Forecast Update
Valid: Tue 24 Nov 2009 19:00 to Wed 25 Nov 2009 06:00 UTC
Issued: Tue 24 Nov 2009 19:58
Forecaster: VAN DER VELDE
A level 1 was issued for the UK for a chance of tornadoes and severe gusts. Excessive non-convective rain amounts are possible.
SYNOPSIS
A cold front passes slowly over the British Isles, and strong moisture transport causes jet another rainy episode for northern UK and Scotland. The front shows a double structure in surface convergence fields, with the jet right between them. The unstable maritime cold airmass will touch upon Ireland and western UK in the early Wednesday morning hours. Forced convection may occur along the eastern convergence band.
At 00Z the right entrance to the >70 m/s 300 hPa jet maximum is over the central UK.
DISCUSSION
12Z GFS model has now tipped over to producing convective precipitation and colder EL heights along the cold front during over UK during the late evening and night. The very strong flow creates potential for wind gusts up to about 35 m/s, but the orientation of winds and shear vectors along rather than perpendicular to the front decreases the potential for a convective squall line. A forced line may nevertheless exhibit wave patterns with small bow echoes oriented more favorably. 0-1 km shear over 20 m/s, 0-6 km shear over 35 m/s and SREH3 over 300 mē/sē in interaction with a (forced) updraft can yield mesocyclones which can produce tornadoes.
The period during which GFS suggests some CAPE, as well as positive QG forcing, is limited to around 20-00Z for the western UK. Around 00Z negative QG forcing (descent) is predicted over the region. Near-neutral lapse rates will remain and continuous strong forcing may at times result in slight realease of instability, in the form of convective cells or narrow lines.
Towards the end of the night, western UK and Ireland receive the cold airmass cells, under a very steep pressure gradient creating a severe large scale wind field. This convection should not be linearly organized and provides little enhancement other than turbulence. Deep and low level shear over W UK will be very large, still, but also unidirectional (lack of veering/SREH), but a tornado cannot be ruled out.
