Storm Forecast
Valid: Thu 15 Nov 2012 06:00 to Fri 16 Nov 2012 06:00 UTC
Issued: Wed 14 Nov 2012 22:04
Forecaster: GATZEN

A level 1 was issued for western Portugal mainly for severe wind gusts and tornadoes.

SYNOPSIS

To the south of the westerly jet, a ridge has build from the Bay of Biscay to the Ukraine. Southern Europe is affected by three cut-off lows over the Balkans, Tunisia, and to the south-west of the Iberian Peninsula. Weak lapse rates and poor low-level moisture limit convection across northern Europe and the Balkans, respectively. The southern cut-off lows are associated with unstable air masses.

DISCUSSION

South Mediterranean Sea

A plume of well-mixed warm air has been advected into the southern Mediterranean. In the boundary-layer, north-easterly winds lead to moisture advection. Due to some QG forcing and upslope flow along the Tunisian coasts, an MCS has formed. Latest models suggest that it will slowly propagate north-eastward, affecting southern Italy and Sicily. Given rather weak low-level convergence, as well as weakening QG forcing, convection is expected to weaken during the period. Therefore, the threat of excessive precipitation will weaken as well. The lack of vertical wind shear limits organized convection and associated severe weather.

Portugal

An intense cut-off low will move eastward reaching the Iberian Peninsula during the day. A strong mid-level jet streak will spread into Portugal, associated with rather strong QG forcing. At low levels, strong southerly winds exceeding 20 m/s at 850 hPa will evolve over Portugal, while easterly surface winds will be present. Strong low-level vertical wind shear will result along the coasts. Main uncertainty is the amount of instability. Lapse rates are expected to be nearly moist adiabatic. While low-level moisture is high over the sea, drier air from the Iberian Peninsula may spread westward, limiting CAPE. Current thinking is that storms from the Atlantic will weaken onshore, but the coasts will likely get thunderstorms. Given the strong vertical wind shear, organized convection is expected, and mesocyclones are forecast with a low potential of severe wind gusts and tornadoes. The overall threat is expected to be low due to the limited low-level moisture.