CONVECTIVE FORECAST
VALID 06Z THU 09/09 - 06Z FRI 10/09 2004
ISSUED: 09/09 09:37Z
FORECASTER: VAN DER VELDE

There is a slight risk of severe thunderstorms forecast across southwestern France, central and north-central Spain

General thunderstorms are forecast across parts of Spain, western France, and extreme southwestern British Isles and across the central Ukraine, the Black Sea, and parts of Moldova.

SYNOPSIS

Strong blocking high pressure area dominates most of Europe with stable conditions. An upper/sfc low is still residing on the Atlantic northwest of the Iberian Peninsula, and causes strong warm air advection over Spain, and thereby causing veering winds with altitude. A weak cold front, or line of convergence, separating Atlantic air from North-African air signifies the western edge of a theta-e ridge swirling around the low, reaching also the Srn British Isles by the end of the period. Over the Ukraine and Black Sea, an upper trough is forecast to induce a sfc trough by CVA. Cold air will spread out over the Black Sea, generating very steep low-level lapse rates.

DISCUSSION

...SLGT risk area...
Deep layer shear has diminished somewhat since the beginning of the unstable period days almost a week ago. 0-6 km bulk shear is generally still around 15 m/s, allowing well-organised multicell storms, especially with the WAA-induced clockwise turning hodographs, which indicates potential for rotating updrafts. Eastern Spain is forecast to be under strong subsidence, and the main trigger seems the convergence line. GFS 12Z forecast 700 hPa fields show some short waves moving along the convergence line over France, and these are associated with strong ascending motion. Given 500-1500 J/kg CAPE, the moderate shear, and relatively dry midlevels (delta -theta-e > 15 K), MCSses and a few supercells could form over Spain and southwestern France that pose a threat of severe gusts and large hail. Over Nrn Spain and SWrn France, where LFCs are lower and low level buoyancies higher, a tornado belongs to the possibilities as well. The 0-1 km shears of around 10 m/s also support this, as well as the enhanced deeper shear forecast by GFS 12Z.

...Srn Ukraine and Black Sea...
A few hundred J/kg CAPE seems to be possible, and will form mostly in lower shear conditions within the upper jet loop around the trough. With increasing low-level instability, low LFCs and steep lapse rates (sfc-1 km), especially over sea, waterspouts seem likely.

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