Storm Forecast
Valid: Wed 09 May 2012 06:00 to Thu 10 May 2012 06:00 UTC
Issued: Tue 08 May 2012 23:26
Forecaster: TUSCHY

A level 1 was issued for parts of E/SE Europe mainly for large hail and strong to severe wind gusts. An isolated tornado event is possible mainly over N-Ukraine and into Russia.

A level 1 was issued for parts of France, Switzerland, S-Germany and Austria mainly for isolated large hail and heavy to locally excessive rainfall.

A level 1 was issued for parts of Benelux, N-Germany and far NW-Poland mainly for isolated large hail and strong wind gusts.

SYNOPSIS

A gradual shift of the main streamline pattern is forecast with pattern recognition showing augmented probabilities for severe thunderstorm development over parts of W/C Europe during the upcoming days.
On Wednesday, a longwave trough to the N and W of Europe reveals a positive tilt throughout the forecast with the build-up of a sharp and deep baroclinic zone, which runs from the Azores towards the Bay of Biscay with a rapid NE-ward expansion during the forecast. In fact, an unusual extensive low-level depression covers all of the North Atlantic north of 30° N with connection to a plume of subtropical air with TPW exceeding 30 mm regarding latest MIMIC analyis. Therefore, a well defined subtropical air mass spreads to the NE and starts to affect parts of W/C-Europe during the forecast period.
Attendant deep WAA assists in geopotential increase over most of SW/S-C Europe, which also results in a cut-off process over SE-Europe.

DISCUSSION

... France, Benelux, Germany, Switzerland, Austria, the Czech Republic and parts of Poland ...

With the steering upper trough still far out to the west of Ireland, the highlighted area resides beneath a strengthening SW-erly mid/upper jet, which pushes a warm front rapidly to the NE, crossing NE-Germany around noon. Behind that front, the air mass features subtropical characteristica with pronounced warming at mid-levels and PWATs increasing to 25 mm or more. As a result, mid-level lapse rates are meager with neutral or slightly unstable profiles forecast. However, yesterday's Td readings already revealed 12-15 °C over France with even higher values towards Aquitaine (France). Further east/NE, roughly 8-10°C were reported with a constant increase noted.

Models diverge regarding final CAPE build-up, especially over France. Whereas GFS/WRF evolve 800-1000 J/kg MLCAPE over most of France and up to 800 J/kg over parts of Germany, EZ still has significantly lower values with local peaks of 500-800 J/kg. This time, we lean more towards the more aggressive GFS/WRF solutions, given adequate BL moisture already in place over France. Also, pockets of locally better lapse rates, bound to the differental positive mid-tropospheric temperature advection regime, may also assist in regionally better CAPE magnitudes.

Next to the lapse rates, a forcing mechanism is missing in the weakly disturbed SW-erly flow regime. Somewhat better structured vorticity lobes atop the English Channel and S-North Sea graze N-France, Benelux and N-Germany, which may assist in some forcing. Otherwise, initiation is probably bound either to the rough topography or to areas, where convective temperatures will be exceeded.

0-6 km bulk shear in the order of 15 m/s over N-France, Benelux and N-Germany, up to 20 m/s over SW-France and at or below 10 m/s over the Alps is expected. Hence, a few well structured multicells are forecast mainly in the northern level 1 and over far SW-France with a decline in storm organization towards the Alps. The main hail risk will be found over SW-France, with a more sporadic risk following the CAPE axis towards the NE (inlcuding C-France and parts of C-/N-Germany). Next to that risk, locally excessive rain accompanies those thunderstorms, given forecast increase in moisture. This risk is probably maximized over S-France, parts of N-Switzerland, S-Germany and Austria, where weak ridging assists in low mean flow values and slow storm motions. 50-% thunderstorm probabilities may be quite aggressive, but most models had at least scattered activity. Also, N-Switzerland/SW-Germany may suffer from limited diabatic heating due to a gradually decaying and ill-defined cluster of showers/sporadic thunderstorms from last night, which may affect those regions until noon.

Confidence in one large level 1 is too low mainly due to the lack of forcing and modest CAPE over most of C/N France and C-Germany. Hence the level 1 areas were issued for those spots, where confidence in scattered thunderstorm initiation is at least adequate.

The overall risk diminishes after sunset, although not much lift is needed to convert the available MUCAPE into stronger but probably elevated updrafts and hence we would not be surprised to see a few overnight storms, too. No severe is forecast with that activity, despite sporadic marginal hail.

... S-UK ...

A warm front gradually lifts to the north during the daytime hours with a warm and moist postfrontal air mass affecting mainly S-UK with low-end instability build-up forecast. A veering profile (especially along and just behind the northward lifting front) and placement beneath the exit region of an approaching 25-30 m/s mid-level jet may assist in a few organized thunderstorms mainly between the late morning and late afternoon hours. An isolated tornado event is possible next to marginal hail and strong wind gusts. No level 1 was yet issued due to the marginal CAPE environment and sporadic initiation forecast in local model data.

During the night hours, the stage is set for a heavy rainfall event, at least partially accompanied by DMC. With numerous vorticity maxima crossing UK from the SW, the influx of a very moist air mass and 850 hPa winds increasing to more than 20 m/s, training/back-building storms are possible. This risk may be enhanced along any mesoscale convergence zone and/or outflow boundary, left by daytime conveciton. Heavy to locally excessive rainfall is forecast with adequate MUCAPE for a few embedded thunderstorms. The flash flood risk is also enhanced by history of past weeks heavy rainfall events.

... E/SE-Europe ...

Most areas see more or less organized multicells (probably determined by mesoscale processes like outflow boundaries or regionally enhanced BL moisture) with 500-1000 J/kg MLCAPE and 10 m/s DLS. However, a few large hail and strong to isolated severe wind gust events are possible with the lower troposphere supporting better structured downdrafts and with moderate mid-level lapse rates in place. Slow storm motion also poses an heavy rainfall risk especially over E-Moldova/Romania, where sea breeze interaction may play a role in longer lived multicells.
The severe risk increases over N-Ukraine into Russia, as 20 m/s DLS and more than 1kJ/kg MLCAPE overlap along a quasi-stationary SW/NE aligned boundary. A few supercells and well organized multicells with large hail and severe wind gusts occur. An isolated tornado event can't be ruled out mainly along the boundary, where LL shear is somewhat enhanced, but overall more robust tornado risk exists beyond our forecast area. Storm coverage and intensity decrease until midnight, but a few storms probably continue until 06 Z with an ongoing isolated large hail risk.