Storm Forecast
Valid: Wed 25 Nov 2009 06:00 to Thu 26 Nov 2009 06:00 UTC
Issued: Tue 24 Nov 2009 23:00
Forecaster: PUCIK

A level 1 was issued for England, Northwestern France, BENELUX, N Germany, Denmark, SW Sweden mainly for tornadoes and severe wind gusts.

A level 1 was issued for Ireland mainly for tornadoes and to the lesser extent, marginally severe wind gusts.

SYNOPSIS

The major feature on the weather maps is a large cyclone over the Eastern Atlantic, which is expected to reside over the area also during Wednesday. On the forward flank of the cyclone, a southerly strong mid and upper level jet is observed with windspeeds over 50 m/s from 500 to 300 hPa layers. Jet will gradually weaken during the day as the cyclone slowly spreads eastwards. Ahead of the cyclone, a ridge will retreat to the east with its axis stretching from the Mediterranean to Central Europe and Scandinavia around 12 UTC. At the surface, center of the cyclone will cross Northern Scotland with pressure under 960 hPa and then turn northeastwards. At 06 UTC, surface cold front will be passing the British Isles and during the day we expect that it will have passed BENELUX and much of France and Germany.

DISCUSSION

...England, Northwestern France, BENELUX, N Germany, Denmark, SW Sweden...

Strong cold front will make its way across the region. Ahead of the front, very strong flow will be present, veering from southerly to south-southwesterly direction with height. At 925 hPa windspeeds above 20 m/s are anticipated and at 500 hPa, values will vary between 40-50 m/s. Models are predicting a belt of enhanced SREH values both in 0-1 and 0-3 km layer. Due to the above mentioned facts, favorable kinematic setup is in store with wind shear values above 40 m/s in 0-6 km layer and from 10-15 m/s in 0-1 km layer.

Strong mesoscale ascent is anticipated in the frontal region and high PV values are advected at upper levels behind the front. One of the detrimental factors to the situation will be a lack of well-developed latent instability. Both GFS and ECMWF predict very marginal values of CAPE, around 200 J/kg in the vicinity of the front. But frontal ascent can at least a bit compensate for the instability. There is a slight chance that with strongly forced convection, meso or misocyclonic circulations manage to develop with attendant threats of severe wind gusts and tornadoes. Due to these threats, Level 1 is issued.

... Ireland, Northern Ireland, Wales, Western England...

In the post-frontal environment, cold mid-levels combined with relatively warm SSTs lead to the marginal destabilisation of the maritime polar airmass. CAPE values around 500 J/kg are expected, mostly over the sea, but models simulate certain values of CAPE also inland. Due to the strong flow at lower levels of troposphere, strong windshear at 0-1 km is forecast, with values up to 15 m/s. Enhanced SREH is also simulated, due to the ageostrophic flow component due to the friction. Slight chance of tornadoes will exist as well as marginally severe wind gusts with stronger convection.