Storm Forecast
Valid: Tue 25 Sep 2012 06:00 to Wed 26 Sep 2012 06:00 UTC
Issued: Mon 24 Sep 2012 23:00
Forecaster: TUSCHY

A level 1 was issued for parts of E/SE France mainly for isolated strong to severe wind gusts and an isolated tornado event.

A level 1 was issued for S-France mainly for isolated strong to severe wind gusts, large hail and heavy rain.

SYNOPSIS

An impressive 979 hPa depression, just about to move ashore along the E-C coast of UK at 22Z (24th), continues to be the main player for the following 24 h. According to the phase diagrams, the warm seclusion process will be finished during the start of the forecast and a very well structured shallow warm-core structure persists until the evening, before gradual weakening begins. Those spots with more or less undisturbed ASCAT measurements show roughly 40 kt near surface winds wrapping around the center with some zonal asymmetrie present. Despite the cut off of the warm/moist supply of modified subtropical air, it still takes some time for that energetic air mass to mix out, so a slow moving and intense/deep cyclonic vortex over the UK will be the major steering mechanism for most of Europe.

Lower geopotential heights and cool mid-levels affect most of Europe and result in unsettled conditions with showers/thunderstorms over numerous areas. Beside a few upper level disturbances, no major trough/rigde is seen during the forecast.

DISCUSSION

... SE/S France ...

The main reason for no higher-end severe risk will be the warm air mass at mid-levels (keeping lapse rates on the lower-end side) and the modified Mediterranean air mass, we have to work with. However, strong onshore flow advects some moisture towards SE France, which assists in roughly 500 J/kg MLCAPE build-up ... probably somewhat higher values will be realistic along the coastal areas. With some type of frontal boundary over C/SW France (first occlusion-type, transforming into a cold front betimes) and the Alps, a channeled SW-erly flow evolves during the forecast with 850 hPa winds exceeding 25 m/s during the evening hours. Limited diabatic heating and prolonged period of lift (isentropic and along the orography) may result in a mess of numerous showers, rapidly clustering into a more stratiform rain area. However, latest WRF keeps CI isolated with indeed sporadic thunderstorm development expected. Any storm would temporarily acquire organization, producing strong to severe wind gusts and marginal hail. An isolated tornado can't be ruled out with expected LL kinematics.

Confidence in isolated severe thunderstorm activity is higher along the coast, where 25 m/s DLS overlaps better CAPE and hence an isolated well organized multicell may produce large hail and strong to severe wind gusts. As a weak impulse at mid-levels approaches from the SW during the evening, winds back to southerly directions, which may produce a somewhat more favorable environment for back-building storms along the S-coast with heavy rain the main risk. Hence the level 1 was expanded far west.

... C-Mediterranean ...

Isolated DMC can evolve all day long, as numerous weak disturbances cross that area. Good CAPE but weak to moderate DLS probably cause weakly to isolated better organized multicells with marginal hail, strong wind gusts and heavy rain. Isoalted waterspouts may occur although strengthening background flow lowers that risk after sunset.

... Germany ...

Despite bad thermodynamics, models persistently show a pronounced convergent flow pattern over C-Germany (extending from N to S). This may assist in marginal moisture pooling, which would be enough for some low-end CAPE build-up. Near parallel background flow to that boundary and 20 m/s shear in the lowest 3 km would be enough for a few fast moving showers/isolated thunderstorms. Strong to severe wind gusts would be the main risk. This is a nowcast topic and therefore no level will be issued that far out with ongoing uncertainties.

... Benelux and NW Germany ...

Influx of maritime air beneath cool mid-levels causes some moderate CAPE of 200-500 J/kg. Some shear/CAPE overlap exists during the afternoon hours and a few better organized showers/thunderstorms are possible with the main activity over the Netherlands. Strong to isolated severe wind gusts will be the main hazard although it would not be the first time to receive a tornado report with that pattern. The risk rapidly diminishes after sunset.

... Rest of the highlighted areas ...

A very broad low lightning area was issued, as sporadic activity may occur. The main activity will occur over offshore areas like the S-North Sea, the Bay of Biscay and the far E-Atlantic.