Storm Forecast
Valid: Fri 20 May 2011 06:00 to Sat 21 May 2011 06:00 UTC
Issued: Thu 19 May 2011 18:18
Forecaster: TUSCHY

A level 1 was issued for most parts of central Europe, mainly for large hail, strong to severe wind gusts and locally excessive rainfall.

SYNOPSIS

Donwstream of an extensive cyclonic vortex, centered between Scotland and Iceland, large-scale ridging affects most parts of Europe. A seasonably warm and humid air mass spreads northeastwards, resulting in a broad area with enhanced thunderstorm probabilities. Weak kinematics/dynamics will be the main "missing" ingredients, precluding a more substantial severe thunderstorm outbreak.

DISCUSSION

A broad level 1 was issued, so hot-spots become blurred and have to be discussed within separate paragraphs. Overall environmental set-up assists in daytime driven thunderstorm development, linked to orographic support.


... CNTRL/NE/E Germany, the Czech Republic and W/N Poland ...

Despite the marginal looking set-up, this region may see an augmented risk of severe thunderstorms. High and low resolution models agree well in the approach of a vertical tilted and weakening vorticity lobe/IPV maximum around noon, fanning out thereafter over NE-Germany/W-Poland. Upper height fields reveal some slight cyclonic curvature, assisting in a modest 300 hPa jet along its downshear axis (which results in a more diffluent pattern over the area of interest). Even more complex pattern evolves at lower level with most models indicating a weak convergence zone, which accompanies the vorticity lobe, moving slowly towards the east.

The air mass is forecast to become quite unstable with MLCAPE in the order of 700 - 1000 J/kg over E-Germany, even increasing over W/NW Poland (ICAPE up to 1500 kJ/m^2 and PWAT well above 30 mm). Despite overall weak gradient flow, shear increases slightly during the day with 0-6 km DLS of 10 - 15 m/s, as upper-level disturbance approaches. Forecast soundings indicate a deep-tropospheric moisture pool with modest lapse rates and weak capping. Hence, initiation around 09Z is not out of the question with thunderstorm coverage and intensity increasing steadily thereafter. The development of a large convective system is anticipated....probably evolving over E/NE Germany during the afternoon hours and affecting W/NW Poland thereafter.

Main concern is the potential for rapid storm clustering and hence strong cold pool development (indicated by high-resolution model data over E-Germany and especially over W/NW Poland). Despite the lack of a more robust mid-level dry layer, wet microbursts are possible next to large hail during the initiation stage of those thunderstorms. Rapid clustering is forecast with the main risk being strong to severe wind gusts. Thunderstorms, which evolve away from the upper support may find themselves in a very weak steering flow environment, increasing the risk for excessive rainfall. The storms move eastwards with a slow decrease in activity after sunset ..... however over Poland they may keep going well into the night.


...Lithuania and Latvia ...

The aforementioned vorticity lobe also affects those areas. However, this upper feature remains well structured with strong gradient flow overspreading the area during the forecast period. 0-6 km DLS of 10 to 15 m/s with 1000 J/kg MLCAPE (due to moisture pooling along a W-E aligned convergence zone and modest mid-level lapse rates) create a favorable environment for multicells and isolated supercells. Large hail and strong to severe downbursts are forecast. Despite only weak signals of somewhat better helical PBL flow due to backing along the convergence zone, 0-3 km CAPE signals remain supportive for an isolated tornado event. Thunderstorm development occurs well into the night hours.

... SW-Belarus to W-Romania all the way to the eastern Alps ...

No dynamic support can be seen, so focus for initiaton will be the topography. Lapse rates at mid-levels remain supportive for abundant mid-level CAPE build-up with ICAPE locally approaching 2000 kJ/m^2. Any thunderstorm will pose a risk for large hail and severe wind gusts, so a level 1 was added, despite regionally confined thunderstorm coverage. Only parts of the level 1 were highlighted with a 50% lightning area. Thunderstorms will continue during the night.

The rest of the lightning areas may see pulsating thunderstorms with locally heavy rainfall, marginal hail and strong wind gusts. The heavy rainfall risk may be augmented SW of Switzerland/SE-France, but forecast QPF data remains well below our level criterion for excessive rainfall.