Storm Forecast
Valid: Tue 13 Jan 2009 06:00 to Wed 14 Jan 2009 06:00 UTC
Issued: Tue 13 Jan 2009 01:36
Forecaster: VAN DER VELDE

SYNOPSIS

After a blocking high pressure area that caused freezing conditions over large parts of Europe, southwestern flow has returned.
Active convective weather is found around the low pressure system between Tunesia and the Balkan. A plume of warm air surges into southern Italy and beyond, with 0-1 km theta-w values of 15°C decreasing to 11°C over the forecast period. Moderate vertical shear and instability are available.
Showers (too marginally electrified for a thunder area) are forecast for the west side of the UK and Ireland, and the west coast of Norway (Bergen).

DISCUSSION

...Sicily, Malta, southern Italy, western Greece...

At the start of the forecast period, convection should be ongoing between Tunesia and Sicily according to GFS. This is when the highest CAPE is present of about 1000 J/kg and capping is forecast to be weakest. Deep layer shear of 20-25 m/s is forecast over 0-6 km and 1-8 km, sufficient to sustain long-lived MCS and development of supercells. 0-3 km SREH is enhanced to >200 m²/s² and a factor of two higher at the warm front itself, where however predicted CAPE is lower or absent.

The main threat of the storms is large hail. Secondary is the possibility of isolated tornado occurrence, given 0-1 km shear is at least 10-15 m/s, relatively low LCL heights, and weak downdraft CAPE. The highest potential is over Sicily and southern Italy. Waterspouts are possible also in weaker shear.

The moderate deep layer shear, however, will retreat to the east (Ionian Sea) during the afternoon. A convective line could form at the predicted convergence line (GFS and LAMMA-WRF) over the Ionean Sea moving to Greece during the night, with some marginal chance of an isolated severe gust due to >20 m/s average winds in the lowest 3 km and 15 m/s 0-3 km shear component normal to theta-e contours.