Storm Forecast
Valid: Fri 04 Sep 2009 06:00 to Sat 05 Sep 2009 06:00 UTC
Issued: Thu 03 Sep 2009 16:29
Forecaster: TUSCHY

A level 2 was issued for NE-Italy, W-Slovenia and W-Croatia mainly for large hail, severe wind gusts, excessive rain and to a lesser extent for tornadoes.

A level 1 was issued for parts of Germany, Netherlands, Belgium, Luxembourg and Switzerland, parts of Austria, Hungary, E-Slovakia, W-Ukraine, W-Romania and most parts of the Balkans mainly for large hail and severe wind gusts.

A level 1 was issued for the western Balearic Islands and east-central coast of Spain mainly for large hail and severe wind gusts.

SYNOPSIS

A strong upper trough over the North Sea moves eastwards and is placed over Sweden and Norway during the end of the forecast. Embedded disturbances cause unsettled conditions for most parts of central and northern Europe. Warm and stable conditions prevail over most parts of the Mediterranean and eastern Europe.

DISCUSSION

... S-Austria, Hungary, W-Ukraine, Slovenia, parts of Croatia and NE-Italy ...

All models show abundant convective signals, but timing of the convective precipitation remains uncertain. ECMWF is a bit reluctant with the main activity during the afternoon hours, GFS has initation all day long and UKMO indicates an heavy rainfall scenario. We stay with GFS and therefore expect a prolonged period with thunderstorms as numerous disturbances cross the area from the west.

The temperatures at mid-levels cool down slightly, whereas the lower levels remain warm and moist due the the blocking effect of the Alps in respect of the cold front passage. This should support a prolonged period of modest CAPE release in a strongly sheared environment. In fact, ECMWF and GFS are in line with up to 2000 J/kg MLCAPE along the NE- Adriatic Sea. Large hail and severe wind gusts are forecast which could reach the criterion "extreme". In addition, a few tornadoes are forecast with the highest risk over western Croatia, central and western Slovenia and extreme NE-Italy. Due to the repeated thunderstorm development over the same area and persistent influx of moist air from the south, excessive rain is forecast, too. The severe risk gradually moves southwards as cold front approaches from the north.

Further towards the east, including S/E-Austria, rest of Slovenia and western Hungary, CAPE decreases somewhat with values in general less than 1000 J/kg MLCAPE. Shear at all levels remains supportive for large hail and severe wind gusts. During the night hours, the cold front finally pushes southwards and so do the thunderstorms.


... Belgium, the Netherlands, Luxembourg, eastern France and Germany ...

The main story during the forecast is the passage of the trough axis and attendant pool of cold mid-level air and the passage of a wave at lower levels (in high gear below 3km). Both features are displaced in time by a few hours, so thunderstorms are forecast in numerous waves and at different places.

The wave is placed over eastern France (roughly 9 UTC), over southern Germany at 12-15 UTC and over the Czech Republic at 18 UTC (based on GFS with ECMWF a tad slower). Both models show potential for thunderstorms, probably deep with cool EML temperatures in GFS. Initiation will be probably bound to the eastward moving cold front with isolated showers/thunderstorms in the weakly capped prefrontal warm sector. A potential scenario is an active, eastward racing cold front with embedded thunderstorms. Degree of shear especially at low levels points to an organized line (LEWP) with a severe wind gust threat. This line ought to decrease in intensity over N-Austria and the Czech Republic during the late evening hours, as CAPE decreases. Timing of this wave is important in respect of how unstable the airmass will be, which finally affects the magnitude of the wind gust risk. Next to that dominant risk, isolated tornadoes and marginal hail can't be ruled out, given degree of shear and some LL CAPE release.

Another area of interst includes Belgium, the Netherlands, Luxembourg and most parts of Germany during the daytime hours. As cold mid-levels overspread those areas from the west, steep lapse rates and adequate surface moisture result in at least modest CAPE release (up to 500 J/kg MLCAPE). Shear decreases gradually to the north, but remains supportive for organized thunderstorms with a hail and wind gust risk. An isolated tornado risk is present due to some directional shear and low LCLs, but the overall risk remains low as directional shear component first increases as CAPE decreases (evening and night hours). Thunderstorms decay after sunset.

Thunderstorm coverage is on the increase over the Baltic Sea during the night hours, as cold thermal trough overspreads the warmer Sea. Moderate CAPE with abundant LL CAPE release but weak shear point to an isolated waterspout risk, althoug no convergence zone can be seen in the models, which could serve as a focus for enhanced spout chances.

The same for the southern North Sea, where a few spouts are possible well in the night hours, although overall set-up does not look too promising with decreasing moisture content and no organized "convective lines" due to the late passage of a backbend occlusion (18-21 UTC).


... East coast of Spain and parts of the Balearic Islands ...

A capped airmass is present (surface dewpoints in the upper tens or lower twenties) and the main mechanism for initiation ought to be a southward sagging cold front during the night hours. It is unclear how fast the front will be but conditions along or ahead of this front improve for augmented thunderstorm chances (e.g. moisture pooling and weakenng CIN). Isolated to scattered thunderstorms are expected during the early night hours onwards with a general motion towards the south/southeast. Shear is beneficial for organized updrafts with 0-6 km bulk shear approaching 25-30 m/s and enhanced directional shear. We see no reason to question GFS CAPE forecast with up to 2000 J/kg MLCAPE (see yesterday 12 UTC sounding 08302) as warm/moist BL and sharp EML gradient still overlap. Large to very large hail and severe wind gusts are the primary risk. However, some LL CAPE release and magnitude of potential updraft strength with rapidly decreasing LCLs also points to a tornado risk. No level 2 was issued due to the limited coverage of thunderstorms. In addition, heavy rain could affect Ibiza and the coastal areas of eastern Spain, but most of that activity ought to stay offshore.