Storm Forecast
Valid: Thu 23 Jul 2009 06:00 to Fri 24 Jul 2009 06:00 UTC
Issued: Wed 22 Jul 2009 22:03
Forecaster: GATZEN

A level 2 was issued for eastern France, most of Germany, nothern Alps, western Poland, and Czech mainly for large hail, severe wind gusts, and tornadoes.

A level 1 was issued for most of France, southern Scandinavia, southern Baltic states, and eastern Poland mainly for large hail and severe wind gusts.

SYNOPSIS

The amplified long-wave trough west of Europe experiences another strengthening as cold mid-level air spreads southward over the Atlantic and the very strong jet streak still intensifies while curving around the troughs base and spreading into western Europe. Downstream, a short-wave trough accelerates eastward and crosses France, Germany, and Poland during the period. At lower levels, a cold front will move across France, Benelux, and north-western Germany in the morning hours. To the south-east, models indicate frontal waves as very warm air masses spread north-eastwards. Strong instability will likely develop in the warm air advection regimes during the period, and another outbreak of severe weather becomes likely.

DISCUSSION

Eastern France, Germany, northern Alps, northern Austria, Poland, and Czech Republic

East of the approaching short-wave trough, a strong mid-level jet spreads into central Europe from the south-west. Strong DCVA is expected to spread south-eastward during the day. At lower levels, a frontal wave will likely move across northern Germany in the morning, while other frontal waves may develop over eastern France spreading across central or southern Germany into southern Poland and Austria. Those frontal waves will likely be associated with strong QG forcing.

The air mass to the south-east of the main front is characterized by rich boundary-layer moisture in excess of 11 g/kg mixing ratio in the lowest kilometre. Additionally, rather steep lapse rates will likely persist ahead of the convection resulting in CAPE values around 500 J/kg in the morning hours that may increase to more than 1500 J/kg in the south-eastern portions during the afternoon.

At 0600 UTC, intense convection is forecast in the range of the northern frontal wave to move across northern Germany into Denmark. A couple of bowing lines or larger bow echoes are forecast due to the strong vertical wind shear and forcing, and severe wind gusts may be possible over numerous places. Another threat will be tornadoes in the morning hours given very strong low-level vertical wind shear and low-level SRH. Large hail is also possible. Relatively weak instability in the morning hours will be the main limiting factor, and a level 2 seems to be not required. Further east, diurnal heating will likely lead to surface-based instability across Poland, where embedded supercells pose a threat of large or very large hail, severe wind gusts, and tornadoes. To the south of the frontal wave, models agree that westerly winds will spread across northern Germany and north-western Poland, leading to cold air advection and temporarily decreasing instability.

At least a second frontal wave is forecast to develop ahead of the approaching trough axis over eastern France, and warm air advection sets in across parts of Germany again while a tongue of very warm air is forecast to spread across the Alps into northern Austria, Hungary, the Czech Republic, Slovakia, and southern Poland. Latest models indicate uncertainties about the development of the associated surface lows. However, some warm air advection and weakening surface winds are likely to the north-east of the developing surface lows that may spread across central Germany as indicated by latest GFS and ECMWF model runs. As a consequence, a level 2 is issued for portions of northern and eastern Germany where CAPE is expected to overlap with strong vertical wind shear. Large hail and severe wind gusts are expected to be the main threat. Rather high low-level buoyancy may be favourable for tornadoes as well, especially when the surface lows will evolve stronger than anticipated.

North of the Alps, instability, low-level vertical wind shear, and veering profiles may increase in the afternoon and evening hours in the range of low surface pressure moving eastward. As convection will likely spread into this region from the west, thunderstorms will likely intensify and become well-organized including long-lived supercells and bow-echoes. Large to giant (5-10 cm) hail, severe winds, and tornadoes are not ruled out when this scenario becomes true. As models indicate a lot of uncertainties, a level 3 is not issued at this time, but an upgrade may be required during the period. Convection will spread into northern Austria, Czech Republic, and southern Poland in the evening, while severe threat will likely continue. After midnight, thunderstorms will likely weaken further east.

Central and southern France

In the range of the trough axis, strong DCVA will cause QG forcing ahead of the approaching surface cold front. Due to diurnal heating, moist low-level air mass will likely become unstable and numerous thunderstorms are expected. As vertical wind shear will be strong, storms will likely be well-organized. Due to slightly backing winds ahead of the surface cold front, supercells my also form. Large hail and severe wind gusts seem to be the main threat. Convective activity will rapidly die from the west as the cold front will move eastward.

Southern Scandinavia

To the north of the frontal wave, clouds and precipitation are likely over southern Scandinavia. However, embedded convection that roots to the boundary-layer will have a potential of producing tornadoes given favourable veering profiles to the north-east of the surface low. Large hail is also not ruled out especially across Denmark. Potential will likely spread north-eastwards during the day, while decreasing low-level buoyancy may reduce the potential gradually.

Southern Alps

Although indications are rather weak, convective initiation is not ruled out as the trough approaches. Given strong vertical wind shear, thunderstorms will likely be severe capable of producing large or very large hail and severe wind gusts.