Storm Forecast
Valid: Thu 13 Oct 2011 06:00 to Fri 14 Oct 2011 06:00 UTC
Issued: Wed 12 Oct 2011 19:29
Forecaster: GATZEN

A level 1 was issued for the Adriatic Sea, Italy, and the Tyrrhenian Sea mainly for excessive rain.

A level 1 was issued for southern Bosnia, southern Serbia, Montenegro, Albania, Macedonia, southern Bulgaria, northern Greece, and northern Aegean mainly for excessive precipitation.

SYNOPSIS

A long-wave trough affects eastern Europe and will expand into the northern Black Sea. To the west, a high over Iberia ridges into the North Sea region. This situation is associated with cold air advection over most of southern and central Europe. Dry low-level air masses will spread into most of the forecast area. Rich boundary-layer moisture remains in the range of a weak surface low near Italy and associated weak southerly winds.

DISCUSSION

Adriatic, Italy, Tyrrhenian

At the south-western periphery of the east European trough, a trough axis extends across the Alps onto southern France. A mid-level jet maximum will be located downstream over the Balkans and will move south-eastward at Thursday. The right entry region of this jet maximum will affect Italy and surroundings and some QG forcing is likely.

At 850 hPa, models indicate increasing cold air advection across the Dinaric Alps into the Adriatic, and temperatures below 10°C are forecast. However, southerly low-level winds will keep the boundary-layer moisture in place, so that CAPE will likely develop. Given lift near the low pressure axis, showers and thunderstorms are quite likely.

Storm that form will be influenced by rather strong vertical wind shear that exceeds 15 m/s in the lowest 3 km in the morning hours. Favourably veering profiles near the Dinaric Alps due to southerly low-level winds and increasing westerly winds at higher levels may support updraft rotation, and mesocyclones are not ruled out. Locally large hail and excessive precipitation is forecast with the stronger storms. Tornadoes are not ruled out especially along the Dinaric coasts where low-level vertical wind shear may be favourable. As the mid-level jet streak spreads south-eastward during the period, weakening of dynamics and vertical wind shear is expected to lead to a decreasing severe threat.

Farther west, additional thunderstorms are forecast over Italy and the Tyrrhenian Sea due to moist low-levels, lift near the low pressure centre, and neutral/steep lapse rates at the trough axis of the mid-level trough. Weak surface winds and about 10 to 15 m/s westerly winds at 700 hPa result in 15 m/s 0-3km vertical wind shear, supporting storm organization. Rich low-level moisture indicates a threat of excessive precipitation. Additionally, locally large hail is not ruled out. Tornadoes are also expected with the greatest probability near the surface low pressure centre east of Sardinia, where latest GFS indicates 0-1km vertical wind shear of 10 m/s.

Southern Bosnia into southern Bulgaria and northern Aegean

The cold front of the east European trough slows down to the south of a building high over eastern Europe. Along the boundary, lift is expected through-out the period and will slowly spread southwards in the evening and night hours. The affected air mass is characterized by rich low-level moisture and rather steep lapse rates that will allow for some CAPE in response to diurnal heating.

While weak low-level winds are forecast, rather strong (15 m/s) westerly winds at 700 hPa will result in 15 m/s 0-3km vertical wind shear that assist for storm organization. Locally large hail is forecast with the stronger storms. Given that the movement vector of the storms has a small angle with the cold front, excessive precipitation is not ruled out.