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Storm Forecast Valid Thu 07 Aug 2008 06:00 to Fri 08 Aug 2008 06:00UTC

06-08-2008 22:56 | Estofex | m.b.t. 07-08-2008 t/m 08-08-2008

Storm Forecast
Valid: Thu 07 Aug 2008 06:00 to Fri 08 Aug 2008 06:00 UTC
Issued: Wed 06 Aug 2008 22:56
Forecaster: GATZEN

SYNOPSIS

Weak ridge over central Europe moves eastward, as intense upper trough spreads into North Sea region, France, and west Mediterranean. There are several vort-maxima that move north-eastwards ahead of the trough axis. Downstream, the strong upper jet curves around the ridge over southern and central Scandinavia into western Russia. Weak short-wave trough will also eject into central Mediterranean. At lower levels, a tongue of warm air mass will advect into Germany ahead of a cold front that will cross most of France during the day. To the west, quite unstable and moist maritime air mass will affect northern France and parts of British Isles. Stable and dry air mass is present over most of eastern Europe.

DISCUSSION

Western Alpine region from eastern France to north-western Italy, and Switzerland

To the south-east of the approaching cold front, models indicate a well-mixed, warm air mass that will likely be unstable given rather rich low-level moisture. Expect that one or two MCS of overnights convection will move north-eastward over eastern France into Benelux, additional thunderstorms will likely develop along the gust front that is forecast to spread into western Alpine region in the morning hours. Convective activity is expected to spread eastward during the period as diurnal heating and upper height falls will support initiation near the Alps. Thunderstorms will likely be well-organized given 15-25 m/s 0-3 km vertical wind shear ahead of the cold front due to a strong south-westerly jet at the 700 hPa level that spreads into northern Italy during the afternoon, and supercells with large hail and severe wind gusts are expected. In the evening hours, upper trough axis will cross eastern France, providing strong DCVA, and low-level convergence may develop over south-eastern France, providing southerly winds to the east. Strong thermodynamics expected over northern Italy overlapped by favourable veering profiles may enhance severe potential especially south of the Alps, where very large hail and isolated tornadoes are forecast. Limiting factor is uncertainty of low-level buoyancy as low-level moisture may be too low. Storms are forecast to spread into north-eastern Italy, where severe potential will only slightly weaken given strong DLS in the range of the mid-level jet.

Eastern France into Benelux, most of Germany, Denmark

Cold front of west European low will propagate eastward during the period. Overnights MCS will likely move northward over northern France and Benelux, while new convection will likely develop along the leading gust front to the east. Strong vertical wind shear is forecast from north-eastern France into western Germany and most of Benelux, and isolated supercells may develop, capable of producing large hail, severe wind gusts and maybe tornadoes given locally strong 0-1 km vertical wind shear in the morning hours.

While the gust front is forecast to spread rapidly eastward into Germany, forcing is forecast to decrease until the trough axis enters eastern France. As a consequence, convective activity may be limited in the morning and noon hours over western Germany, while initiation of convection is more likely over eastern France. Although latest models differ slightly, it seems that a frontal wave will build later on over central Germany, supporting a MCS that spreads into Baltic Sea during the night hours. Given well-mixed and dry air mass to the north-east of this system, severe wind gusts seem to be the main threat along the leading gust front. Isolated tornadoes are not ruled out as low-level vertical wind shear is strong.

Over southern Germany, latest models indicate a convergence line that will likely support low-level moisture pooling over southern Bavaria, and quite strong instability is expected. In the afternoon/evening hours, approaching gust front as well as QG forcing ahead of the trough axis will likely support a linear MCS moving north-eastward into Czech Republic during the night. Severe wind gusts are forecast with this MCS that may be widespread north of the Alps. Large hail is not ruled out with more isolated cells. Additional thunderstorms may develop along the gust front over northern Austria late in the period, and severe wind gusts and locally large hail are not ruled out.

South-eastern British Isles

In the range of the trough axis, moist maritime air mass is expected to destabilize due to daytime heating. Weak vertical wind shear is forecast as well as quite strong forcing and low-level convergence, and non-supercell tornadoes may develop, especially where sea-breeze fronts will interact.

Hagel(-schade) | Onweer/blikseminslag | Tornado/hoosverschijnsel(en) | Wind/storm (-schade)

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