Storm Forecast
Valid: Thu 20 Oct 2011 06:00 to Fri 21 Oct 2011 06:00 UTC
Issued: Wed 19 Oct 2011 17:34
Forecaster: GATZEN

A level 1 was issued for the north Adriatic Sea mainly for excessive rain and severe wind gusts.

SYNOPSIS

The rather intense European trough will move north-eastwards into western Russia at Thursday, leaving a southern cut-off behind that will be centered over the Alpine region. The polar air over central Europe will spread into the west Mediterranean Sea and the Adriatic. Rich low-level moisture is currently present to the south of the frontal boundary over the Mediterranean, and CAPE will likely develop ahead of the cold front given steep low-level lapse rates due to the warm sea surface. The tongue of rich low-level moisture may also spread north-eastwards into the Ionian Sea. Although isolated tornadoes and strong rain are not ruled out, rather weak vertical wind shear seems to limit the severe potential so that a categorical risk is not issued. Farther north-east, southerly low-level winds over the Adriatic will enable moisture advection across the northern Adriatic Sea, where colder air masses will also lead to increasing lapse rates across the warm sea surface.

DISCUSSION

Northern Adriatic

Ahead of a weak surface low centered over central Italy a south-easterly flow will be associated with moisture pooling over the north Adriatic. Weak WAA and DCVA underneath the right entry region of a mid-level jet streak will be associated with QG forcing, and lift is expected especially near the frontal boundary that will slowly progress southward. Current thinking is that increasing lapse rates will enable CAPE in the order of 500 J/kg, and thunderstorms are expected.

Models indicate that the unstable air mass will overlap with 15 m/s 0-3km bulk shear at the eastern flank of the approaching low. Organized storms are forecast posing a threat of excessive rain and severe wind gusts. Especially in the proximity of the frontal boundary favourably veering profiles are expected what may support supercells. Tornadoes and large hail are not ruled out. Limiting factor will be reduced low-level buoyancy as shown by latest GFS model output as well as the fact that isolated cells are unlikely as thunderstorms are forecast to cluster rapidly.

To the north of the frontal boundary, excessive rain becomes the dominant threat that will be mostly non-thundery due to the weak instability, though. Late in the period, the frontal boundary moves southward and leaves the area of strong vertical wind shear. CAA and strong and dry Bora winds will reduce the instability over the north Adriatic in the evening hours.