CONVECTIVE FORECAST
VALID 06Z SUN 22/02 - 06Z MON 23/02 2004
ISSUED: 21/02 19:50Z
FORECASTER: DAHL

General thunderstorms are forecast across the westen and central Mediterranean ... coastal regions of Portugal and Spain.

SYNOPSIS

SW European upper low will merge with the digging North Atlantic upper trough late in the period ... this trough is expected to reach the North Sea towards Sunday morning. This will result in a broad WSWLY upper flow over the Mediterranean regions late in the period. Though SFC low ATTM covering SW portions of Europe is progged to continue weakening ... imposing WAA regime will be maintained at its E periphery through the period ... pushing mainly dry air masses from the Sahara desert into the central Mediterranean regions.

DISCUSSION

...West and central Mediterranean ...
Plume of steep low/midlevel lapse rates is embedded in the SLY/SWLY flow over the central Mediterranean ... which has resulted in a deep EML across central Mediterranean regions. W of this plume ... moist marginally unstable subtropical-Atlantic air mass is present and also advected N/NE. TSTMS fave formed within the Saharan plume ... and along the main cold front at the W edge of the Atlantic air mass over NW Algeria.

Main problems remain uncertainties about the thermodynamic profiles ... especially PBL-moisture depth over the central Mediterranean ... and difficulties with the position/timing of the various vort maxima embedded in the upper flow.

Current thinking is that air mass in the thermal-ridge axis will be quite dry ... and TSTM threat may be quite limited. Even if low-level moisture turns out to be higher than currently expected ... a strong cap is present ... and chance for SFC-based convection appears to be quite remote ATTM. Though any storms that develop will likely be elevated then ... dry environment may promote strong downdrafts ... which may penetrate the shallow cool SFC-based layer ... and produce a few strong wind gusts ... some possibly being marginally severe. Also ... some hail may occur. Potential foci for TSTM development will be the nose of the theta-e plume where low-level WAA is maximized ... and regions DCVA-related lift ahead of the various small vort maxima.

Main cold front is progged to curve across the west Mediterranean Sea at the beginning of the period ... DCVA-related lift overspreading this boundary will likely maintain convection along the front. CAPE should be fairly meager owing to weak lapse rates ... but deep-layer shear on the order of 40 to 50 knots will likely be present. Saturday's rawinsonde data suggest that mesoscale wind fields accompanying the vort maxima are quite stronger than what is offered by the models ... and 0-4 km shear may well be in the oder of 60 knots over the NW Mediterranean late on Sunday. Any TSTM that forms will have potential for producing severe straight-line winds ... and possibly small hail. However ... weak thermodynamic support precluding a SLGT ATTM.

Farther west in the deep polar air beneath the upper thermal low ... cellular convection will likely persist. Chance of SVR TSTMS remains rather low given weak shear/instability.

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