Storm Forecast
Valid: Sun 03 Jun 2012 06:00 to Mon 04 Jun 2012 06:00 UTC
Issued: Sat 02 Jun 2012 17:15
Forecaster: PISTOTNIK/GROENEMEIJER

A level 2 was issued for South-Central Germany and the Western Czech Republic mainly for large hail and excessive precipitation, and to a lesser extent for tornadoes.

A level 2 was issued for Southernmost Germany and Northern Austria mainly for severe wind gusts and large hail.

A level 1 was issued for the Eastern Czech Republic and Western Slovakia mainly for severe wind gusts.

A level 1 was issued for Northern Italy mainly for large hail and excessive precipitation.

SYNOPSIS

A pronounced frontal zone divides Europe into an unseasonably cool Northern and a very warm Southern half. At the surface, the almost stationary and further sharpening frontal boundary runs from the Southern British Isles via Belgium and Central Germany to the Czech Republic.
While the model pool has closed in on a wide agreement concerning the general pattern, uncertainties on the mesoscale still remain considerable. Several shallow waves are expected to form and travel eastward along this boundary. To its South, a very warm air mass is present in the Southeastern quadrant of Europe, whereas cooler Atlantic air masses spread over France and the Iberian Peninsula behind a diffuse cold front.

DISCUSSION

... South-Central Germany, Czech Republic (noon to afternoon hours) ...

The most supposable scenario is that the dominant wave along the sharp frontal boundary will move from Northern France (06z) via Central Germany (12z) to the Czech Republic (18z). Depending on its final degree and track, a robust severe weather threat may evolve in its warm sector, as the warm and humid air mass draws near the axis of the pronounced mid- and upper level jet streak (Westerlies of 25 to 30 m/s at 500 hPa).
Whereas GFS and WRF models forecast the evolution of widespread CAPE in the 800 to 1500 J/kg range across the warm sector in response to diurnal heating, ECMWF offers weaker and more patchy CAPE signals. Model confidence in convective initiation seems to be similarly diverse. All these uncertainties are reflected in very different forecasts of the coverage of convective precipitation in the warm sector. It is noteworthy, though, that all the finer limited area models show precipitation signals that can only be interpreted as tracks of long-lived and organized thunderstorms.
Current thinking is that insolation and reduced vertical mixing of boundary layer moisture in the continuing warm air advection regime may lead to the build-up of CAPE around 1000 J/kg over the Southern half of Germany and later over the Czech Republic. Deep-layer shear between 20 and 30 m/s and 0-3 km SRH around 200 m^2/s^2 are more than sufficient for storm organization. Any thunderstorms forming in this environment are supposed to quickly turn into multicells or supercells with an attendant threat of large hail, excessive precipitation and (to a minor degree) severe wind gusts. One or two tornado events are possible as well, in particular in closer vicinity to the warm front where the condensation level drops to <1000m and low-level shear rises to 10 m/s or even more.
Despite the unclear coverage, the severe weather threat seems to be robust enough to issue a level 2 for a corridor from South-Central Germany to the Czech Republic. Convective activity is forecast to weaken and to become incorporated into the large-scale rain field of the warm front shield, while moving further East over Poland.

... Switzerland, Southern Germany, Austria, Slovakia (evening hours) ...

The advance of a cooler maritime air mass from the West will probably trigger another round of thunderstorms, starting over Switzerland, Western Austria and Southern Germany in the afternoon hours and intensifying along its way further East. Moderate instability (CAPE expected to be around 1000 J/kg) and moderate vertical wind shear (DLS around 15 m/s) are thought to be sufficient for at least multicellular storms with a primary threat of large hail. The formation of a cold pool in the wake of the storms may also foster the formation of an MCS or a convective line even despite the lack of large-scale forcing. Consequently, rather widespread severe wind gusts are a distinct scenario, which is why the level 2 was extended to Southernmost Germany and Northern Austria.
Further East, remnants of an EML (situated over Southern France and the Western Mediterranean region today) will be advected over the Eastern Alpine region towards Hungary and Slovakia. Hence capping will probably be strong enough to suppress convection during the day despite the build-up of CAPE in the range of 1000 J/kg or more in association with a rapid boundary layer recovery, and it is unclear how far East the evening convection will manage to push. MCSes in these environments have a history of sticking to the Northern Alpine fringe and travelling due eastward for a long time. This is why the level 1 was extended to Eastern Austria, Eastern Czech Republic and Western Slovakia as well, even though the majority of forecast models only shows the passage of a dry wind shift line overnight and no convective precipitation any more.

... Northern Italy (overnight) ...

Later on, the ill-defined cold front will encounter an air mass with low to moderate (elevated) CAPE and moderate vertical wind shear, when it arrives in Northern Italy in the night hours. A few stronger storms with large hail and excessive precipitation are possible despite the unfavourable timing.