Storm Forecast
Valid: Thu 12 Sep 2013 06:00 to Fri 13 Sep 2013 06:00 UTC
Issued: Wed 11 Sep 2013 21:45
Forecaster: GATZEN

A level 2 was issued for central Serbia mainly for large hail, severe wind gusts, and tornadoes.

A level 1 was issued for the central Balkans and western Romania mainly for large hail, severe wind gusts, tornadoes, and excessive precipitation.

A level 1 was issued for the western Ukraine and Romania mainly for large hail.

A level 1 was isued for the Adriatic region mainly for large hail, axcessive precipitation, severe wind gusts, and tornadoes.

A level 1 was issued for the north African coasts and the south Mediterranean mainly for large hail and severe wind gusts.

SYNOPSIS

Main synoptic-scale feature is a broad cut-off low slowly digging south-east from central Germany to northern Austria. Two main vort-maxima will travel with this trough. The first is associated with a rather intense short-wave trough that moves across the central Mediterranean and Adriatic before it becomes negatively tilted and moves on into the Balkans and Poland. The second is connected to the cut-off center and slowly spreads across southern Germany and the Alps into the Adriatic. Cold air advection will set in at the western and southern flank of the cut-off, spreading across most of the western and central Mediterranean. Further east, warmer air masses with better moisture will remain from the southern Adriatic into the central Balkans and Black Sea region. This moisture may overlap with increasing lapse rates ahead of the trough axes. Over northern Europe, ridging is expected ahead of a new Atlantic trough.

DISCUSSION

Central Balkans

Together with a 25 m/s mid-level jet streak, a negatively tilted short-wave trough will cross the Balkan region on Thursday afternoon. A strong south-westerly flow will evolve ahead of this trough axis leading to strong vertical wind shear. Lapse rates will increase given strong DCVA and even some low-level WAA in the noon hours. Boundary-layer moisture is also quite good as indicated by latest Beograd and Szeged soundings with 10 g/kg 0-1 km mixing ratio. The overlap of lapse rates, moisture, and favourably veering profiles (0-3 km SRH 150 m²/s²) will be supportive for organized convection.

Main uncertainty is the low-level diabatic heating during the morning and noon hours prior to the passage of the trough axis. Numerous thunderstorms forming the night before across the Adriatic will leave clouds that inhibit insolation. Therefore, expect a rather cool and moist boundary-layer with northerly winds across the northern Balkans and a better mixed, south-westerly flow in the southern Balkans. Latest GFS also indicates a frontal boundary between both air masses across the central Balkans.

Given the low-level convergence together with increasing lapse rates and reduced CIN ahead of the approaching trough axis, thunderstorms are expected in the moist air mass. Although they tend to be elevated, some of the storms will likely root into the cooler boundary-layer. These storms will have a rather good potential to become severe and tornadic supercells accompanied by large hail and severe wind gusts are possible from Bosnia-Herzegovina and Montenegro into central Serbia and western Romania. Given the uncertainty o low-level warming that influences the coverage of severe storms, a level 1 threat was issued except for the frontal boundary where storms are quite likely. With better daytime heating across the central Balkans, a level 2 upgrade for a broader region will be needed. Storms may merge and upscale growth into an MCS, possibly with a more pronounced wind threat, is possible in the evening and night hours, before the convection becomes elevated and spreads into the western Ukraine and eastern Poland, where heavy precipitation is possible.

Ahead of the maim activity, additional storms are forecast to develop especially across Romania and the Ukraine. Main reason is a frontal boundary with warm air advection and low-level convergence ahead of the approaching trough. Given weaker moisture and especially due to weaker vertical wind shear, severe potential is expected to be lower compared to the central Balkans. However, large hail seems to be likely enough to issue a level 1 threat as well. Additionally, excessive rain and a tornado are not ruled out.

Italy, Adriatic, and surroundings

Together with the axis of the short-wave trough, a cold front will cross the area from the west. Drier and colder low-level air will therefore enter this region during the period. Ahead of the cold front, better low-level moisture will overlap with increasing lapse rates and weak CAPE is expected. Especially along the cold front that provides low-level lift, thunderstorms are forecast. With moderate vertical wind shear underneath the strong mid-level jet streak, some organized storms are expected, capable of producing large hail, severe wind gusts, and tornadoes. The rather weak low-level moisture indicated by latest soundings is the limiting factor and only a level 1 is issued. Storms will cease in the wake of the cold front across most of Italy, but remaining moisture across the Adriatic will allow for ongoing activity until the late night hours, when a second vort-max will enter the region from the north. Therefore, locally excessive precipitation is also not ruled out especially along the Balkans, where upslope flow will support intense convection.

North African coasts to southern Italy

Along the cold front that moves south, some storms will develop given plenty of low-level moisture and rather steep lapse rates. Convective activity will decrease during the period as cold air advection and DAVA becomes dominant. Given the strong deep layer vertical wind shear in the vicinity of a mid-level jet streak that spreads eastward during the day, some storms will be capable of producing large hail and severe wind gusts.