Storm Forecast
Valid: Sun 29 May 2016 06:00 to Mon 30 May 2016 06:00 UTC
Issued: Sun 29 May 2016 00:45
Forecaster: PISTOTNIK

A level 1 and level 2 were issued for Germany, the W Czech Republic, N Austria and N Switzerland mainly for large hail and excessive precipitation, and to a lesser extent for severe wind gusts.

A level 1 was issued for Belgium and N France mainly for excessive precipitation.

A level 1 was issued for N Italy mainly for large hail and to a lesser extent for severe wind gusts.

A level 1 was issued for parts of Belarus, Russia and the Ukraine mainly for excessive precipitation and large hail.

SYNOPSIS

An elongated area of low 500 hPa geopotential and rather cool maritime air extends from the Norwegian Sea across the British Isles to N Spain and France and also spreads towards the Alpine region. Ahead of it, a short-wave trough crosses N Italy, Slovenia and Austria in the first and S Germany and the Czech Republic in the second half of the forecast period. Strong westerly mid-level flow is present across the W and central Mediterranean region.
Over the Eastern half of Europe, a blocking anticyclone sits over Scandinavia and geopotential gradients are weak otherwise. Southerly flow pumps very warm air onto much of the continent.

DISCUSSION

... France, Switzerland, Austria, Germany, BeNeLux, Czech Republic, W Poland ...

A deepening surface cyclone moves from E France into the S half of Germany. Exact position of its core, or possibly multiple cores, is difficult to predict and may heavily depend on the regional distribution of daytime heating and previous convective activities. Forecast uncertainties are considerable.
Aided by plentiful evapotranspiration, CAPE on the order of 500 to 1500 J/kg will again be realistic. Isolated thunderstorms may still be left over from the previous night or may form already before noon. Storm coverage will increase in the afternoon and evening. With weak winds and plentiful moisture (forecast 2m dewpoints are mostly between 15 and 18°C), heavy precipitation and localized flash floods will be the main risk.

A common denominator across the model pool is a shield of warm air advection on the N and E flank of the surface low. The associated delay in vertical mixing will likely allow the highest CAPE values and may also create veering wind profiles, despite an overall low magnitude of wind speeds. Storms may therefore organize into multicells with an enhanced risk of moderately sized (2-4 cm) hail.
Warm air advection and the approaching short-wave trough could also provide enough large-scale lift to favor the formation of one or several large thunderstorm clusters towards evening, which may persist well into the night. Wind gusts along the leading edge of such a system may at least locally become severe. An isolated tornado is not ruled out, either, in case of favorable mesoscale developments (e.g., a storm moving over a pre-existing outflow boundary).
Overall, a level 2 is issued for central Germany, where fine-meshed limited area models agree on plentiful and widespread signals of convective precipitation, and where more concentrated flocks of severe weather events are anticipated.

Another area of interest is the northern Alpine rim. If upvalley winds can still overcome the strengthening southerly background flow, a narrow belt of high CAPE (e.g., locally up to 2500 J/kg according to WRF) and enhanced vertical wind shear may be maintained. A few well-organized storms could bring large hail, downbursts and flash floods in this case.
However, precipitation signals in the fine-meshed models are limited. It therefore appears equally likely that the southerly flow across the Alps will win and that dry Foehn winds, or at least a quick diffusion of low-level moisture, will limit thunderstorm potential.

... W Mediterranean across Italy and N Balkans into Hungary and Slovakia ...

Saturday's sounding data showed very warm air in mid-levels (850 to 700 hPa), which acted as a capping inversion on top of a moist boundary layer. Rising motions ahead of the short-wave trough will erode this cap, but it is still questionable if it can be broken.
0-3 km shear around 15 m/s would be adequate for multi- and supercells. A level 1 is issued to cover a conditional risk of large hail and severe wind gusts in case surface-based storms form. Best chances for at least isolated initiation exist over the N Appennin mountains or along sea breeze fronts from the Adriatic Sea.
Otherwise, elevated and weakly electrified convection may develop from Altocumulus clouds ahead of the trough axis (regardless of whether there is land or sea underneath). It may also spread into Hungary and Slovakia overnight without a particular severe weather risk.

... E Europe ...

Diurnal heating of warm and moist air will create widespread CAPE again under very weak vertical wind shear, probably on the order of 500 to 1500 J/kg. Regionally higher CAPE values, like predicted by GFS on a daily basis, were not confirmed by Saturday's soundings (apart from Rostov to the east of our forecast domain), and are doubted on Sunday again.
In the absence of any synoptic disturbances, at least scattered thunderstorms will form over orographic features or old outflow boundaries and follow a pronounced diurnal cycle. Localized flash floods and marginally large hail are possible with strong pulse storms.

... Ireland and S England ...

Scattered thundery showers will form as the cool maritime air is exposed to daytime heating. One or two non-mesocyclonic tornadoes are not ruled out due to low cloud bases and weak vertical wind shear.