Storm Forecast
Valid: Thu 29 Oct 2015 06:00 to Fri 30 Oct 2015 06:00 UTC
Issued: Wed 28 Oct 2015 21:53
Forecaster: GATZEN

A level 1 was issued for the Tyrrhenian Sea, central and southern Italy, the Adriatic, and Ionian Sea mainly for excessive rain and to a lesser extend severe wind gusts. Large hail and tornadoes are not ruled out.

A level 1 was issued for the southern Mediterranean mainly for large hail.

SYNOPSIS

A quasi-stationary ridge axis stretches from Central Europe to south-west Scandinavia and the North Sea. A sharp negatively-tilted Atlantic trough cuts off into the central Mediterranean leaving a lifting short-wave trough that crosses the North Sea and northern Germany until the end of the forecast period. In its wake, a strong south-westerly flow spreads into the British Isles between an amplified Atlantic trough and ridging over south-western Europe. Another ridge weakens over south-eastern Europe. A polar trough remains further north-east.

Main focus of convective initiation is the central Mediterranean where warm and moist air masses are present ahead of the forming cut-off trough. Over western Europe, rich low-level moisture is expected ahead of the Atlantic trough, but lapse rates will be weak at the anticyclonically sheared flank of the jet. Most of northern and eastern Europe are dominated by dry air masses.

DISCUSSION

Tyrrhenian Sea into Italy, Adriatic Sea, Ionian Sea and south Mediterranean

The approaching cut-off low will be associated with QG forcing mostly due to DCVA, spreading across the Tyrrhenian Sea south-eastwards. Further west, cold air advection will lead to QG sinking. Affected air mass is moist with 0-500 m mean layer mixing ratios around 10 g/kg as indicated by latest LIRE Pratica Di Mare ascend. Lapse rates will increase in the following hours, especially at low levels due to stretching, and CAPE evolves offshore of Italy by the morning hours. Weak capping and lift ahead of the cold front, along sea-breeze convergences, and upslope flow across parts of Italy will support early initiation. Storms will spread south-east ahead of the approaching cold front, affecting southern Italy and the Ionian Sea in the evening and night hours. The Adriatic Sea is also affected, but storms may be rather low-topped to the north-east of the passing cut-off.

Vertical wind shear will be strongest ahead of the cold front, where CAPE and shear will also overlap. A few 100s J/kg CAPE overlapping with 20 m/s 0-6 km bulk shear can be expected from the Tyrrhenian Sea (morning) to southern Italy and the Ionian Sea (evening and night). Multicells are forecast, capable of producing excessive rain and severe wind gusts. A few hail events may be possible if rotating updrafts can form. Rather weak low-level vertical wind shear does not support tornadoes, but a few events are not ruled out, e.g. near the coasts where shear profiles may be better locally. Across the Adriatic, excessive rain threat is dominant due to weaker flow and rather persistent storms.

Late in the period, storms can also be expected across the south Mediterranean. An elevated mixed layer will be associated with larger CAPE and vertical wind shear will be also strong with 20-25 m/s in the lowest 6 km. Expect a few supercells to form along and ahead of the approaching cold front, capable of producing large hail. Excessive rain will be another issue that is highest across the Ionian Sea.