Storm Forecast
Valid: Thu 26 May 2011 06:00 to Fri 27 May 2011 06:00 UTC
Issued: Wed 25 May 2011 22:12
Forecaster: GATZEN

A level 1 was issued for north-eastern Spain, south-eastern France, and the Alpine region mainly for large hail.

A level 1 was issued for central and northern Germany mainly for severe wind gusts.

A level 1 was issued for northern Morocco mainly for large hail.

A level 1 was issued for southern and western Turkey mainly for severe wind gusts.

SYNOPSIS

A trough moves eastward over north-eastern Europe. In its wake, another trough spreads eastward across the North Sea on Thursday. It is associated with a tongue of warm air advecting into central Europe. To the south of the frontal zone, weak dynamics dominate southern Europe. Slightly stronger winds are forecast over Morocco and to the south of Turkey head of weak low-pressure systems.

DISCUSSION

North-eastern Spain to Germany

Together with the approaching trough, a low pressure system will move across the North Sea on Thursday. With south-westerly winds ahead of the cold front of the low, warm air masses from the west Mediterranean will advect into Germany. The warm front is expected from the German North Sea coast to the Czech Republic at noon. The warm air mass is forecast to become slightly unstable ahead of the cold front given some low-level moisture increase and synoptic lift ahead of an approaching south-westerly mid-level jet streak. Latest models indicate differences in the amount of CAPE over central and northern Germany, but current thinking is that at least weak instability will be available ahead of the approaching cold front.

Along the cold front, low-level convergence will be relatively weak. Additionally, cold air advection may set in rapidly near the surface trough as indicated by latest models. Given the strong linear forcing at the eastern flank of the jet streak and cold front, a convective line seems to be most likely to develop. The angle between the cold front and the mean flow will be quite large, and some line echo wave patterns or bowing segments are expected that can be associated with severe wind gusts.

The main uncertainty will be the amount of instability and low-level moisture. Latest ECMWF indicates only weak instability and therefore weak convection that may not manage to merge into a linear system. But given 15 m/s at the 850 hPa level and probably bowing segments, severe wind gusts are also expected with this scenario. On the other side, the higher GFS moisture may result in a linear MCS capable of producing severe wind gusts at some more places while tornadoes along the leading gust front can not be ruled out.

Farther south, instability is more certain near the Alps and over south-eastern France and north-eastern Spain. Ahead of the cold front, CAPE of 1000 J/kg is possible. As the mid-level trough approaches, the deep layer bulk shear increases to 15 m/s. Storms are forecast to develop in the afternoon/evening hours and will likely organize. Large hail seems to be the main threat, while isolated severe wind gusts cannot be ruled out. The best potential exists near north-eastern Spain, where easterly low-level winds will likely result in favourably veering profiles that can support supercells.

In the evening and night hours, the cold front becomes more parallel to the mean flow in the southern part, and storms are expected to become gradually elevated. Excessive precipitation is not ruled out locally, while the severe threat is forecast to weaken. In the northern parts of the cold front, instability is forecast to disappear quite rapidly and storms are expected to decay.

British Isles

In the wake of the trough, a tongue of rather moist maritime air spreads into England from the north. Daytime heating is expected to result in weak instability release, and scattered showers and thunderstorms are forecast. The relatively strong low-level vertical wind shear that reaches 10 m/s in the lowest kilometre may favour the development of tornadoes, and a few events are not ruled out. A level one seems to be not warranted, though. Storms are forecast to decay in the evening hours.

Northern Morocco

A mid-level jet streak ejects from an Atlantic trough west of northern Africa and will spread into southern Iberia from noon into the night hours. At lower levels, this is associated with relatively strong westerly winds along the coasts of Morocco that will reach 15 m/s at the 700 hPa level. Latest models suggest that these westerly winds will advect rather cool and moist maritime air masses into Morocco. Given a very warm air mass above this maritime air mass, lapse rates are expected to be poor. Additionally, the low-level forcing is forecast to be rather weak given weak to the east of the trough. However, daytime heating may be strong enough to build some deeper instability in the afternoon hours, and some thunderstorms are not ruled out. Convection that forms is expected to become organized given the rather strong bulk shear. Multicells will be capable of producing isolated large hail. Supercells may also be possible as the SRH increases to about 100 m²/s² in the lowest 3 km, but current thinking is that the weak forcing and instability will limit the potential of long-living storms.

East Aegean and western Turkey

Upslope flow of moist maritime air masses is expected in response to diurnal heating over Turkey on Thursday. With weak CIN indicated by latest models, storms are forecast to develop during the day. While the bulk shear will be limited with only 10 m/s in the lowest 6 km, storms will likely be not too well-organized. However, pulse storms may produce large hail and excessive precipitation locally. Additionally, decreasing mid-level theta-E values may result in an increasing microburst risk, and a few severe wind gusts are not ruled out. A marginal level one is issued. Storms may go on during the night hours as the trough centre will mode eastward.

Spain, Italy, and the Balkans

Diurnal heating is expected to result in storms near mountains and sea breeze convergences. In the weak flow, organized storms are expected to be unlikely. However, strong pulse storms are not ruled out capable of producing isolated large hail, severe wind gusts, and excessive precipitation.