CONVECTIVE FORECAST
VALID 06Z SAT 09/10 - 06Z SUN 10/10 2004
ISSUED: 08/10 19:21Z
FORECASTER: DAHL

There is a slight risk of severe thunderstorms forecast across central and S France ... extreme NE Spain.

General thunderstorms are forecast across France ... Spain ... SW Germany ... Switzerland ... SW Mediterranean Sea.

General thunderstorms are forecast across Turkey and the Black Sea.

SYNOPSIS

Upper trough off the Iberian W coast ... is progged to continue to close off into an upper cut-off low ... and move into the Gulf of Biskay by the end of the FCST period. Elongated vort max at its E periphery ... should spread across E Spain and France on Saturday. Building upper ridge over the N Atlantic is expected to move into the North Sea towards late Saturday night ... with downstream long-wave trough progressing eastwards into N-central and NE Europe. At low levels ... intense SFC low will likely persist over the SW Biskay ... with trailing cold front progged to curve from the Bretagne across central France and E Spain into the Strait of Gibraltar ... with the warm-frontal boundary stretching from N France into S Germany/Austria by Saturday 12Z.

DISCUSSION

...SLGT area...
One focus for convective development will likely be the cold front over France. Ahead of this front ... narrow plume of weakly unstable suptropical/Atlantic air mass is present as of Friday evening ... while farther E an EML is present atop Mediterranean moisture ... yielding somewhat more interesting thermodynamic profiles though instability remains rather weak. Exact nature of Saturday's thermodynamic profiles is somewhat uncertain ... but steep mid-level lapse rates are like to exist over portions of France and the SW Mediterranean ... yielding strong capping and some instability. 500 hPa flow is FCST to be on the order of 20 m/s ... increasing over S France to 25+ m/s after 21Z. SWLY 850 hPa flow will be on the order of 10 m/s per MM5 ... slightly increasing late in the evening. In accordance with this ... GFS advertises 30+ m/s 0-6 km shear over S France ... and 10 m/s 0-1 km shear. Main DCVA-related UVV's are progged to overspread the front over central France and NE Spain after 18Z ... but degree of ascent should be sufficient to compensate for the somewhat adverse timing. Also ... convection may develop ahead of the cold front along outflow boundaries laid out by Friday's convection and beneath patches of DCVA that are expected to spread across the pre-frontal air mass. Current thinking is that TSTMS will (re)develop over France ahead of the cold front ... and later in the day over NE Spain and central France along the cold front. Greatest severe threat should exist with the storms along the cold front where shear should be maximized. Main convective mode should be short/bowing lines with the potential for severe straight-line winds ... though a few mesocyclones may occur as well ... additionally posing a threat for large hail and for a tornado or two especially late in th evening. TSTMS may spread into Switzerland and SW Germany towards early Sunday morning.

...NW Spain...
Mainly elevated TSTMS may occur farther N and NW along the warm-frontal boundary and closer to the low center withing theta-e plume spiraling into the center. If SFC-based convection managed to form over NW Iberia ... potential for rotating updrafts and attendant severe weather would be quite high given 0-3 km SRH in excess of 400 m2/s2. However ... current thinking is that any TSTM will likely be imbedded in stratiform precip and elevated.

...SW Mediterranean Sea...
Small/fast vort max is progged by GFS 12Z over the W Mediterranean early Sunday morning. This would enhance the TSTM threat along the cold front over the SW Mediterranean towards the end of the FCST period. Strong enough shear profiles for severe will likely be in place ... but later OBS have to be awaited to assert existence of this vort max.

#