Storm Forecast
Valid: Thu 19 Jul 2007 06:00 to Fri 20 Jul 2007 06:00 UTC
Issued: Wed 18 Jul 2007 22:15
Forecaster: GATZEN

SYNOPSIS

Main feature over Europe is a strong southwesterly flow between subtropical ridge from Algeria to Black Sea region and developing cut-off low over southern British Isles/Bay of Biscay. Another trough moves eastward over Scandinavia. Associated strong jet streaks are present over Bay of Biscay region and from southern Baltic Sea to western Russia. Several vort-maxima are also embedded in the upper flow. At the 850 hPa level, very warm air mass characterized by well-mixed low to mid levels is present over central Balkans, while slightly cooler low-level air is present to the north as indicated by latest soundings over northern Balkans and Austria. During the period, warm air advection is expected over France, and Alpine region ahead of mentioned upper cut-off low.

DISCUSSION

France

Low-level moisture has strongly increased in the range of the frontal boundary as indicated by latest Lyon ascend. Expect that this process will go on during the period, and expect that diurnal heating is rather strong, quite high (1000 J/kg) CAPE will likely develop over central/eastern France during the day. Ahead of the cut-off low, a strong (40 m/s) southerly 300 hPa jet streak moves over Bay of Biscay/northwestern France and spreads eastward into central France in the night hours. Given QG forcing due to DCVA, a frontal wave is likely to develop over central France, and low-level winds are expected to back leading to warm air advection. Thunderstorms are forecast to form along the frontal boundary/old outflow boundaries and will likely intensify during the afternoon/evening hours while spreading northeastward into central, and eastern France. Given strong (around 20 m/s) deep layer vertical wind shear and increasing favorable veering profiles over eastern France, chance for supercells and bowing lines seems to be quite good. Severe wind gusts, large hail and maybe tornadoes with more isolated storms are possible with supercells over central, and eastern France. Limiting factor is uncertainty of CAPE due to low-level clouds that inhibit diurnal heating. Farther north, strong warm air advection is forecast in the evening/night hours over northeastern France. Strong low-level veering winds are forecast north of the frontal boundary, and storms that may root to the boundary layer will likely become mesocyclones with the chance of producing tornadoes. Limiting factor is quite stable/dry low-level air mass as well as high coverage of unorganized storms. Thunderstorms will likely merge into one or two MCS in the evening hours spreading into Benelux and western Germany given persistent QG forcing, while severe threat will slowly weaken due to low-level stabilization. Severe wind gusts are expected to be the main threat when well-organized convective systems will form indeed.

Northern Alpine region, southern Germany, Czech Republic

There are indications that boundary layer moisture has started to increase well underneath several inversions north of the surface front. On Thursday, latest model output agrees that strong diurnal heating and low-level evapotranspiration will lead to very warm/moist low-level air mass over the affected region, capped by a relatively weak inversion of well-mixed hot air mass. Although low-level moisture of latest GFS model run seems to be very optimistic, current thinking is that high CAPE around 1500 J/kg will likely develop over a broad region. Although large-scale QG forcing is expected to be weak in the range of upper ridge axis, embedded vort-maxima will likely be present and help convection to develop in the range of mountains, old outflow boundaries and low-level convergence lines. Expect that thunderstorms will develop in the range of the northern Alps, chance for supercells is relatively good given moderate (15 m/s) deep layer vertical wind shear and also some veering. Thunderstorms will move northeastward in the afternoon/evening hours, capable of producing large/very large hail, and isolated severe wind gusts. Chance for tornadoes does not seem to be good given quite weak low-level vertical wind shear as well as relatively weak low-level buoyancy. A tornado is not ruled out with every supercell that forms, though. While QG forcing is expected to be limited, thunderstorms will likely weaken during the night hours.

Southern Poland, Belarus/northwestern Ukraine, parts of western Russia

Along the frontal boundary, some instability will likely develop during the day, especially when strong diurnal heating is possible. Although QG forcing will be weak, a few thunderstorms are expected especially in the range of old outflow boundaries, where low-level moisture is enhanced. Given moderate deep layer vertical wind shear, a few supercells are not ruled out, capable of producing large hail, severe wind gusts and possible a tornado. Given low coverage of storms and mentioned limiting factors, decide to place a level 1 along the frontal boundary.

Eastern Finland, northwestern Russia

In the range of approaching upper trough, warm/moist low-level air mass and cold mid-level will likely create strong low-level buoyancy over eastern Finland/northwestern Russia. Thunderstorms will likely form. Given weak vertical wind shear, a few waterspouts/tornadoes are forecast to form. Best potential seems to exist in the range of the surface low center, where low-level convergence is likely.