Storm Forecast
Valid: Thu 19 Apr 2012 06:00 to Fri 20 Apr 2012 06:00 UTC
Issued: Wed 18 Apr 2012 22:39
Forecaster: GATZEN

A level 1 was issued for northern Ukraine to western Russia and eastern Belarus mainly for severe wind gusts.

A level 1 was issued for north-eastern Spain and west Mediterranean mainly for large hail.

A level 1 was issued for the southern Adriatic region mainly for large hail and severe wind gusts.

SYNOPSIS

Low geopotential has spread across most of Europe. Cold mid-level air masses are present to the north of a strong jet stream from Iberia to northern Africa, southern Italy, and Turkey. At the eastern flank of the low, an intense negatively tilted short-wave trough will move northward across the Ukraine.

DISCUSSION

Northern Ukraine into western Russia and Belarus

The intense short-wave trough moves northward at Thursday and leads to strong warm air advection from the south-east. 850 hPa temperature will likely rise to more than 10°C over the south-eastern Ukraine. Moderate lapse rates are likely overlapping with a tongue of rich boundary layer moisture that advects westward ahead of a cold front. Mixing ratios of 8 /kg are expected by latest GFS model run.

Given the strong synoptic forcing and diurnal heating at low-levels, CAPE will likely build during the day ahead of the approaching cold front. Strong low-level convergence is expected and thunderstorms are forecast along the conf front, spreading northward. These storms will likely organize along the cold front forming a squall-line. With about 20 m/s south-easterly winds at 700 hPa, severe wind gusts seem to be well possible along the leading gust front, especially with bowing segments. These are not ruled out given the strong vertical wind shear of 25 m/s in the lowest 6 km. Additionally strong low-level vertical is expected ahead of the cold front. A tornado cannot be ruled out. Limiting factor will be limited low-level instability further north, and chance of severe storms will likely be limited to the cold front itself. Late in the period, nocturnal stabilization of the boundary will lead to an increasing low-level jet ahead of the cold front. However, given the rather poor lapse rates and moisture, storms are expected to weaken gradually.

Iberian Peninsula to west Mediterranean

In the wake of the European trough, a strong north-westerly jet extends across south-western Europe. Jet streaks embedded in the flow will cause QG forcing. Although the lapse rates and low-level moisture are rather poor, diurnal heating will likely be strong enough to build some weak CAPE as expected by latest GFS. Some showers and thunderstorms may develop. Given the strong deep layer vertical wind shear, large hail is not ruled out with these storms. The storms will likely weaken from the west given decreasing moisture.

Southern Adriatic Sea region

A strong west-southwesterly flow affects the central Mediterranean. A jet streak curving around the base of the European trough will cause QG forcing spreading into the Balkans late in the period together with warm air advection from the south-west. At low levels, a southerly flow is indicated by latest models that will be associated with increasing boundary-layer moisture over the southern Adriatic Sea region.

Current thinking is that convection is likely along the mountains east of the Adriatic Sea. Upslope flow will likely enable deep moist convection that will be possible from the afternoon into the night hours. Deep layer vertical wind shear of 20 m/s will be adequate for organized storms, and locally severe weather is forecast. Large hail and severe wind gusts seem to be the main threat. Excessive precipitation is also not ruled out.