Extended Forecast
Valid: Thu 26 Aug 2010 06:00 to Fri 27 Aug 2010 06:00 UTC
Issued: Tue 24 Aug 2010 19:42
Forecaster: TUSCHY

A level 1 was issued for parts of France/Germany, Luxembourg, Belgium and the Netherlands mainly for severe wind gusts, tornadoes and a few large hail events.

SYNOPSIS

Progessive upper-level wave train affects most parts of Europe with a strong wave crossing the English Channel, S-North Sea and Denmark during the forecast. A gradually developing depression at lower levels and its broad warm sector result in a chance for organized convection over parts of western Europe. The hot and stable weather prevails over the Mediterranean with unsettled conditions over E-Europe.

DISCUSSION

... Belgium, the Netherlands, Luxembourg, parts of France and Germany ...

Overall predictability of this event increased in the past, but model discrepancies still exist, especially with ECMWF, which takes this depression into the North Sea a tad later. Nevertheless, the structure of this depression reveals striking similarities, with an SW-NE elongated depression. There remains a chance that this vortex degenerates into an open wave-like structure, but this would have only marginal effects on the final kinematic environment. NCEP ensemble also support that scenario with strong clustered members regarding different geopotential height fields. Favorable phasing between the eastward shifting upper-level short-wave over the English Channel/S-North Sea and a NE-ward surging ridge over the central Mediterranean cause a markedly increase of the isohypse gradient over the area of interest, pushing shear parameters to a nievau for organized deep moist convection.

Future model runs will still cause considerable fluctuations regarding the final wind field strength (especially if weaker/slower ECMWF solution asserts itself), but for now we use GFS/WRF, which did a good performance (assesing the kinematic environment) in past events. 0-6 km bulk shear of 20-30 m/s and 0-3 km shear of 20-25 m/s are already favorable for organized updraft structures. However, the main concern arises when studying the BL wind field, which reveals strong shear of roughly 15 m/s and 200 m^2/s^2 SRH-1, so well organized LL mesocyclones will be possible.

Main uncertainty will be the thermodynamic profile of the lower troposphere for that day. This will be another event, where a well mixed maritime air mass and an EML from Spain start o mix over France/S-Germany. In addition, this depression taps into a pool of partly modified subtropical air over the E-Atlantic, so BL moisture will be more than adequate. A broad warm sector with 300 - roughly 800 J/kg MLCAPE evolves with values decreasing from Benelux eastwards. Also, EL temperature forecasts reveal quite warm cloud top temperatures, which could further limit electrified convection. Hence, a broad level 1 was issued, to cover a severe wind gust risk with downward mixing (20-25 m/s in lowest 1-3 km AGL), an augmented tornado risk and isolated large hail. Later model data will be used to discern potential concentrated swaths with enhanced severe and a possible subsequent upgrade.

Thunderstorms are also forecast over extreme W-Russia, but right now, either shear or CAPE or both parameters won't support organized convection.