Storm Forecast
Valid: Wed 29 Jul 2009 06:00 to Thu 30 Jul 2009 06:00 UTC
Issued: Tue 28 Jul 2009 16:22
Forecaster: TUSCHY

A level 1 was issued for NW-Germany, E-Netherlands and E-Belgium mainly for severe wind gusts.

SYNOPSIS

The progressive weather pattern continues over parts of west/northwest Europe. A strong upper trough crosses UK during the day and enters the North Sea during the evening hours. A surface depression and the attendant cold front serve as foci for unsettled and windy conditions. Hot and stable conditions persist over the Mediterranean with warm and unstable conditions over eastern Europe beneath a weak upper trough.

DISCUSSION

...E/NE-France, Luxembourg, Belgium, the Netherlands and NW/N-Germany, 18 UTC onwards ...

Global models agree in a rapidly eastward translating upper trough, which crosses the area of interest during the night hours from the SW. Placed under the left exit region of a vigorous mid-/upper jet, a deepening surface depression over the southern North Sea will push an active cold front to the eastnortheast, crossing Belgium and Luxembourg between 18 - 00 UTC and NW-Germany until 06 UTC. The position of the depression is well captured in the NCEP-ensemble with slight oscillations regarding the strength still present. Rest of the models are in line with only marginal discrepancies regarding timing of the cold front passage.

Main issue is the strength and quality of the return flow ahead of the cold front. GFS forecasts an increase of surface dewpoints of roughly 10°C in 24h which seems too aggressive given current dewpoint reports further upstream. However, a tightening gradient ahead of the line ought to increase advection to the north, so coupled to ongoing evapotranspiration and moisture pooling along the front itself, at least mid-tens will be reasonable to forecast over NW-Germany, Luxembourg and E-France. Mid-level lapse rates steepen somewhat during the forecast and a small prefrontal tongue of 100-500 J/kg MLCAPE will likely persist during the night ahead of the cold front.

Marginal thermodynamics may be offset by very strong dynamics, as the trough draws near during the night. In addition, an unseasonable strong IPV anomaly accompanies the trough and there are indications that an active dry slot crosses the area of interest during the night and overtakes the surface cold front. This effect could offset the generally expected minimum of convective activity after midnight with those marginal MLCAPE values. In addition to that, GFS also inclines the trough in a highly negative tilt, which causes the cold mid-levels to rapidly spread eastwards atop the still warm and moist surface layer and therefore increases MLCAPE release again after 03Z.

To harmonize all that, we expect the cold front to support isolated thunderstorm initiation over NE-France around sunset with increasing activity over E-Belgium, the eastern Netherlands and NW-Germany thereafter. A wavy, leading convection line is forecast along the cold front, given strong shear (25m/s at 0-6km, 20m/s at 0-3km) with a pronounced severe wind gust risk. This hazard could become more widespread over NW-Germany. An isolated tornado event can't be ruled out despite low SHR1 values but even a slight deviant storm motion could result in significantly better directional shear. The hail risk, although present, plays an subordinate role. The complete area was placed under a low probability thunderstorm area (not yet made public) with higher probabilities, where conditions look the best.

A strengthening jet below 700 hPa causes an enhanced severe wind gust risk over the Netherlands after 03 UTC. The thunderstorm risk looks too marginal right now to expand the level 1 more to the west.

E-France has to be monitored for initiation with better instability but marginal forcing.

... Rest of the general thunderstorm areas ...

An isolated large hail event could occur south of Switzerland but otherwise shear and instability remain too marginal for anything severe.