Storm Forecast
Valid: Wed 10 Oct 2007 06:00 to Thu 11 Oct 2007 06:00 UTC
Issued: Wed 10 Oct 2007 06:33
Forecaster: GATZEN

SYNOPSIS

On Wednesday, a high west of Iberian Peninsula ridges into British Isles and Germany, while a cut-off low is expected to travel southward over France at the western flank of low geopotential over central Mediterranean. While the cut-off will move into Iberian Peninsula, high pressure is forecast to build from British Isles to central Europe. At low levels, cool and rather dry air mass has spread into most of Europe except for the eastern Mediterranean. Warm maritime air mass is advected into British Isles during the period.

DISCUSSION

Southern France, northwestern Spain

Ahead of the approaching cut-off low, a tongue of warm and moist low-level maritime air mass spreads south-eastward into southern Bay o Biscay, northwestern Spain, and southern France. This air mass will likely be characterized by rich low-level moisture and low LCL heights. Above the boundary-layer, steepening lapse rates are likely as the cut-off low approaches. Latest soundings indicate rather stable air mass, and strong instability is not expected.

In he morning hours, a strong mid-level jet streak is forecast to move southward at the western flank of the cut-off low, and QG forcing is likely at the cyclonic flank of this jet streak. Additionally, low-level convergence is expected along the cold front that will reach northern Spain around noon, and a convective line is likely to form. South of this cold front, strong vertical wind shear and favorable veering is forecast by latest models. With westerly winds at low levels and rather strong north-westerly winds aloft, about 12 m/s 0-1 km vertical wind shear, 20 m/s 0-6 km vertical wind shear, and more than 200 J/kg 0-1 km SRH are forecast by latest GFS model run. Thunderstorms that may form will likely include rotating updrafts, and given low LCL heights and rather strong LLS, storms that are more isolated are forecast to produce tornadoes and large hail. Severe wind gusts are also not ruled out.

Limiting factor is questionable instability. Low-level air mass may be too cool for strong low-level buoyancy as well as for deep convection. Current scenario is that the convective line will be rather shallow with only a few lightning strikes initially. Better instability is forecast later in the day ahead of the front, where chance for thunderstorms will gradually increase, while vertical wind shear is forecast to decrease.

Later in the period, convective activity will likely spread into west Mediterranean, as strong upper jet streak moves southward. Latest soundings indicate rather steep mid-level lapse rates and some instability that will likely increase in the evening, and night hours. Thunderstorms are forecast to spread southward along the cold front. Ahead of this front, strong vertical wind shear of 25 m/s DLS, 10 m/s LLS, and favorable veering are forecast by latest models, and embedded supercells are expected. Large hail and severe wind gusts are forecast. Tornadoes are not ruled out, but threat seems to be not too high.

Greece

Ahead of the Mediterranean trough, increasing vertical wind shear is expected over Greece. Thunderstorms are expected to develop and may organize, while threat for severe weather is rather low given only weak vertical wind shear and buoyancy.

Central Mediterranean

Underneath the low geopotential, strong low level buoyancy and weak shear are forecast, and isolated waterspouts may occur.