Storm Forecast
Valid: Sun 11 Dec 2011 06:00 to Mon 12 Dec 2011 06:00 UTC
Issued: Sat 10 Dec 2011 20:44
Forecaster: TUSCHY

SYNOPSIS and DISCUSSION

No major shifts in the weather pattern are forecast compared to the past 24 hours. Another sharp upper trough and attendant thermal trough move in from the NW and affect Ireland, Scotland, UK and parts of NW France during the forecast. Latest IR images already indicate quite active cold sector convection with sporadic lightning detected. This activity will increase as cold mid-level air moves atop 11°C SSTs. Strongest convection is forecast to evolve beneath the thermal trough axis where shear relaxes, so short-lived storms with sleet/marginal hail will be the main hazard. Nevertheless, a few severe wind gusts may accompany stronger convection over N-Ireland and Scotland, as 850 hPa winds exceed 20 m/s.

Another area with DMC is forecast over the W/CNTRL Mediterranean. A flat upper trough moves in from the west with 500 hPa temperatures dropping to less than -20 °C atop SSTs of ~ 20°C. Focus for initiation will be a broad confluence zone, which splits up the W-Mediterranean from SW to NE. This assists in some moisture convergence within the highlighted area and repeatedly shower/thunderstorm development is forecast.
Of interest will be an area east of Corsica at 21 Z onwards. The upper level trough axis approaches from the west and places that area beneath an increasingly diffluent streamline pattern. At the same time, models like GFS/EZ agree in the development of a stronger W-E aligned convergence zone east of Corsica with modest (~15 m/s) boundary-parallel LL winds. This may result in a few slow moving showers/thunderstorms with an heavy rainfall risk along the westward facing coast of CNTRL-Italy. There are no hints yet that rainfall amounts, matching our level 1, may occur but this area has to be monitored. Otherwise, shear remains too weak for an organized severe risk.