Storm Forecast
Valid: Thu 26 Sep 2013 06:00 to Fri 27 Sep 2013 06:00 UTC
Issued: Wed 25 Sep 2013 22:12
Forecaster: GATZEN

A level 1 was issued for Portugal mainly for excessive rain and tornadoes.

A level 1 was issued for souhern Ukraine mainly for severe wind gusts.

SYNOPSIS

An intense long-wave trough remains across east Europe. A strong mid-level jet is present along its periphery including a strong vort-max travelling into the northern Balkans and western Black Sea during the period. Across western Europe, a ridge is forecast. Iberia is increasingly affected by an Atlantic cut-off low.

DISCUSSION

Portugal

Ahead of the approaching cut-off low, a very moist tropical air mass is advected northward. Although lapse rates are initially poor in this air mass, but will increase due to lift as the mid-level trough moves slowly east. The rather cool mid-level air mass in the range of the trough axis and the warm sea surface may allow for CAPE in the order of 1000 J/kg according to the latest GFS, so that at least weak instability is expected to be likely.

Along the cold front that will be nearly parallel to the mid-level flow, training storms are expected that will spread into Portugal late in the period. These storms may produce excessive rain along the coasts. Additionally, the very strong low-level wind field exceeding 20 ms at 850 hPa will result in strong low-level vertical wind shear. As a consequence, embedded supercells are forecast capable of producing tornadoes and locally large hail and severe wind gusts. Overall activity will likely go on at Friday morning.

Northern Balkans to western Black Sea

Southern Poland, Slovakia, northern Romania, and surroundings

An intense mid-level vort-max will travel eastward reaching the Ukraine on Thursday evening. It will provide strong DCVA. However, the affected air mass is rather dry and cool over most of the area. Main focus will be a moist air mass south of a frontal boundary extending from southern Poland to the Ukraine on Thursday morning. Latest models indicate that weak CAPE is possible as lapse rates increase east of the short-wave trough axis.

Although chances of deep moist convection are limited due to the weak lapse rates and rather poor moisture, storms that may develop will likely organize given very strong vertical wind shear in excess of 30 m/s 0-6 km bulk shear and favourable low-level hodographs. Supercells capable of producing severe wind gusts and tornadoes are not ruled out. However, indications are too weak for a categorical threat level over most of the region at this time. The situation must be monitored closely as better moisture and daytime heating can result in a set-up supportive for organized storms that may merge into bow echo. The best potential exists near the Black Sea, where low-level buoyancy and moisture are much better.

Western Alps into southern Germany

At the western flank of the east European trough, the right entry of a strong jet streak will affect the Alpine region. At lower levels, a frontal boundary will move southward slowly, allowing for rich low-level moisture during the afternoon hours, when daytime heating will also result in rather steep lapse rates. Latest GFS indicates weak CAPE during the afternoon. Along and south of the cold front, storms are forecast to develop that will move eastward. These storms are forecast to become organized given the strong vertical wind shear around 20 m/s in the lowest 6 km. Multicells and supercells are not ruled out, capable of producing large hail and severe wind gusts. A tornado is not ruled out. The overall threat is expected to be limited due to the rather weak moisture and lapse rates and weak low-level buoyancy, so that a level 1 was not issued.