Storm Forecast
Valid: Thu 27 Nov 2008 06:00 to Fri 28 Nov 2008 06:00 UTC
Issued: Wed 26 Nov 2008 23:01
Forecaster: GATZEN

SYNOPSIS

Intense cut-off trough placed over south-western Europe merges to another intense long-wave trough that amplifies over north-western Europe. To the south-east of both troughs, strong jet streaks are present. While most of Europe is dominated by rather cold air masses or stable maritime air mass, convectively mixed air mass is expected over north-western Europe. Instability will also develop over west Mediterranean, where focus of convective activity is forecast.

DISCUSSION

West Mediterranean

As strong mid-level jet streak over northern Africa ejects into the Mediterranean, models indicate that low-level warm air mass will spread north-westwards in the lee of the Atlas mountains. This will be associated with a strong return of boundary-layer moisture as indicated by latest model output. Quite intense forcing in the range of deepening surface cyclone will likely be associated with increasing instability during the period. Latest observations indicate more low-level moisture over central Mediterranean that will likely advect westwards on THU.

Most critical ingredient for convection will be instability, and GFS may overestimate CAPE over west Mediterranean Sea. Although low-level moisture will likely exceed 8-10°C, values of 14°C seem to be too optimistic at this time. However, at least weak instability is expected in the range of strong upper lift due to strong ageostrophic low-level flow. Current thinking is that stratiform precipitation will intensify during the period over south-west Mediterranean as embedded convection develops due to increasing instability. Convection will likely be elevated at first given rather dry boundary-layer.

Later in the period, surface cyclone starts to move northwards over west Mediterranean. Along the warm front, intense precipitation with embedded thunderstorms will spread northwards. In the wake of the warm front, relatively weak southerly winds are expected in the range of convectively mixed air mass that will likely be unstable underneath the cyclonic flank of the upper jet streak. Showers and thunderstorms will likely develop given strong DCVA and some warm air advection. Potential for severe convection is highest near the northward moving warm front, where strong low-level vertical wind shear may be favourable for a few rotating updrafts capable of producing tornadoes, hail, and severe wind gusts. Threat of severe convection will decrease to the north due to decreasing low-level moisture and low-level buoyancy. Intense precipitation will be another threat along the warm front.

North-western Europe

Strongly forced convective line is expected to move south-eastwards over British Isles. Weak low-level instability may be present, but warm equilibrium level will likely limit potential for thunderstorms. Locally severe wind gusts and brief tornadoes are not ruled out completely, but deep moist convection is forecast to be unlikely.