Storm Forecast
Valid: Wed 06 Aug 2008 06:00 to Thu 07 Aug 2008 06:00 UTC
Issued: Tue 05 Aug 2008 21:57
Forecaster: GATZEN

SYNOPSIS

Active weather period continues over Europe. Another strong upper trough moves eastward on WED. The axis of this trough will stretch from British Isles to Bay of Biscay and further to Iberian Peninsula. A strong mid-level jet spreads north-eastward ahead of this trough curving around a ridge over central Europe. To the south, a westerly flow is present over the Mediterranean, and embedded short-wave trough will move into Italy during the period. A well developed EML is expected to spread into France ahead of the western trough, and latest soundings indicate steep lapse rates up to the 500 hPa level. Over eastern Europe, cold air advection dominates in the wake of a low pressure system over western Russia.

DISCUSSION

Parts of France, Belgium, the Netherlands, north-western Germany

Quite warm EML will spread across France east of the propagating trough. 850 hPa temperature is expected to reach around 20°C over central France in the afternoon hours. Given quite dry and relatively cool boundary-layer, a strong capping inversion will likely suppress deep convection most of the day except of some thunderstorms that are forecast along a warm front near the North Sea that will likely be elevated due to the cool boundary-layer. In the evening hours, upper height falls are forecast over western and northern France in the range of the propagating upper trough. Models also indicate that some QG forcing will result in low surface pressure over central France, and moisture flux convergence is forecast to lead to rich boundary-layer moisture from south-western France to central France in the evening hours, leading to CAPE values in the order of 1500 J/kg. In the evening hours, a cold front will move south-eastwards into north-western and western France, and thunderstorms will likely develop along this front, spreading north-eastwards. Thunderstorms that form will likely evolve into supercells given strong vertical wind shear and favorable veering profiles. 0-3 km vertical wind shear will be in the 10-15 m/s rage ahead of the approaching cold front in the evening hours. Large hail and severe wind gusts are expected to be the most significant threat. Thunderstorms may merge into a linear MCS ahead of the cold front given quite intense cold pools. Thunderstorms will likely spread into central France during the night hours, while severe threat is expected to remain. To the north, low-level moisture development is more questionable. However, it seems that quite strong QG forcing evolves at the northern edge of a tongue of warm air mass spreading northward just east of the approaching cold front. This will likely result in convective initiation to the north of the surface low located over central France in the evening hours. Strong vertical wind shear will be present and supercells are forecast, capable of producing large hail and severe wind gusts. An isolated tornado is not ruled out from northern France to Belgium and the Netherlands, where locally favorable veering profiles may materialize in the range of the warm front. Latest models indicate that the best potential may be present over Belgium, where strong southerly low-level jet is forecast in the evening/night hours. Thunderstorms will likely merge into an MCS moving north-eastwards along the North Sea coast into north-western Germany, while right-moving clustering storms will likely spread into western Germany in the night hours. Severe threat is expected to weaken given limited instability, but severe wind gusts are not ruled out along the leading gust front.

South-eastern British Isles

A moist maritime air mass is forecast ahead of the approaching cold front over British Isles. To the north of low pressure over France, low-level convergence will likely be present in the afternoon/evening hours, while warm air advection goes on ahead of the cold front. Instability is rather questionable given rather cool boundary-layer. During the afternoon/evening hours, strong DCVA will spread eastwards to the north of strong mid-level jet streak, and convection will likely develop. Current models agree that convection will be more elevated, but a surface-based storm is not ruled out that will likely profit from strong vertical wind shear exceeding 25 m/s in the lowest 6 km. Multicells and supercells are not ruled out, capable of producing strong or severe wind gusts. Tornadoes are also possible with isolated, surface-based storms given rather strong 0-3 km vertical wind shear. Limiting factor is uncertainty of instability, and observations of low-level buoyancy have to be checked in the afternoon hours.

Northern Italy

10-15 m/s DLS and strong instability release with up to 1000 J/kg MLCAPE will be present as a short-wave crosses the area from the west during the midday hours. Song diurnal heating and low-level convergence due to sea-breeze fronts will likely allow for initiation. Large and isolated very large hail, but also severe wind gusts due to strong downdrafts will be the main threat from those storms. Convective activity will likely weaken in the evening hours.