Extended Forecast
Valid: Thu 20 Aug 2009 06:00 to Fri 21 Aug 2009 06:00 UTC
Issued: Tue 18 Aug 2009 21:51
Forecaster: TUSCHY

A level 1 was issued for Belgium, the Netherlands, Luxembourg, NW and central Germany, Denmark and parts of Norway and Sweden mainly for large hail and severe wind gusts and to a lesser extent for tornadoes.

SYNOPSIS

A 985 hPa vortex east of Iceland is the steering mechanism for most parts of the European weather. Intense WAA downstream causes hot conditions over central Europe with scattered thunderstorms along the frontal zone. Cold and stable conditions preclude thunderstorms over eastern Europe.

DISCUSSION

... Benelux, W/NW-Germany, Denmark, parts of Norway and Sweden ...

An unseasonably strong trough regarding its expansion and intensity is forecast with model guidance being in line, showing a sub-990 hPa between Iceland and Scotland. Impressive WAA is forecast downstream, which overspreads an area all the way up to north Sweden and Norway. The upper trough continues to move eastwards on a slow rate and acquires a slight negative tilt. Along its eastern fringe, an active frontal zone pushes eastwards in the form of a cold front, which runs from the Azores all the way to the west coast of Norway. This front approaches Benelux during the evening hours and W-Germany during the morning hours (21st August). However, models agree in the development of a sharp prefrontal convergence zone ahead of the cold front, which will be the focus for the most widespread thunderstorm initiation.

This airmass modification is accompanied by steep mid-level lapse rates and a seasonably moist airmass at lower-levels. GFS again is the outlier with dewpoints in excess of 20°C over Benelux and NW-Germany, but we stick with this model for now due to the prolonged time for the return flow with mid-/upper tens already in place over France (evening hours of the 18th August). Beside the advective component, strong nature of the forecast convergence zone indicates good moisture pooling along this zone, probably maximized over Benelux and NW-Germany. Dewpoints decrease somewhat to the north and south of those areas. MLCAPE will be maximized over Benelux ( probably greater 1kJ/kg) with lower values to the north and south.

The 0-6km bulk shear vector is parallel to the frontal zone over Benelux and north-northeastwards and supportive for rapid thunderstorm organisation. Latest indications point to an enhanced supercell risk over Benelux, extreme NW-Germany and Denmark but later model outputs will be evaluated before issuing higher probabilities. Large hail, severe wind gusts are the primary risk, the latter one could become more serious, if 20m/s 0-3km shear indeed verifies. The tornado risk still depends on the strength of a weak surface trough, which is not present in all models. Instability north of Denmark decreases, although enough elevated instability remains for a large hail risk all the way to central Norway. A flash flood risk arises beneath nort-northeastward moving storm clusters.

Initiation is quite complex, given numerous, strong short waves, which cross the frontal zone and the intensifying prefrontal convergence zone over the southern part of the level 1 and the elevated nature over the northern parts. Numerous rounds of thunderstorms are probably the most likely scenario, with the most active one occuring during the afternoon and evening hours.

An uncertainty could be the near front-parallel storm motion, which could cause rapid storm clustering. However, even with this scenario in mind, the severe risk still exists during the initiation phase, where thunderstorms are still more discrete.

... Eastern France, Switzerland, parts of the Alps and rest of Germany ...

During the day, daytime driven thunderstorms occur along the southern Alps and parts of Switzerland. Shear is too weak for anything organized, but a level may be issued due to slow storm motion and therefore an augmented flash flood risk. For now, the risk looks too marginal to go with a level 1 in an extended outlook.

During the evening and night hours, thunderstorms also evolve over E-France, SW/S and central Germany, along the eastward preceding convergence zone and cold front. Instability is on the decrease but the severe risk increases to the north, where shear is the strongest and also over SW-Germany along the cold front during the morning hours. Isolated large hail /severe wind gusts are possible but only central Germany was included into a level 1, where coverage of wind/hail reports could be more widespread.

... UK, Ireland and Scotland ...

Beneath cold mid-levels, daytime driven thunderstorms are forecast. Shear is too marginal for anything severe, but the potential for augmented LL CAPE release has to be monitored, which dictates the tornado / funnel risk.