Storm Forecast
Valid: Thu 07 May 2015 06:00 to Fri 08 May 2015 06:00 UTC
Issued: Wed 06 May 2015 22:52
Forecaster: GATZEN

A level 2 was issued from northern Greece to southern Bulgaria and north-western Turkey mainly for large or very large hail, excessive precipitation, and to a lesser extend severe wind gusts and tornadoes.

A level 1 was issued across the south-eastern Balkans and the Aegean region mainly for large hail.

A level 1 was issued for north-eastern Spain and south-western France mainly for large or very large hail and to a lesser extend severe wind gusts.

SYNOPSIS

Low geopotential centered across north-western Europe yields a strong mid-level westerly flow at its southern flank. One strong jet streak is currently over the north Mediterranean ahead of a short-wave trough. The short-wave trough and jet will rapidly progress east, where a downstream ridge decays during the period. A plume of well-mixed desert air will be replaced by a cold front at the same time. In the wake of this cold front, cool maritime air masses have spread into most of Europe, leading to weak low-level moisture. Highest moisture will remain in the wake of Iberia and the Alps. Lapse rates will start to recover due to diurnal heating.

DISCUSSION

South-eastern Balkans and Aegean Sea region

A cold front will move into the forecast area on Thursday morning. It is expected to slow down in response to a lee trough across the Aegean Sea and will stretch from southern Serbia to southern Bulgaria by Thursday noon. Rich low-level moisture will further increase due to low-level convergence and upslope flow. This moisture is expected to overlap with steep lapse rates sampled by latest Zagreb sounding. CAPE may exceed 1500 J/kg as indicated by latest GFS model.

Initiation is forecast due to upslope flow and frontogenetical forcing. Additionally, increasing DCVA is expected in the afternoon hours due to the approaching short-wave trough. Storms will organize given 20 m/s deep layer bulk shear and 10 to 15 m/s 0-3 km bulk shear. Favourably veering profiles are forecast by GFS especially during noon, supporting supercells. Therefore, large or very large hail is forecast. Supercells may also be capable of producing severe wind gusts and tornadoes. Storms will merge to clusters given increasing DCVA in the afternoon, forming one or two MCSs capable of producing excessive precipitation and locally large hail and severe wind gusts. Tornadoes are not ruled out, but low-level vertical wind shear is mostly weak.

Northern Spain to southern France

A strong westerly mid-level jet stream affects Iberia through-out the period. Downstream of an approaching short-wave trough, ridging is expected on Thursday, leading to sunny conditions and increasing lapse rates. Over the eastern parts, upslope flow of moist maritime air masses is forecast due to the thermal low over Spain.

As the short-wave trough approaches in the evening hours, easterly low-level winds will lead to an overlap with steep lapse rates, and CAPE can develop from the western Pyrenees to the Bay of Biscay. DCVA will help to weaken the cap, and initiation is forecast in the evening hours. Given strong deep layer bulk shear, well-organized storms are possible, capable of producing large or very large hail. However, storms will gradually become elevated as the boundary layer cools in the evening, and chance of severe storms will also decrease. Ongoing DCVA and frontogenetical forcing ahead of an approaching cold front will support thunderstorms until the end of the period. These storms may merge into a well-organized MCS given strong vertical wind shear. Bowing lines may form, capable of producing locally severe wind gusts.