Forecast Update
Valid: Thu 17 Aug 2006 15:00 to Fri 18 Aug 2006 06:00 UTC
Issued: Thu 17 Aug 2006 14:52
Forecaster: GATZEN

SYNOPSIS

Intense long-wave trough present with an axis from Scandinavia to western Iberian Peninsula ... with closed low over Bay of Biscay. To the east ... subtropic ridge has formed over central Mediterranean ... where very warm air mass (around 25°C @ 850 hPa) from northern Africa spreads northward. Between both features ... very strong upper jet is forming from Morocco to Alpine region SW of intense vort-max that rotates around Biscayan trough center. At lower levels ... cold front/occlusion of Biscayan surface low pressure system moves north-eastward crossing Iberian Peninsula and France today. Ahead of this cold front ... moist air mass is present over most of western Europe ... with mixing ratios of 10..12 g/kg over eastern/northern France, south-eastern British Isles, western Germany, southern North Sea region, Alpine region ... and 12..16g/kg over western Mediterranean. West of the cold front ... cool and convectively mixed maritime air mass spreads eastward into British Isles and France during the period. Low-level moisture has mixed out especially over most of Iberian Peninsula and south-western France.

DISCUSSION

North-eastern/eastern France, Benelux, western Germany

Ahead of the main vort-max/short-wave trough ... southeasterly winds are present over eastern and northern France ... creating strong vertical wind shear. Latest soundings indicate that instability has formed over a broad region ... and thunderstorms have already formed over northern and eastern France. Current thinking is that convective activity should go on during the next hours ... and chance for mesocyclones and supercells should be significantly enhanced. Thunderstorms should merge into one or two MCS in the evening hours due to increasing QG forcing along the cold front that move eastwards into Germany and Benelux ... capable of producing severe wind gusts and insolated large hail/tornadoes ... but severe potential should gradually weaken during the night hours over Germany. West of the cold front ... cool maritime airmass should allow for additional thunderstorms. Enhenced vertical wind shear wil be present ... and isolated severe wind gusts and tornadoes should be possible.

South-eastern France, northern Italy

Latest soundings indicate that significant CAPE has formed over northern Mediterranean, south-eastern France, and northern Italy in the range of warm air mass south of the mean frontal boundary. Moist low-level airmass is advected north-westward into France ... where thunderstorms have already formed along a warm front/convergence line. Quite strong QG forcing is expected to continue during the next hours as upper vort-max/jet streak travels north-eastward over northern Mediterranean/northern Italy ... and models predict deep convection. Limiting factors should be many clouds as well as fog over parts of Italy ... inhibiting insolation. As vertical wind shear will be strong ... with 10 m/s LLS and 30 m/s DLS expected by latest GFS model run ... thunderstorms should organize into multicells and mesocyclones ... capable of producing large hail and severe wind gusts. Given inversion over most places ... chance for tornadoes is expected to be rather weak .. but an isolated event ... probably a strong one ... is not ruled out. Later in the period ... main vort-max of Biscayan trough reaches the region ... providing strong forcing along the propagating cold front. Although amount of instability is unclear ATTM due to afternoon convection ... CAPE should be positive over most places ... and given moist low-level air mass and very strong vertical wind shear ... at least isolated/embedded cells should produce severe events ... and chance for tornadoes should be enhanced. Additional thunderstorms may form later on over northern Mediterranean in the range of the vort-max that may affect Italy in the morning hours. Severe wind gusts should be the main threat ... while isolated large hail is not ruled out and potential for tornadoes should be slightly enhanced.

Turkey

Steep lapse rates and impressive EML is present over Turkey. Every storm that will form during the next hours will have a potential for downbursts. Chance for severe weather seems to be rather isolated ... and we don't issue a cathegorical risk.