Storm Forecast
Valid: Fri 08 May 2009 06:00 to Sat 09 May 2009 06:00 UTC
Issued: Thu 07 May 2009 20:17
Forecaster: TUSCHY

A level 1 was issued for N-Germany, NW-Poland and S-Sweden mainly for large hail and severe wind gusts.....

A level 1 was issued for SW-France mainly for large hail and strong to severe wind gusts.....

A level 1 was issued for S-Germany and NW-Austria mainly for large hail and strong to severe wind gusts.....


SYNOPSIS

A sub-975hPa cyclone is situated north of Scotland during the period. Strong ridging and WAA occurs downstream, affecting most parts of the W/CNTRL Mediterranean and SW/W-Europe. Stable conditions therefore persist in that regions. A cut-off low over the W-Black Sea results in unsettled conditions over the E-Ukraine, whereas mostly stable conditions occur over NE-Europe.

At the surface, an active boundary runs from SW-Europe all the way to Sweden and serves as a focus for thunderstorms. A cold front approaches from the W/NW during the period and stalls out over S-France, while remaining progressive over NE-Germany/Sweden.

DISCUSSION

...E-France, N-Switzerland and SW/CNTRL/E-Germany between 06Z-roughly 09Z...

A pronounced lead impulse, embedded in a moderate southwest flow, is placed over central/east Germany at 06Z whereas the position is a compromise between AFWA,GFS,WRF and ECMWF, the latter one being the most aggressive one. Nevertheless, the impulse outruns axis of highest theta-e over SW/W-Germany, so just a very isolated thunderstorm is forecast over E/SE and central Germany during that time. Ongoing upslope flow and a near uncapped warm/humid airmass assists in scattered showers/thunderstorms over SW/W-CNTRL Germany and NW-Switzerland, which build northward betimes along a pronounced theta-e tongue. Winds at mid-levels are already on the increase, but the risk of severe remains low due to limited instability/weak synoptic forcing and probably elevated nature of storms. Strong wind gusts and marginal hail are possible with stronger storms.

... N-Germany, NW-Poland and S-Sweden from 09Z onwards ...

An eastward moving cold front is situated over NW-Germany during the morning hours/noon and showers/thunderstorms will start by that time. Initiation should occur both along the cold front and in the prefrontal airmass, where surface pressure falls. Persistent SW-flow keeps moisture advection going over N/NE-Germany all the way up to S-Sweden. The lack of better mid-level lapse rates ought to limit instability release but 200-500 J/kg MLCAPE look reasonable.

Beside aforementioned early initiation along the front, thunderstorms also evolve further east(NE-Germany) as we hit the convective temperature during the day. Shear constantly increases from the west, so an isolated supercell, embedded in the predominant multicell mode, is possible with an attendant large hail/severe wind gust risk. Abundant 0-3km CAPE release is forecast, but very slim directional shear in that layer keeps tornado probabilities very low.

ECMWF features the most interesting solution for the cold front passage right now, which affects NE-Germany and S-Sweden around 18Z. The progressive upper trough axis sweeps in from the west between 15Z-18Z and picks up the cold front, which is placed over N-CNTRL Germany. Dependant on the quality of the airmass ahead, a line of thunderstorms/showers (potential LEWP-type) can evolve with a severe wind gusts/isolated large hail risk. ECMWF still paints an atmosphere with some potential instability release ahead of the front and given the strong nature of the forcing, we went ahead and placed a wide swath in a level-1 for severe wind gusts. Beside rapidly diminishing instability over S-Sweden, cold front passage could still be active regarding not necessarily electrified deep convection and severe wind gusts are forecast,too. We have to monitor the degree of moisture advection as better LL moisture would culminate into an higher severe wind risk.

... S-France, Switzerland and parts of Austria all day long over highlighted regions in France and at the latest after 12Z-15Z over the rest of the area ...

Despite the exaggeration of GFS regarding surface dewpoints (locally more than 5°C overrated over SW-France), the boundary layer indeed yields dewpoints in the lower to mid tens. A weak Spanish plume fans out over the area, but lapse rates are not too steep, so MLCAPEs of 500-locally 1kJ/kg build along a SSW-ENE aligned convergence zone, later re-enforced by a southward sliding cold front, which also stalls out somewhere over S-France/W-Alps during the night. The airmass is weakly capped at best, so thunderstorms initiate over S-/SE-France and Switzerland all day long and also over S-Germany/most parts of the Alps during the early afternoon hours. The messy picture gets complete as initiation occurs due to ENE-ward traveling short-waves, upslope flow along the mountains and persistent convergence along the boundary over S-France.

Wind increases somewhat during the afternoon hours with DLS of 15-20m/s, so isolated large hail and strong wind gusts are likely, especially during the more discrete phase of those storms. Rapid clustering occurs and the main hazard then shifts to heavy rain with the following arguments:

i) flow at the lower levels is aligned normal to the boundary with mid-/upper winds blowing near parallel to it, so convergence along southward moving outflow boundaries favors new cell growth with some training possible

ii) ECMWF and GFS both hint on a moist atmosphere at the lowest 3km, especially over W/NW Switzerland with an increasingly deep warm cloud layer

iii)just weak to moderate shear occurs, which keeps entrainment effects low

A few spots just west of Switzerland and also along the W-Alps could indeed see rain amounts, which match our criterion, but this depends mostly on the location of those convective clusters, so we did not issue a level-1 for that hazard. The level-1 over S-Germany/NW-Austria and the other one over SW-France imply mainly large hail/isolated severe wind gusts. Especially over SW-France, thunderstorm coverage might be more sporadic, but more isolated nature should bring hail chances up. Thunderstorms diminish after sunset as CAPE vanishes.

... Ireland and N/CNTRL-UK...

A humid/well mixed postfrontal airmass affects those regions and daytime-driven thunderstorms are forecast. Beside weak directional/speed shear at all levels, 15-20m/s at 850hPa mix down easily, so showers/thunderstorm can produce strong to isolated severe wind gusts. Both the coverage and intensity of those thunderstorms remains too marginal for a level-1. Thunderstorms decrease around sunset.

... CNTL/E-Ukraine...

500-1000 J/kg MLCAPE develop over the area in a weakly sheared environment, so despite an isolated large hail risk, no severe risk will occur with that activity. Thunderstorms leave the area of responsibility during the late afternoon, while weakening.