Storm Forecast
Valid: Thu 27 Oct 2016 06:00 to Fri 28 Oct 2016 06:00 UTC
Issued: Thu 27 Oct 2016 06:12
Forecaster: GATZEN

A level 2 was issues for the southern Tyrrhenian Sea, southern and central Italy, Sicily, and the Ionian Sea region including western Greece mainly for excessive precipitation and to a lesser extent large hail, severe wind gusts, and tornadoes.

A level 1 was issued for the central and southern Mediterranean mainly for excessive precipitation and large hail.

SYNOPSIS

High geopotential has developed over western Europe, yielding a north-westerly flow over most of the forecast domain with a broad trough over eastern Europe. At its south-western periphery, intense short-wave troughs rapidly move into the central Mediterranean, where a closed mid-level vortex evolves at the end of the forecast period. This is associated with intense deep moist convection through-out the period.

DISCUSSION

Central Mediterranean

Rich low-level moisture is present over the central and south Mediterranean, with dew points between 20 and 24°C. This moisture is capped by an elevated mixed layer indicated by latest Trapani and Decimomannu ascents. A cold front currently reaching northern Algeria, Tunisia and Sicily is therefore not associated with convective storms. In the wake of the cold front, a short-wave trough currently over the Tyrrhenian Sea rapidly digs south. Strong DCVA acting on the air mass will result in rapidly weakening cap, and latest Ajaccio sounding indicates impressive overlap of low-level moisture and steep lapse rates with almost no cap.

Mesoscale convective systems have already formed over central and southern Italy. Currently, these are quite well-organized due to remaining strong vertical wind shear ahead of the approaching trough centre. During the period, vertical wind shear is expected to decrease, whereas strong QG lift will result in almost no cap, allowing for widespread convective storms both along the cold front and further north. Main convective mode will be clusters of storms near the trough centre and training MCSs along the cold front, where vertical wind shear is better.

Main severe weather threat is excessive rain that increases late in the period when the system encounters even better moisture / warm sea surface temperatures across the Ionian Sea. Given high CAPE values up to 2 KJ in the southern part of the convective regime, large hail is also expected. Tornadoes are not ruled out.

Late in the period, ongoing convective storms are forecast de to a second mid-level vort-max ejecting from the European trough. High resolution models indicate the formation of a well-developed MCV and warm-core cyclone over the Ionian Sea late in the period. Severe winds will become more likely especially along the coasts of southern Italy and Sicily. Additionally, the excessive rain threat continues.

Norway

Strong mid-level jet streak / mid-level front will affect Norway late in the period. Steep lapse rates form due to strong stretching and rather warm sea surface. Low-level moisture is very limited, so CAPE will be marginal. However, upslope flow is strong, and showers / thunderstorms are forecast. Severe wind gusts are the main threat, however, contribution by convective storms is expected to be low.