Storm Forecast
Valid: Wed 28 May 2008 06:00 to Thu 29 May 2008 06:00 UTC
Issued: Tue 27 May 2008 22:41
Forecaster: GATZEN

SYNOPSIS

A sharp and quite intense upper trough is present over western Europe, with a short-wave trough axis extending from west of the British Isles to central Algeria. To the east of this through, subtropical high over east central Mediterranean ridges into central Europe and North Sea. During the period, the southern part of the trough turns northward over west Mediterranean, while the geopotential over Central Europe slightly increases. At lower levels, a tongue of subtropical air spreads into Germany, and steep mid-level lapse rates indicated by latest ascends are expected from central Mediterranean to Balkans, and Germany. Models also indicate rich low-level moisture in the range of the warmest air from northern Balkans to Benelux, and diurnal heating will likely result in rather high CAPE. Low-level dry air mass remains in the range of surface high pressure from Scandinavia to Poland and Ukraine. Maritime air mass is present over western Europe.

DISCUSSION

North-western Germany to Alpine region and northern Balkans

Focus of expected severe potential is expected to be present in the range of the warm subtropical air mass, where rather high instability will likely overlap with quite strong mid-level jet streaks flanking the ridge over central Europe. A 40 m/s southerly jet streak is expected from western Mediterranean to eastern France and Benelux, where 20 m/s DLS is expected, while strongest DLS is forecast over western Alps reaching 30 m/s 0-6 km vertical wind shear. 15 to 20 m/s DLS is also forecast near the frontal boundary over the Balkans in the range of strong north-westerly jet. Weak DLS is forecast in the range of the ridge axis over central Balkans.

Some uncertainties exist about initiation over most of the region, as quite strong capping inversion may be present. Latest models indicate that quite strong QG forcing will be present ahead of propagating jet streak from eastern France to Benelux. In the range of the frontal boundary, quite widespread convection is expected during the day spreading northwards over eastern France, western Germany, and Benelux. Given strong vertical wind shear, a MCS seems to be likely, and strong to severe wind gusts as well as intense precipitation are expected. More isolated convection at the eastern and southern flank of the MCS may also produce large hail and severe wind gusts. To the south, new convection will likely develop over western Alpine region spreading northward into south-western Germany. Convection may me embedded in a stratiform rain band initially and may be elevated over some places, but given strong DLS, a few mesocyclones are forecast to produce severe wind gusts and isolated severe hail. Further east, convective activity will be more isolated given quite large CIN and weaker forcing. Models indicate a convergence zone from central Germany to northern Austria and another north of the Alps over southern Germany. Along both convergence lines, low-level moisture flux convergence is expected to lead to high CAPE, but there are uncertainties about low-level moisture at this time. Current thinking is that some isolated storms will develop along the main convergence line from north-western Germany to western Czech Republic and maybe further south-east over Austria. Best potential for large hail is expected in the northern part of the convergence as DLS decreases to the south, but every isolated storm that form will have the potential of severe hail given high CAPE and at least moderate DLS. Local flash flooding is also not excluded. Tornadoes are not ruled out in the range of the convergence zone, although quite large CIN and relatively weak LLS will be limiting factors. Convective activity will spread north-eastwards during the night, while severe thread will likely decrease.

British Isles

At the cyclonic flank of the upper jet streak, a tongue of warm/moist air mass is advected into the British Isles. Although most storms will be elevated, a few surfaced-based storms may develop during the morning hours that will encounter quite strong low-level helicity, and a few tornadoes are not ruled out in the northern England region.

Central Mediterranean

In the range of negatively tilted upper trough, strong QG forcing, strong vertical wind shear and rather high SRH are expected from Tunisia to northern Italy region. With steep lapse rates in the warm air mass east of a propagating cold front, diurnal heating will likely result in CAPE over the land. The cool sea surface will limit surface-based storms. Current thinking is that widespread stratiform rain with embedded convection will affect the region, while surface-based storms capable of producing large hail, severe wind gusts and a few tornadoes will be possible over the land.

Northern Hungary, Romania

Along the cold front, some convection may occur that has a potential for large hail given strong DLS.

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