 35 m/s mid-level jet support ongoing thunderstorm development along the leisurely eastward moving surface cold front over SW/W France. Deep mid-tropospheric moisture profiles, an highly diffluent upper flow regime with a 10 m/s SE-erly LLJ support training/back-building thunderstorm clusters with an heavy rainfall risk. An even higher rainfall risk (probably approaching excessive rainfall amounts) exists over NE Spain and far SW France (mainly the coastal areas), as a 15 m/s LLJ feeds very moist air into a NW-SE aligned cluster of storms along the cold front. This back-building cluster slowly shifts N/NE during the daytime hours with a gradual decrease in intensity during the evening hours. Southern most storms (tail-end charlies) will be able to produce large hail and an isolated tornado given an improving shear environment between Barcelona and Marseille during the daytime hours. We issued a broad level 1, with the aforementioned coastal areas probably approaching a borderline level-1/2 situation. For now, we stick with the level 1 due to the progressive nature of this cold front/attached cluster of storms.

During the late afternoon hours onwards, the severe risk also increases over N/C-France and far W-Switzerland with enhanced moisture pooling and PW values in excess of 30 mm. Slow moving thunderstorms will be able to produce heavy rainfall amounts with local flash flood problems. Especially W-Switzerland/extreme E France will see the most supportive mid/upper dynamics for numerous large evening/overnight clusters of storms with heavy rain and strong to isolated severe wind gusts. These clusters gradually spread into SW Germany during the night with a lowering severe risk.