Storm Forecast
Valid: Sat 06 Sep 2008 07:00 to Sun 07 Sep 2008 06:00 UTC
Issued: Sat 06 Sep 2008 07:32
Forecaster: SCHLENCZEK
SYNOPSIS
An occluding surface low over the British Isles will move slowly northeastward. Its cold front stretches from SW Portugal via SW France and S Belgium towards N Germany, separating cold polar air over the East Atlantic from hot and moist air over eastern Europe. In the vicinity of the cold front, a strong 60 m/s jet streak at 300hPa will translate to the north / northeast. During the afternoon, a shortwave trough will overspread SE France / N Italy where some low-end instability is expected. Thunderstorms may develop over the Baltic States, E Belarus and W Russia in the vicinity of the warm front associated with the depression over UK. A well-defined EML is present over southern parts of Italy and central Mediterranean. Moderate instability will form during the day but it will remain strongly capped. Cold air convection with showers and thunderstorms is expected over the North Sea in the wake of the trough.
DISCUSSION
...Extreme S/SE France / N Italy...
Ahead of the cold front, some low-end instability is expected to form. During the afternoon, a small shortwave trough will cross the region and winds at all levels will increase. Convective initiation should lead to one or two linear systems that may contain some mesocyclones as SRH3 between 150 and 300 J/kg will be in place. Deep layer shear in order of 25 m/s and wind speeds around 20 m/s at 700hPa may allow isolated severe gusts. T/Td in order of 20/15 °C and therefore low LCL heights and 10 m/s low level shear suggest that an isolated tornado cannot be discounted either and at least a marginal level-one threat should be warranted.
Recent soundings show a rather strong cap over southern France and northeastern Iberia and also over central Italy. If forcing is strong enough for convective initiation especially over southern France / western Mediterranean,strong deep layer shear (25 m/s), moderate CAPE and SRH in order of 300 - 400 J/kg will allow some supercells with large hail / damaging gusts or perhaps a severe MCS to form and an upgrade will be required.
...SE Germany / W Czech Rep...
During the late morning / early afternoon, convective initiation ahead of the cold front will form isolated multicells or perhaps a linear MCS that may produce isolated severe gusts as about 15 m/s at 700hPa are present. Some storms may profit from 20 - 25 m/s deep layer shear and could develop rotating updrafts. Isolated large hail is not ruled out as well. Best chances for tornadoes should exist in the vicinity of the Erz Mountains where low level lapse rates in order of 10 K / km, locally enhanced LLS and strong LL buoyancy are forecast. Storms should move into SW Poland in the evening hours where less instability will be in place and severe threat should be too marginal for a threat level.
...North Sea, Denmark, S UK, Netherlands, N Germany...
In the wake of the trough, upper level cooling and diurnal heating of the boundary layer will lead to some hundred J/kg of CAPE in a weakly sheared environment. Showers and thunderstorms will likely develop and an isolated funnel / brief tornado is not ruled out given low LCL heights and steep LL lapse rates. Most of the CAPE should be surface-based and storms should diminish after sunset.
...NE Belarus, Baltic States, W Russia...
Up to 1000 J/kg MLCAPE are expected to form near the warm front of a large low pressure system. Towards the south, a strong capping inversion should preclude convective initiation but further north, some multicells may develop. Deep layer shear in order of 15 m/s should allow some small hail / gusty winds but organized severe thunderstorms are rather unlikely.
