Storm Forecast
Valid: Sun 23 Nov 2008 06:00 to Mon 24 Nov 2008 06:00 UTC
Issued: Sat 22 Nov 2008 23:22
Forecaster: SCHLENCZEK
SYNOPSIS
At the southern tip of a very intense 90 m/s jet streak that points towards Ireland, cyclogenesis is expected, leading to a 985 hPa surface low over the southern North Sea. The cold front / upper trough of this feature may allow convection over the British Isles and northwestern France. There is a high risk of widespread severe / isolated extreme non-convective gusts over northern UK in the second half of the forecast period.
The weather in eastern-central Europe is dominated by an intense 960 hPa cyclone. In the warm sector over parts of Russia, relatively warm and moist air is advected towards the Baltic States. This feature is surrounded by a 90 m/s jet streak at 300 hPa. The associated upper trough leads to unsettled conditions over the eastern Mediterranean.
DISCUSSION
...Ireland, Wales, SW-central England...
Some CAPE will likely be produced near the cold front over southern UK and the Channel region where rich BL moisture is present and LCL heights stay relatively low. An approaching upper level vort max should provide sufficient forcing for convective initiation. Good kinematic conditions with about 15 m/s low level shear, 600 J/kg SRH1 and 25 m/s background flow at 850 hPa will allow one or two convective lines with embedded mesocyclones, capable of producing severe gusts and possibly a few tornadoes. The cold front will cross the region in the morning / early afternoon.
Later in the period cold upper air overspreads the area, leading to a few hundred J/kg of CAPE. Showers and thunderstorms will form in a weakly-sheared environment with potential for organized severe weather being negligible. In the late afternoon / evening, an intense upper shortwave trough associated with the northwestern edge of the surface low will cross the region with sufficient QG forcing for ascent. A very strong gradient flow with wind speeds near 35 m/s at 850hPa will establish and a narrow convective line will likely produce severe gusts. SRH1 in order of 400 J/kg and 15 - 20 m/s low level shear may allow embedded mesovortices and a few tornadoes are not ruled out. Peak activity is expected between Sunday 15 UTC and Monday 03 UTC.
...Adriatic Sea...
In the northern part of the Adriatic Sea, very cold upper air and relatively warm SSTs will lead to low-end instability. Deep layer shear in order of 40 m/s should aid storms to organize into multicells, some of them may develop shallow mesocyclones. An isolated waterspout is not ruled out but overall threat will stay below the level-one threshold.
...Eastern Mediterranean, SW Turkey...
Ahead of the large upper trough over eastern Europe, about 500 J/kg MLCAPE are present in a region with 10 - 20 m/s deep layer shear with most of the shear in the 0-3 km layer. Some weak QG forcing is in place and about 200 J/kg SRH3 should allow some mesocyclones. Storms will likely produce small hail and isolated severe gusts.
...W Russia...
Latest GFS outputs show some signals for instability near the cold front of the large cyclone. Impressive kinematic conditions will persist in that region, i.e. more than 50 m/s deep layer shear, 15 - 20 m/s low level shear, strong QG forcing and about 400 J/kg SRH3. Multicells will likely organize into one or two lines that may produce quite a large number of severe gusts and perhaps an isolated tornado. The time-frame should be limited to Sunday 06 UTC - 12 UTC as instability and LL wind will weaken during the afternoon.
