Storm Forecast
Valid: Mon 11 Aug 2008 06:00 to Tue 12 Aug 2008 06:00 UTC
Issued: Sun 10 Aug 2008 18:00
Forecaster: TUSCHY
SYNOPSIS
An extensive cyclonic vortex over NW Europe and attendant short-waves dictate the European weather for the next 24 hours. A very strong wind field at all levels goes along with this low pressure and where strong winds and instability overlap, organized thunderstorms will be possible. Of special attention will be an open wave, crossing the N-Bay of Biscay during the afternoon hours, racing eastwards during the night hours as this feature poses a potential severe weather threat. Hot an stable conditions prevail over the Mediterranean and E-Europe.
DISCUSSION
... SE-France, N-Italy, Switzerland, S/SE Germany, parts of the Czech Republic and Poland ...
A quasi-stationary frontal boundary serves as focus for scattered to widespread thunderstorm initiation. In addition, there are various upper lifting mechanisms as short-waves eject out of the base of the main upper trough. The surface boundary, roughly running from SW-France over S-CNTRL France, S-Germany to CNTRL Poland during the start of the period, shifts slowly northward in response to an approaching upper trough. This boundary/warm front separates a pretty dry and well mixed airmass to its north from a more humid one to its south. However, steep mid-level lapse rates should help for a belt of 500-locally more than 1000 J/kg MLCAPE along the boundary but blotchy BL moisture dispersal is reflected in an unhomogeneous instability distribution. Timing of the main jet is late (evening hours), as instability already diminishes, but there is a time-frame between the early afternoon and late evening hours, where modest instability, 15-20m/s DLS and enhanced storm relative helicity overlap so we issued a broad level-1 for parts of all those regions due to an isolated large hail and strong wind gust threat. During the evening hours, LL shear increases significantly over SE-France and S-Germany but at the same time, instability vanishes rapidly due to BL stabilisation. However, GFS takes a strong vorticity maximum over those areas at 18Z and this could balance vanishing instability at least temporarily. We therefore include an isolated tornado risk especially for thunderstorms, which develop along the slowly northward moving frontal boundary/warm front and generally over S-France, where LL shear/instability overlap will be the best.
... Central - East France ...
The attention then turns to a racy eastward moving open wave, which enters France after 21Z from the west and approaches S-Belgium during sunrise. Overall the global models are consistent in timing and placement of this feature and confidence in its development is growing. The strength of this wave will play a major role in the final thunderstorm coverage. Current predictions show a broad developing warm sector and this mirrors in the forecast soundings from this area, featuring a warm mid-level airmass above a cool BL. If this verifies, convection will be mainly elevated but not much modification of the lowermost levels is needed for surface based convection. The airmass ahead and south of this feature need close monitoring and an update may be needed during the late evening/early night hours.
Despite those uncertainties, the stage is set for a few thunderstorms as drier high-tropospheric / lower stratospheric airmass overspreads the moist LL and as mid-levels start to cool down, a plume of steeper mid-level lapse rates develops over central France and a few hundred J/kg CAPE could be released. Wind field is extraordinary strong for this time of year with DLS at or above 30 m/s and winds of 25 m/s at 850hPa. LL shear becomes very intense with 15-20 m/s and SRH-1 values climb well above 200 m^2/s^2. Forecast soundings confirm that with intense speed/directional shear and low LCLs. Any thunderstorm, which can root into the highly sheared boundary layer has a high chance to rotate with a distinct tornado threat. Dependant on the final instability release, even a strong tornado will be possible.
... SW Finland ...
A small window for a few thunderstorms exists between 06Z and 12Z as a cold front encroaches on SW-Finland from the SW. A moist BL and a slight cool-down of the atmosphere results in a concentrated swath of low-end instability release. A weak vort. max. and the NE-ward moving surface front should help to spark a few thunderstorms over the highlighted area. Forecast soundings confirm the chance for deep convection and so do EL temperatures near -40°C. Strong directional shear and low LCLs point to an isolated tornado threat over S-Finland. The risk diminishes inland as instability values ease quite rapidly.
... Sweden and Norway ...
As pool of cold mid-levels overspreads those regions and as diurnal heating starts, up to 600 J/kg MLCAPE will be released. DLS is quite weak, but locally up to 15m/s should be enough for a marginal and isolated large hail risk with strong wind gusts. In addition, LL CAPE values are enhanced over central Norway/Sweden and a short spin-up can't be ruled out. The activity will diminish after sunset.
