CONVECTIVE FORECAST
VALID 06Z WED 18/08 - 06Z THU 19/08 2004
ISSUED: 18/08 00:49Z
FORECASTER: GROENEMEIJER

There is a slight risk of severe thunderstorms forecast across the southeastern UK, the low countries, Denmark, a large part of Germany, much of France and Switzerland.

General thunderstorms are forecast across much of western Europe, central Europe, the Baltic States, southern Scndinavia, and a part of Russia as well as the area around the Black Sea.

SYNOPSIS

Wednesday at 06Z... a mid/upper low pressure area is located about 200 km SW of Ireland. A ridge is located approximately along a line from southern Italy to Belarus and a trough over the Black Sea. An intensifying SWly jet is present over western continental Europe. Most attention this forecast periods is given to a shortwave trough forecast about 400 km west of northern Portugal, that is expected to move rapidly NEward reaching Britanny at 18Z and the German Bight by 06Z Thursday morning.

DISCUSSION

...slight risk area...
Across most of the slight risk area a slightly unstable air-mass is expected as was the case yesterday. Around 500 - 1000 J/kg of CAPE will likely form in these areas in response to diurnal heating. Scattered storms will likely form as a result, with the highest coverage over western Germany, the Low Countries, the UK part of the area and the northern half of France. As deep-layer shear increases these may well include supercells. Despite the low CAPE, these may produce isolated large hail and strong or severe winds. Given that low-level shear is expected to be around 10 m/s and LCL heights are rather low, there is also a threat of tornadoes with these storms.

During the early afternoon, strong QG upward motion will begin to affect western France. It seems likely that widespread convection will form as a result, that will quite rapidly organise into linear MCSs. Corfidi vectors and the speed of the shortwave indicate that bow echos may develop that propagate downwind at a speed of 20-25 m/s to the northeast. Severe wind gusts will likely occur in quite a number of places. A threat of tornadoes will also be present with this system. GFS (12 and 18Z) indicate, the activity will be near Britanny at 18Z, over the Low Countries at 00Z and over northern Germany at 06Z, UKMO (12Z) is approximately 3 hours faster. An upgrade to moderate risk may be given for a specific area in the path of the the convective system(s) as more thermodynamic information about the pre-storm environment becomes available.

...remainder of the UK, Ireland...
Weaker shear than in the risk area is expected here. Still a few rather strong multicell clusters are expected in the area, that may cause a few marginally severe events. Low LCL heights suggest funnel clouds, water- or landspouts could occur.

...western Mediterranean area...
Weaker forcing for upward mtion will likely affect the indicated area so that few storms are expected in the area. Since rather high CAPE being in place over the western mediterranean and deep-layer shear is forecast to be quite strong, any storm that does develop may become severe and pose a threat of large hail and strong to severe wind gusts.

...Southern Ukraine, Black Sea...
Near the trough over the Black Sea area clustered convection is expected to continue during the perios in response to synoptic scale ascent. Clustered convection may produce isolated severe events, but low shear hinders the formation of well-organised severe storms. Therefore, overall severe threat does not warrant a risk category.

...northern Poland, southern Baltic Sea...
A weak mid/upper trough is expected to aid the formation of convective storms across the indiacted area. As low-level shear is expected to be relatively strong (about 10 m/s 0-1 km shear) there is some potential for short-lived rotating updrafts that may cause a few brief severe events. overall threat however, seems to be to low to warrant a risk category.

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