Storm Forecast
Valid: Fri 05 Sep 2008 06:00 to Sat 06 Sep 2008 06:00 UTC
Issued: Thu 04 Sep 2008 18:13
Forecaster: TUSCHY

SYNOPSIS

Majority of models agrees in track and strength of a powerful depression SW of UK, moving NE-wards during the day, while crossing UK from the SW. The depression will fill during the day, but still remains a strong feature, affecting NW-Europe. Ridging keeps most places of S/SE and E-Europe warm/hot and stable.

DISCUSSION

... SE/E-France and SW-Germany ...

Complex pattern of surface fronts, as warm-front lifts northward over E-France during the morning/midday hours and cold front approaches from the west during the afternoon hours. Behind this front, a very warm/humid BL airmass spreads to the north, covering those areas during the afternoon hours. As mid-level lapse rates exceed 7°C/km and at least marginal diabatic heating pushes surface temperatures to at or above 25°C, pockets of moderate instability release will likely develop with readings of 500 J/kg. For sure, thermodynamics will be the main concern during this set-up, as shear will be very strong (DLS 25m/s, LL 15-20m/s and 150-250 m^2/s^2 SRH-1) and LCLs won't be too high (around 1km during the midday hours but decreasing afterwards).

Despite those favorable parameters, there remains a few uncertainties left. Numerous UVV maxima cross the area during the day, resulting in a potential prolonged period of initiation, so a thick cloud shield and stable BL due to outflow air could keep instability release even less than currently anticipated. Even with limited activity during the day, a slowly eastward moving cold front, serving as another focus for initiation during the afternoon and evening hours, should be more or less a solid line of potential trailing storms due to a front-parallel background flow, so discrete thunderstorm development is questionable. Nevertheless, a level-1 will be introduced and an upgrade for parts of SE/E-France may become necessary, if either instability or storm mode will be more supportive for an enhanced tornado threat. Otherwise, large hail and severe wind gusts will accompany stronger storms.

The wind gust risk could become more widespread over extreme E-France / W-Germany, if storms line-up, as wind field at lower levels remains strong. Rapidly decreasing instability and aforementioned orientation of the cold front preclude higher probabilities for now.

... W/NW-France and S-UK....

Intense surface depression just SW of UK slowly migrates NE-wards. Up to 500 J/kg SBCAPE release in the highlighted area are likely and some shear at all levels, although mostly displaced from best instability fields, locally overlaps with some low-end CAPE and as LCLs stay low, an isolated tornado report will be possible. Marginal hail and strong wind gusts accompany stronger storms.

.. NE-Poland and W-Belarus...

Locally more than 1000 J/kg MLCAPE, but weak shear overlap and an isolated large hail risk will be possible and hence a level-1 was included.