Storm Forecast
Valid: Thu 26 Jun 2008 06:00 to Fri 27 Jun 2008 06:00 UTC
Issued: Wed 25 Jun 2008 21:38
Forecaster: GATZEN

SYNOPSIS

West European trough weakens, as subtropical ridge over Iberian Peninsula spreads into eastern Atlantic. Ahead of the approaching trough axis, very strong upper level jet spreads across Germany, Poland, Belarus, and Russia. QG forcing is forecast especially underneath the right entry region of this jet ahead of the trough axis. At lower levels, a frontal boundary extends from Bay of Biscay to central France, southern Germany, Czech Republic, to western Ukraine. To the south of this boundary, unstable air mass is present due to rich low-level moisture and a well-developed EML.

DISCUSSION

Eastern Alpine region

A very unstable air mass is expected to be present during the period over the eastern Alps. Latest soundings indicate an EML and rich low-level moisture, and daytime heating will likely result in CAPE values of 2000 J/kg. During the day, approaching trough axis will likely lead to QG forcing, while increasing convergence along the low-level frontal boundary and outflow boundaries of overnights convection will likely be focus of new initiation. Thunderstorms that form will likely become well-organized given moderate DLS about 15 to 20 m/s in the lowest 6 km as well as favourable veering profiles. Giant hail and severe wind gusts will be the most significant threat with supercells, but tornadoes are also possible given locally large low-level buoyancy and 15 m/s 0-3 km vertical wind shear. Latest GFS shows that focus of severe convection will be over southern and eastern Austria. From southern Germany to northern Austria, at least weak instability is expected in the range of the frontal boundary, and thunderstorms are expected capable of producing large hail and severe winds given moderate to strong (20 m/s) DLS. Coverage of storms is forecast to be lower that over southern and eastern Austria. Further south-east, vertical wind shear is forecast to decrease over northern Balkans, where thunderstorms will be slightly weaker. However, large hail and severe wind gusts are not ruled out given high instability and weak to moderate DLS. Thunderstorms will likely merge into one or two MCS moving eastward into Hungary during the night, where severe threat will gradually weaken due to low-level stabilization and ceasing convection as QG forcing will weaken.

Eastern Slovakia, south-western Ukraine, and northern Romania

Along the frontal boundary, another wave will travel eastward during the period. The wave that was located over Germany on WED will reach south-western Ukraine during the day. At mid-levels, the frontal wave will again be accompanied by a jet streak that provides moderate DSL and some QG forcing. Expect that overnights convection will re-intensify during the day given increasing instability. However, some uncertainties exist about sunshine heating and low-level buoyancy along the nose of warm air mass. Latest GFS does indicate that quite strong instability will indeed overlap with strong low-level vertical wind shear in the lowest 3 km. If this scenario will indeed come true, supercells are likely to develop, capable of producing large or very large hail and severe wind gusts. Isolated cells may also be tornadic given moderate low-level vertical wind shear and favourable veering profiles. Thunderstorms will likely merge into an MCS moving eastward into northern Romania, where large hain and severe wind gusts may be possible during the night hours.

Northern Iberian Peninsula, France

Weak trough axis will also move into Iberian Peninsula and France. Upper height falls will likely weaken the cap in the range of the frontal boundary, and low-level convergence may be strong enough for initiation. Current thinking is that at least isolated storms will develop, moving eastwards into extremely southern France. Given 15 m/s 0-3km vertical wind shear and also strong DLS, supercells are expected to develop. Large hail and damaging wind gusts will be possible with this convection as low-levels are forecast to be well-mixed. Convective activity will likely weaken after sunset as QG forcing weakens.

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