Storm Forecast
Valid: Thu 08 May 2014 06:00 to Fri 09 May 2014 06:00 UTC
Issued: Wed 07 May 2014 23:21
Forecaster: GATZEN

A level 1 was issued for the southern British Isles mainly for tornadoes.

A level 1 was issued for the southern Balkans mainly for excessive precipitation.

SYNOPSIS

A relatively strong westerly jet is directed to the British Isles, northern France, and Germany, with several embedded vort-maxima. Over central Europe, the jet forms a delta ahead of an east European trough that extends into the east Mediterranean. A first cold front has moved into Poland and the Balkans that will become quasi-stationary during the period. The associated maritime air mass is characterized by rather weak lapse rates around 6 K/km, but also better moisture as indicated by latest Wroclaw sounding. Better lapse rates that overlap with ridge moisture are still present over the southern Balkans and Italy.

DISCUSSION

Southern British Isles

With the westerly flow, a frontal system will affect the British Isles. Moist maritime air masses are expected to spread into the southern portions during the morning hours. Lapse rates will be rather poor initially. In the afternoon, QG forcing is expected due to weak low-level warm air advection and mid-level cooling at the cyclonically sheared flank of a jet streak. With the help of some diurnal heating, and given the rich moisture content, weak CAPE is quite likely.

Forcing will be rather limited given westerly or south-westerly low-level winds with no well-developed convergence zone behind the warm front. However, high resolution models indicate more southerly flow with the chance of convergence lines and slightly upslope flow, assisting in storm initiation. Storms that form will profit from favourably veering profiles and substantial vertical wind shear (in the lowest km 12 m/s bulk shear and 120 m²/s² SRH). Isolated to scattered multicells and supercells are forecast that will move east during the afternoon and evening. Given the strong low-level vertical wind shear and high moisture and associated low-level buoyancy, the main threat will be tornadoes. Additionally, intense cells which develop cold pools may produce severe wind gusts. The overall threat is expected to warrant a level one.

In the evening, severe potential rapidly decreases as the storms spread east into the North Sea area and drier air masses are advected south-east behind a cold front.

Eastern Italy

In the delta of the west-European jet, a short-wave trough will spread south-eastward and affects the Adriatic region. Cold air advection will affect the area, but also quite strong DVCA is expected, resulting in QG forcing that spreads into southern Italy during the day. Mid-level cooling and diurnal heating will result in steepening lapse rates coupled with some moisture pooling along the sea-breeze front over Italy near the Adriatic Sea. Initiation is forecast in the noon hours and storms are forecast to spread southward with the main QG support, whereas cold air advection will lead to weakening storms from the north in the afternoon.

With vertical wind shear around 15 m/s in the lowest 6 km storms may organize into multicells capable of producing large hail. Strong gusts and heavy rain may occur as well, but I is questionable if the thresholds for a level 1 will be reached. Due to the weak CAPE and limited vertical wind shear, a level 1 was not issued.

Southern Balkans

Widespread thunderstorms are expected during the day. Weak vertical wind shear will limit storm organization, but given QG forcing, clusters of storms may form that can produce wxcessive precipitation.

Denmark to northern Poland

Thunderstorms are forecast in the range of a trough axis spreading north-east during the period due to rather cold mid-levels and good low-level moisture. In the wake of the trough axis, vertical wind shear starts to increase, but may not affect the storms that become quite shallow due to mid-level warm air advection. Marginal hail and strong gusts may occur, but a level 1 seem to be not warranted.

Spain

A rather moist air mass is expected across central Spain and the Pyrenees on Thursday. Diurnal heating will lead to steep low-level lapse rates and some CAPE. Low-level convergence and upslope flow is expected to lead to initiation of isolated storms. Weak vertical wind shear and CAPE are expected, and organized storms are not likely. However, locally inverted-v-profiles may assist for an isolated dry microburst, before the activity weakens after sunset.