Storm Forecast
Valid: Thu 16 Jul 2009 06:00 to Fri 17 Jul 2009 06:00 UTC
Issued: Wed 15 Jul 2009 19:57
Forecaster: GATZEN

A level 2 was issued for portions of northern France mainly for severe wind gusts.

A level 1 was issued for France mainly for severe wind gusts and large hail.

A level 1 was issued for south-eastern England mainly for excessive rain.

A level 1 was issued for north-eastern Spain mainly for severe wind gusts.

A level 1 was issued for western Russia mainly for large hail.

SYNOPSIS

An amplified long-wave trough over the eastern Atlantic yields a deep south-westerly flow from the Iberian Peninsula across western and central Europe into western Russia. Some short-wave troughs and mid-level jet streaks embedded in the main flow will travel north-eastward during the period, while the main trough still amplifies and cuts off over the British Isles. A strong mid-level jet is forecast to evolve at the edge of the trough that will point towards the Benelux countries at the end of the forecast period.

At lower levels, a well-defined frontal boundary stretches from Iberia to Germany. While the cold front will move eastwards in the southern portions, warm air advection is likely to the east of the approaching trough, where an elevated mixed layer currently over the Mediterranean will overspread quite moist low-level air masses in the range of the frontal boundary.

Further north-east, another large frontal wave will move across the Baltic Sea region. Warm and moist air masses will be advected into the western Russia region ahead of the associated mid-level trough. To the south, only weak mid-level winds are forecast in the range of a ridge from the Mediterranean to the Ukraine, but weak mid-level troughs will likely lead to some forcing.

DISCUSSION

France and surroundings

Severe convection becomes likely especially across eastern France ahead of the amplifying trough. On Wednesday, an elevated mixed layer originating from the Iberian Plateau will be advected northward overspreading a moist boundary-layer over France, and GFS and ECMWF agree that rather strong instability of 1000 to 1500 J/kg is possible. QG forcing will cause low surface pressure moving from southern France north-eastward in the afternoon and evening hours along the nose of the warm air. Low-level convergence along the northern and western flank of the warmest air mass may be sufficient to break the cap in the evening hours over western and north-western France. Storms will likely rotate especially in the northern portions where favourable veering profiles are expected, and supercells are forecast capable of producing very large hail and severe wind gusts mainly.

In the late evening hours, strong linear forcing will evolve over central and eastern France ahead of the main vort-max. Models do agree on a strong southerly low-level-jet over eastern France reaching 15 (ECMWF) or even 25 (GFS) m/s at the 850 hPa level. Expect that convection will develop in this regime with 20 m/s 0-6km, 15-20 m/s 0-3km, and 10 m/s 0-1km vertical wind shear, well-organized mesoscale convective systems are forecast. Bowing lines may form with a threat of severe wind gusts, and large hail is also forecast especially in the range of intensifying convection to the south of the convective systems. In the northern portions, a tornado is not ruled out given the strong low-level vertical wind shear, but clustering of the convection will likely limit the potential. Excessive rain is also not ruled out in regions affected by the mesoscale convective systems. At the southern region of the convection, severe wind gusts are possible given well-mixed air masses from northern Spain to southern France.

While the convection moves eastward across most of France during the night and will enter the Benelux countries, threat of severe wind gusts may increase especially in the northern portions, and a level 2 is required. Further south-east into western Germany and the western Alps, severe threat will gradually decrease due to decreasing low-level instability.

England

As the mid-level jet streak approaches over The Channel, strong QG forcing is expected over England in the evening hours. At the surface, a convergence is expected to develop, and rather moist low-level air mass may become slightly unstable given moist-neutral profiles. Given favourable veering profiles to the east of this convergence, embedded mesocyclones may develop, and a tornado is not ruled out completely, but limiting factor will be weak instability. For the south-eastern portions, excessive rain may occur at the northern flank of the French MCS.

Eastern Spain

Ahead of the approaching trough, hot and well-mixed air masses will affect eastern Spain. Along the eastern coasts, moist low-level maritime air mass spreads westward and upslope flow is expected. A strong capping inversion will likely suppress any convection. However, thunderstorms that may develop over the mountains will have a potential of severe downbursts given well-mixed and dry low-level air masses. Strong vertical wind shear may also be favourable for large hail.

Alps

In the range of the Alps, QG forcing and low-level convergence is expected in the warm air advection regime. Given quite moist air mass, some showers and thunderstorms are forecast. Moderate vertical wind shear is forecast and large hail is not ruled out. While the capping inversion will likely suppress convection in the afternoon and evening hours, best potential for a few severe hail events seem to exist over the Alps, where isolated thunderstorms may develop. Convective activity is forecast to weaken after sunset.

Western Russia

Ahead of a frontal wave and mid-level short-wave trough, models indicate increasing southerly winds over western Russia that will advect warm air masses northward. In the boundary-layer, rather weak moisture will increase during the day from the west, and weak instability is expected to develop. Thunderstorms that form may organize given quite strong 0-3km vertical wind shear. However, this potential decreases from the west during the day. A few large hail events are not ruled out.

Finland

Ahead of a short-wave trough over the Baltic Sea region, a strong mid-level jet enters southern Finland during the day. At lower levels, the nose of rather warm air over eastern Europe moves over Finland, where QG forcing is expected. As low-level moisture will likely increase in the afternoon hours thunderstorms are forecast to develop. Although vertical wind shear is rather weak, slightly enhanced low-level vertical wind shear may be favourable for a few severe storms capable of producing marginally severe hail.