Storm Forecast
Valid: Thu 15 Apr 2010 06:00 to Fri 16 Apr 2010 06:00 UTC
Issued: Wed 14 Apr 2010 21:22
Forecaster: GATZEN

A level X was issued for (area name) mainly for (severe weather type(s)). (repeat for each area)

SYNOPSIS

Weak geopotential stretches from the Atlantic Ocean west of Iberia to the Bay of Biscay and further into eastern Europe. Cut-off lows embedded in this trough are present to the west of the Iberian Peninsula, across France, over Hungary, and south-western Russia. Strong mid-level jet streaks are present to the south of the trough centres and will spread into the Iberian Peninsula and the central Balkans during the period. Rather strong mid-level winds are also expected across the Mediterranean. At lower levels, the boundary-layer moisture has recovered especially to the east of the Alps ahead of the low pressure centred over the Czech Republic, while the polar air is rather dry over most places. Rich low-level moisture is expected to spread into the southern Iberian Peninsula with southerly winds.

DISCUSSION

Southern Portugal and south-western Spain

The trough centred to the west of Iberia is not expected to move significantly eastward. A mid-level jet streak is forecast to travel around the trough and enters the Iberian Peninsula during the period. Strong DCVA may remain off-shore for most of the day, while the transport of moist and rather warm air will go on during the period to the east of a cold front crossing southern Portugal in the morning hours. Latest soundings indicate that this air mass is dominated by neutral and moist profiles and only weak instability is present. During the period, daytime heating is expected to create CAPE in the range of 500 J/kg over the Iberian Peninsula. Initiation seems to be quite likely in the range of the cold front that leads to significant low-level convergence and lift as well as later in the period in the range of the following trough. Storms that manage to develop will profit from 15 m/s deep layer vertical wind shear, and multicells are forecast to develop. These storms may produce isolated large hail. The chance of excessive rain is rather low given the high propagation speed of the storms. In the evening hours, increasing low-level vertical wind shear and low-level helicity may be supportive for rotating updrafts, but as the boundary-layer cools rapidly, tornadoes are not expected.

Central France, northern Hungary to south-eastern Poland, and south-western Russia region

In the range of the cut-off lows, diurnal heating will likely lead to steep low-level lapse rates and increasing instability due to the cool mid-level air below the 500 hPa level. Given rather moist low-level air, rather strong buoyancy may build at low-levels, and some strong updrafts may develop during the day. Although vertical wind shear is expected to be weak over most portions, the stronger cells may be capable of producing isolated (marginally) large hail. A level 1 seems to be not warranted at this time. Storms will quickly decay after sunset.

Northern Tunisia, Sicily, southern Italy, central Balkans to Bulgaria

At the surface, a convergence zone is present from northern Africa to the central Balkans. To the south of this zone, more southerly to easterly low-level winds are present. The low-level air mass is rather cool but the moisture content is not too bad. Above the boundary-layer, a south-westerly flow is present that advects rather warm air to the north-east, and widespread lift is expected especially from southern Italy to the central Balkans. Main question is about the development of instability. Strong diurnal heating may be needed and is most likely over northern Tunisia and Sicily. Over the Balkans, more stratiform precipitation and clouds may limit the potential of instability significantly. Despite this negative factor, the weak low-level convergence is also a limiting factor over most places. Best chance seems to exist over northern Tunisia in the evening hours, when a weak cold front will reach the coasts. Strong deep layer vertical wind shear is present and will favor multicells and supercells. Locally large hail is not ruled out. Especially over the Balkans, strong precipitation is likely, but this is expected to be of stratiform nature, mostly.