Storm Forecast
Valid: Wed 29 Aug 2007 10:00 to Thu 30 Aug 2007 06:00 UTC
Issued: Wed 29 Aug 2007 09:39
Forecaster: GATZEN

SYNOPSIS

Long-wave trough placed over northern Europe, while high geopotential covers the Atlantic and Mediterranean. A cut-off low W of Iberian Peninsula is expected to merge with the European trough and will propagate eastward during the period. Ahead of this feature, very warm air mass spreads into Mediterranean and Balkans. A strong frontal boundary stretches from Bay of Biscay to Alpine region, and northern Balkans. This region is expected to be the focus of intense convection.

DISCUSSION

Southern France, northern Italy, Alpine region

Ahead of the short-wave trough/cut-off now over northwestern Spain, DCVA and WAA is present over western Mediterranean. Axis of warm and moist low-level air mass extends from western Spain to southern France, and northern Italy just to the west and north of very warm and well-mixed desert air mass over western Mediterranean. Given a well-developed EML above the moist low-level air mass, high CAPE will likely form in response of diurnal heating.

Latest model output suggests two regions with QG forcing today. First, DCVA is expected underneath the right entrance region of a strong anticyclonic curved jet streak that propagates eastward over southern Germany. This will likely affect southern France and the Alps during the day. Second, another jet streak is expected to propagate northeastward ahead of the short-wave trough. It will spread across northwestern Spain in the evening hours. Over southern France/western Alps, a coupled jet is expected by this time. Given persisting WAA during the period, strong QG forcing is likely.

Latest observations indicate that widespread convection is present over southern France, Alps, and northwestern Spain at this time. Given rather cool boundary-layer air mass, most convective cells are expected to be elevated. However, given some surface heating especially over southeastern France, and northwestern Spain, some updrafts may also root to boundary layer. Over southwestern France, low-level clouds inhibit strong diabatic heating at this time.

During the next couple of hours, convection is forecast to go on over southeastern France and Alpine region. As low-level heating increases, convective cells may root to the boundary-layer. With weak low-level winds and rather strong south-westerly wind aloft, as well as favorable veering profiles, resulting in 20-30 m/s DLS, locally 10 m/s LLS, and 150-250 J/kg SRH, thunderstorms that root to the boundary-layer will rapidly become well-organized and may evolve rotating updrafts. Supercells that form will likely produce (very) large hail. Isolated severe wind gusts are not ruled out with the stronger storms. Given low LCL heights/high low-level moisture, tornadoes are possible in regions where diabatic heating is present, increasing low-level buoyancy. Limiting factor is rather weak low-level vertical wind shear.

During the day, another focus of intense convection is forecast with the second region of strong upper lift. Current thinking is that thunderstorms will likely initiate along/north of the Pyrenees, spreading rapidly northeastward. Given very favorable profiles, and high instability, well-organized convection is expected. Mesoscale convective systems, including bowing lines and embedded supercells are forecast, capable of producing (very) large hail and severe wind gusts. Given locally favorable low-level veering, a few tornadoes are also forecast, especially over southeastern France, where low-level southerly winds will likely increase ahead of the convective system.

Thunderstorms are expected to spread into the Alps, and northern Italy, where large hail, severe wind gusts and a few tornadoes are still expected.

Additionally, severe flash floods and flooding are expected with the stronger storms throughout the period.