Extended Forecast
Valid: Tue 12 Aug 2008 06:00 to Wed 13 Aug 2008 06:00 UTC
Issued: Sun 10 Aug 2008 21:36
Forecaster: TUSCHY
SYNOPSIS
Not much changes ahead as major upper trough is still in place over NW-Europe. Numerous disturbances rotate around this trough, resulting in unsettled conditions over most parts of central and N-Europe. Still hot and stable conditions over the Mediterranean and SE/E-Europe.
DISCUSSION
... N - Germany ...
As discussed in the day-1-outlook, the main feature of interest is an open wave, which crosses N-Germany during the early morning hours ( NW-Germany) and early afternoon hours ( NE-Germany) from the WSW. The main difference compared to the activity over France the night before will be better timing, as diabatic heating will help to diminish the effect of the inhibiting warm mid-level airmass and surface based convection becomes increasingly likely already during the morning hours. The area of concern will be the wide warm sector, characterized by a warm and humid airmass, where surface winds will be mainly from the S/SSW. The main concern right now is the astonishing intense wind field for this time of year as a 45m/s speed max. at 500hPa goes along with this wave, providing divergence in its left exit region, also under the favorable area of a 60m/s streak at 300hPa. This wind field creates strong directional and speed shear with high helicity values in the inflow layer of surface based storms as LL shear will be in the order of 12-15m/s and SRH-1 values above 200 m^2/s^2, slowly decreasing over NE-Germany betimes. Exact strength of this wave is still unclear and hence the final strength of the shear but there are indications for thunderstorms to become supercellular with a distinct tornado risk over N-Germany during the forecast period. Later outlooks will evaluate the tornado risk with new data.
... N-Poland, Lithuania, Latvia and Estonia ...
An exact forecast for those areas becomes increasingly difficult as the final placement and strength of the severe weather risk depends on smaller-scale waves, which vary a lot in strength. During the past few model runs of GFS, there was a corridor, running from NW-Poland over the SE Baltic Sea and Lithuania, Latvia to Estonia, where LL shear and SRH-1 values were augmented. The final strength of the instability axis is more uncertain and hence no level area was yet issued. If both-shear and instability-overlap, there will be a risk of tornadoes and strong wind gusts.
... United-Kingdom ...
A maritime airmass overspreads the area and as diabatic heating commences, a few hundred J/kg instability will be released. Locally good LL CAPE and somewhat enhanced LL shear over S-UK indicate a risk of an isolated tornado. The risk is too low for a level-1 right now.
.. S-France ...
A tongue of better LL moisture advects northward over extreme S-France, resulting in a concentrated area with some low-end instability release. DLS in the order of 25-30 m/s is more than adequate for storm organisation and an isolated large hail event can't be ruled out. An upgrade may be issued later-on, if more instability can be released than currently anticipated.
