Storm Forecast
Valid: Thu 02 Apr 2009 06:00 to Fri 03 Apr 2009 06:00 UTC
Issued: Wed 01 Apr 2009 14:35
Forecaster: GATZEN

SYNOPSIS

Relatively low geopotential is present over southern Europe as broad upper cut-off low is situated over the south-western Mediterranean. To the north, a ridge extends from the Atlantic across the British Isles, Germany, and the Balkans to the eastern Mediterranean. While cold air masses dominate in the range of the south-western trough as well as the eastern Europe, diabatic heating has warmed the polar air mass in the range of the ridge.

DISCUSSION

Adriatic to northern Balkans

At the eastern periphery of the Mediterranean trough, the axis of a short-wave trough propagates northward over the northern Balkans during the day. It will provide quasi-geostrophic forcing, while mid-level lapse rates will likely decrease. At lower levels, latest observations show rather moist low-level air mass over the Adriatic region. To the north of a frontal boundary, moisture flux convergence and diurnal heating will contribute to instability, and thunderstorms seem likely to form. Slightly veering profiles and some vertical wind shear in the range of a weak mid-level jet streak may assist for some multicells or mesocyclones. One limiting factor is the weak instability during the period and rather weak vertical wind shear, and chance for marginal hail or weak tornadoes seems to be too low for a categorical risk level.

Central to northern Italy

Italy is affected by the eastern flank of the Mediterranean cut-off low. A negatively tilted upper short-wave trough embedded in the southern flow will move northward and is forecast to cross the western Alpine region late in the period. In the range of this trough, differential cyclonic vorticity advection will likely contribute to lift and mid-level lapse rates are forecast to improve significantly. Additionally, moderately strong easterly winds at mid-levels will lead to unidirectional deep layer vertical wind shear of around 20 m/s in the lowest 6 km.

At low levels, a cold front that is orientated parallel to the flow will remain quasi-stationary over northern central Italy. To the north, latest observations indicate rather rich boundary-layer moisture that will likely spread further west during the period. Current thinking is that relatively rich boundary-layer moisture and steepening low-level lapse rates will be associated with increasing low-level instability during the day. Locally upslope flow and quasi-geostrophic lift are likely and showers are forecast. Deep convection with thunderstorms may form especially near the cold front over northern central Italy, where colder mid-levels are forecast in the range of the short-wave trough axis.

Multicells may form due to unidirectional shear profiles. Limiting factor is widespread stratiform precipitation ahead of the cold front predicted by latest models, but given rather warm low-level air mass, shallow instability seems to be likely at least. Severe potential is low given unfavourable veering profiles. Local flash flooding cannot be ruled out. Convection is expected to spread into western Italy and will likely decease during the night.