Extended Forecast
Valid: Fri 21 Aug 2009 06:00 to Sat 22 Aug 2009 06:00 UTC
Issued: Wed 19 Aug 2009 21:09
Forecaster: TUSCHY

A level 1 was issued for most parts of Germany, Switzerland, Austria and the Czech Republic mainly for large hail, severe wind gusts and excessive rain amounts.

SYNOPSIS

An eastward moving cold front crosses parts of central Europe with slow forward progression along the Alps. Strong WAA downstream causes hot conditions far north also over Norway and Sweden. Hot and stable conditions persist over the Mediterranean with CAA over eastern Europe.

DISCUSSION

... South / east and northeast Germany, parts of the Czech Republic, Austria and Switzerland ...

Eastward trailing cold front sparks scattered to widespread thunderstorms during the forecast period. The cold front position is captured well by all models, which push the front out of Germany until 06Z with the fastest motion over north/central Germany and a trailing part over southern Germany. Numerous mid-level impulses, embedded in the cyclonic flow cross the area during the forecast from SW but the focus agian is the prefrontal convergence zone and the surface front.

The quality of the prefrontal boundary layer looks promising with dewpoints in the upper tens and even 20°C can't be ruled out. EML atop that airmass fans out but remains still steep enough for 500 - 1000 J/kg prefrontal MLCAPE release over NE/E-Germany and somewhat higher values over S/SE-Germany.

Shear in the lowest 3km but also 6km is moderate with 15m/s and 20m/s respectively, but that's more than adequate for organized thunderstorms along the cold front. Preceding convergence zone could outrun that favorable shear, especially over SE Germany, but better instability may compensate that to some degree.

Thunderstorm initiation again is a bit diffuse that far out with isolated to scattered thunderstorms ongoing at 06Z. Later the day, conditions improve rapidly especially over southwest / central / east and northeast Germany and a mixture of multicells and supercells is forecast with large hail and strong to severe wind gusts. Initiation over SE-Germany may be delayed a bit due to foehn conditions, but thunderstorms also develop during the late afternoon and evening hours with an hail/wind risk. Thunderstorms also affect the Czech Republic and western Poland during the evening and night hours with a gradually diminishing severe risk as instability weakens.

The level 1 also covers most parts of Switzerland and the Alps mainly due to excessive rain. However, large hail and strong wind gusts are forecast dependent on how far south and east the jet expands. Model discrepancies are still present.

Uncertainties remain:
* where is the cold front situated at the start of the forecast. GFS decelerated the progression of the front in the past few runs, but convection from the night before could push the surface front further east than expected.
** coverage of elevated thunderstorms over Sweden. The level 1 may be expanded well to the north due to MUCAPE/shear overlap with an hail risk
*** Parts of the level 1 may see an upgrade, most likely a swath from southwest/central/east and northeast Germany. Uncertaintiy of the final position of the front, degree of insolation and debris of mid-/high cloud layer of the storms the night before preclude that upgrade

... Belgium and the Netherlands (15-21UTC) ...

As cold mid-level airmass overspreads those regions, some low-end instability is forecast. Shear in the 0-6km layer is strong enough for a few organized thunderstorms with an hail and severe wind gust risk. A level 1 may be introduced in the day-1 outlook.

... UK and Ireland ...

Scattered thunderstorms beneath cold mid-levels are expected. Shear is weak, so main attention is the boundary layer and attendant CAPE release. Latest data don't support a more widespread funnel/tornado risk.