Outlook for Sat...
Storm Forecast
Valid: Sat 14 Nov 2009 06:00 to Sun 15 Nov 2009 06:00 UTC
Issued: Sat 14 Nov 2009 00:06
Forecaster: TUSCHY
A level 2 was issued for parts of the English Channel and the coastal areas of S-UK mainly for severe wind gusts and to a lesser extent for tornadoes.
A level 1 was issued for most parts of the UK, surrounding the level 2 mainly for severe wind gusts and to a lesser extent for tornadoes.
SYNOPSIS
Extensive cyclonic vortex over far NW-Europe steers numerous disturbances around its fringes, which affect western and N-Europe during the forecast. Ridging over the Mediterranean suppresses convection, whereas cooler/stable conditions persist over E-Europe.
DISCUSSION
... S-UK and the English Channel ...
The main story of the day will be the passage of the upper trough axis with the attendant intense depression/pressure gradient over S-UK/the English Channel.
The wind forecast is straightforward as 35-40m/s at 850hPa will mix down easily towards the surface, so damaging and potential life-threatening wind gusts affect the southern part of the UK until 18 UTC, exiting the area thereafter towards the east. Some weakening of the wind field is forecast over far SE-UK, but kinematics are still adequate for isolated extreme wind gust events.
The tornado risk is already more uncertain but most likely maximized along the coastal areas of S/SE-UK, where some onshore moisture causes marginal CAPE. Strong tornadoes will be possible, if more persistent updrafts can evolve. However this brings us to the main uncertainty: convection.
The region will be placed beneath the weakening sting jet, where intrusion of dry, high-level air and constantly evaporatively cooled airmass inhibits deep convection. This is also seen in warm EL temperature forecasts and slim convective precipitation signals. However, dry slotted areas could also see some insolation and not much BL modification is needed along the coast for deeper convective updrafts. A strong vorticity love also crosses S-UK during the daytime hours, so despite slim signals, at least supportive conditions for deep convection can't be ruled out along the coast and SE-UK.
Probably the most likely scenario will be a rapidly E/NE-ward racing forced line of convection over S-UK. Conditions onshore become worse, so the strongest activity remains confined along the south coast (probably enhanced by diurnal heating) and the level 2 was expanded inland, where the tornado risk is enhanced next to the damaging wind gust threat. As convection is forecast to play at least a partial role in this severe wind event, a level 2 became necessary. The overall risk diminishes from west to east during the day. The level-2 was expanded well towards the west, as latest data indicates a slow-down of the eastward progression of the strongest winds.