Storm Forecast
Valid: Thu 23 Sep 2010 06:00 to Fri 24 Sep 2010 06:00 UTC
Issued: Wed 22 Sep 2010 16:40
Forecaster: GATZEN

A level 1 was issued for eastern Spain mainly for large hail, severe winds and excessive rain.

A level 1 was issued for northern France and the Benelux States mainly for tornadoes.

SYNOPSIS

High geopotential over north-eastern Europe and a weak trough over the Mediterranean cause a weak mid-level flow across Europe. From the Atlantic a mid-level trough moves eastward and will affect western Europe during the forecast period. The southern part of this trough will accelerate and rapidly cross the Iberian Peninsula. The northern trough is expected to move to England. Relatively warm air masses present over the Mediterranean will advect into central Europe. To the north-west of a frontal boundary extending from southern Scandinavia to the North Sea and northern France cold air advection is expected at the end of the forecast period.

DISCUSSION

North-eastern Spain

Rich boundary-layer moisture (13 g/kg) is present over the Mediterranean Sea and will spread into north-eastern Spain ahead of a surface low approaching over southern Spain. Low-level convergence and upslope flow are likely during the day, and diurnal heating will help to build CAPE. Additionally, upward vertical motion is expected from the morning on east of the approaching trough. Especially in the afternoon hours, when an intense mid-level jet streak will spread into eastern Spain, strong DCVA is forecast over north-eastern Spain.

Initiation of storms is expected in the noon or afternoon hours. With strong deep layer vertical wind shear (about 20 m/s in the lowest 6 km), storms are forecast to organize and pose a threat of large hail and severe wind gusts. These storms will likely merge into mesoscale convective systems as the QG forcing will be strong and a persistent and concentrated low-level convergence is expected. Excessive rain seems to be the main threat after sunset that will gradually shift to the Balearic Islands during the night hours.

Northern France and the Benelux States

A short-wave trough curves around the approaching mid-level trough and affects northern France on Thursday. To the east of the trough axis, this short-wave trough becomes slightly negatively tilted. A 20 m/s mid-level jet streak will spread eastwards over northern France and is expected over western Germany at the end of the forecast period. DCVA is forecast at the cyclonically sheared side of the jet streak.

At lower levels, latest observations indicate that the boundary-layer is moist with a low-level mixing ratio of 8 g/kg. On Thursday, a southerly low-level flow ahead of the approaching frontal system will be associated with warm air advection from France and the southern Benelux States into the North-Sea region, where moisture pooling is forecast. The lapse rates are expected to improve slightly in the afternoon hours when the mid-level trough approaches, and instability is forecast by latest models.

Precipitation is likely from the morning on in the vicinity of the frontal boundary near the North Sea. Warm air advection will go on and convection that may develop in the course of the day will likely be elevated as the boundary-layer is cool. Surface-based instability will develop to the south of this precipitation in response to diurnal heating and increasing low-level moisture. As the mid-level jet approaches, a low-level convergence/cold front will approach aver France and moves into the Benelux States during the afternoon. Surface-based convection is forecast that may merge to clusters later on.

With about 15 m/s bulk shear in the lowest 3 km, storms will likely become organized. Although hodographs are not too favorable, at least some low-level helicity is forecast and supercells are not ruled out, especially near the frontal boundary over the North Sea region, where latest GFS forecasts curved low-level hodographs. Isolated supercells may be capable of producing large hail and severe wind gusts. In the evening hours, the low-level vertical wind shear increases to 10 m/s, supporting a weak threat for tornadoes.

The main convective mode will likely be clusters moving north-eastward. Embedded mesocyclones will pose a threat of isolated tornadoes or hail. Convective activity will gradually weaken during the night as the boundary-layer cools.