Storm Forecast
Valid: Tue 17 Nov 2009 06:00 to Wed 18 Nov 2009 06:00 UTC
Issued: Tue 17 Nov 2009 02:56
Forecaster: VAN DER VELDE

A level 1 has been issued for parts of Ireland and United Kingdom mainly for a chance of tornadoes.

A level 1 has been issued for Denmark mainly for a chance of tornadoes and later gusts.

SYNOPSIS

High pressure with a large scale anticyclonic flow pattern covers southern Europe, while depressions affect the British Isles and southern Scandinavia with cooler and at times unstable weather with showers and some isolated thunder. A long stationary frontal zone running from Portugal to Poland marks boundary between the warmer Mediterranean airmass and the polar airmass. On the warm side of the front, mid level lapse rates actually steepen dramatically due to warm air advection with the strong low level winds (T850 10°C) and 500 hPa temperatures decreasing to -18°C.


DISCUSSION

...Denmark....

Weakly unstable airmass passes during morning and early afternoon, bordering a jet streak to its south partially in overlap.
Deep layer shear values of 15-35 m/s and low level shear (0-1 km) of 10-13 m/s with some SREH seem in support of an isolated tornado. Supportive forcing should come from the left exit of the jetstreak. At the end of the period, new convective activity, likely linearly organized, will arrive to western Denmark with potential of severe gusts, mainly.

...Ireland, UK...

At about 15Z an Atlantic shortwave trough with unstable air should arrive to this area, backed by a PV anomaly, and a jet (left exit) to its south. Although overlap with CAPE is small, with up to 35 m/s DLS and 15 m/s LLS interfering with any convection a potential for tornadoes exists. Over the North Sea, both convective and non-convective gusts could reach over 25 m/s.

...Portugal, western Spain...

At 06Z GFS places some instability along the frontal region, dissolving after 09Z. DLS, LLS and SREH show elevated values. However it doesn't look like the ideal timing for any convection over the still cool land, the forcing near the stationary front should be weak, EL temperatures are quite warm. Insufficient reason for a level 1.

...France, Switzerland, southern Germany...

If it were summer, absolute moisture content would have been almost twice what is available today (now only 6-7 g/kg in lowest 1000 m) and widespread storms could have developed in a strongly sheared, high wind speed environment. Nevertheless, GFS as well as Meteoblue and LaMMA NMM mesoscale models develop a few hundred J/kg mostly surface-based CAPE and negative LI north of the line Toulouse-Strasbourg. Forcing is likely too weak to overcome any capping and mixing of unstable parcels.