Storm Forecast
Valid: Mon 23 Jun 2008 06:00 to Tue 24 Jun 2008 06:00 UTC
Issued: Sun 22 Jun 2008 21:47
Forecaster: GROENEMEIJER

SYNOPSIS

Monday morning at 06:00 UTC, a major feature on the weather map is a upper-level trough over southern Scandinavia with an intense westerly jet located on its southern flank. An shortwave trough near the Kattegat rotates rounds its base across the Baltic Sea reaching Karelia on Tuesday morning. It is followed by a second shortwave trough. A trough west of Portugal preceded by a strong SSWly jet streak ahead moves northeastward. A frontal zone that forms the boundary between relatively cool maritime air to its north and warm and moist air to its south is expected near a line from Bordeaux via Strasbourg to Gdansk. Over Poland and the Czech and Slovak Republics it moves east to southeastward, whereas its more or less stationary further west and returns northward across France.

DISCUSSION

Spain...

Across central and northern parts of Spain, abundant insolation is expected to allow 500-1000 J/kg MLCAPE to form during the afternoon. Scattered storms are expected to develop as a result. Given that deep-layer shear of 20-25 m/s is expected, some storms will probably develop into supercells. Steep lapse rates just above the freezing level and high cloud bases support the development of large hail with these storms. Additionally, a few strong downbursts are possible. The limited coverage that is expected is the main reason not to issue a level 2 for this area.

Southwestern, southern and central France...

A difficult forecast for this area, in which high MLCAPE, posisbly in excess of 2000 J/kg should develop during the afternoon within the air-mass south of the front. Models suggest that the front should return northward during the late afternoon. At that time, moderate values of storm-relative helicity are expected to develop east of a surface cyclone over SW France, especially in the vicinity of the front. These facts and strong shear in the 700-500 hPa layer would in principle support the development of intense rotating updrafts with very large hail possible. However, numerical models are hesitant with the development of surface-based storms, because of a lack of upper-level support. The consensus of model guidance is that storms will probably not develop before mid-evening when the Spanish trough approaches. If more than isolated storms indeed develop, a level 2 may well be needed at a later moment. Presently, the uncertainties preclude a level 2 to be issued.

South Germany, northern Alps, Czech and Slovak Republics...

Scattered storms will likely redevelop along the front during the afternoon across these areas. MLCAPE on the order of 1000-1500 J/kg and 20 m/s deep-layer shear should suffice for supercell development among other convective modes. Primarily the supercells will be able to produce large hail and isolated severe gusts.

Poland, Kaliningrad, south Lithuania...

Ongoing convection over NW Poland in vicinity of the surface cold front will move eastward during the day. As it does so, it encounters only modest values of CAPE, but wind shear will be strong. It appears that one or more convective lines will be likely, possibly containing short-lived rotating updrafts. The wind will porbably the main risk, but some large hail and an isolated tornado will also be possible. Current thinking is that the coverage will be low enough not to warrant a level 2, mainly because of the low CAPE values.

Southern Alps...

A moist low-level air-mass with surface dew points near 20 C will should remain in place across the Po-Valley, so that up to 2000 J/kg of MLCAPE will likely redevelop during the day. Isolated storms are expected to form across the southern Alpine flanks with a potential to produce large hail.

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