CONVECTIVE FORECAST
VALID 06Z SAT 21/02 - 06Z SUN 22/02 2004
ISSUED: 20/02 23:17Z
FORECASTER: DAHL
General thunderstorms are forecast across western and central Mediterranean Regions ... west-Iberian coastal regions.
SYNOPSIS
Extensive closed upper low currently centered just W of the Gulf of Biscay is covering SW portions Europe and the N African states Morocco ... Algeria and Tunesia. Deep SWLY flow is present E of this feature ... pushing quite warm air from N Africa northwards into the western Mediterranean Regions. Upper trough ATTM over the Black Sea ... will dig south ... and reach the E Mediterranean States late in the period. At low levels ... high pressure and/or stable continental air masses will dominate much of northern ... eastern and southeastern Europe. Large SFC low will remain in place across SW Europe ... though models coherently assume some weakening of this feature during the period.
DISCUSSION
...Western and central Mediterranean...
Some uncertainty with respect to the thermodynamic profiles is posed by the dearth of radiosonde data across N Africa ... Some 12Z soundings show quite steep low-/mid-level lapse-rates which are currently advected into the western Mediterranean (ref. DAOR, In-Salah, Beni-Mellal ascents) ... but these may not be representative of the theta-e plume which appears to have originated from the subtropical Atlantic ... any may thus be characterized by somewhat weaker lapse rates but deeper/richer PBL moisture (as suggested by Tenerife and Funchal 12Z ascents).
SFC wind fields suggest that air mass currently over the south/central Med will be advected underneath the 850 hPa theta-e plume late in the day. Heraklion, Crete, 12Z sounding shows that SFC-based moisture is quite shallow ... as do other soundings across the central Mediterranean. Confidence that this air mass will moisten drastically over the next hours is rather low ATTM.
Friday's LIEE ascents show systematic increase in low-level moisture, though, which looks to be associated with the northward advection of the subtropical/Atlantic air mass ... Current thinking is that this relatively deep/rich SFC-based moisture will be limited to the W portions of the Mediterranean Sea.
Several ill-defined vort maxima are simulated at the E periphery of the SW-European upper low ... which is supported by latest SAT imagery ... GFS and GME 12Z runs are in fair agreement that strong DCVA will overspread the western ... and later in the period the central Mediterranean regions. Current thinking is that slightly elevated TSTMS ... possibly congealing into one or more larger clusters ... will cross the western Mediterranean regions during the day.
Scattered TSTMS may also form primarily along the nose of the theta-e plume where WAA will be maximized ... this activity could spread into the central/south-central Mediterranean and Italy Saturday evening/night. Given advection of rather dry low-level air into these regions ... activity may struggle to become SFC-based.
500 hPa flow will likely be in the 40 to 50 knots range ... possibly augmented near small-scale vort maxima ... and it looks that a few marginally severe TSTM events may occur. ATTM threat for SFC-based storms in the WAA regime is considered rather low ... and effective storm-relative veering in the inflow should not be too impressive for the majority of the cells. It appears that marginally severe wind gusts will be the main threat ... along with some hail. Greatest chances for SVR appears to exist with the west-Mediterranean system.
Some uncertainty exists with this scenario ... but especially the TSTMS expected to cross the W Mediterranean during the first half of the forecast period ... may turn out to require a SLGT with later outlooks.
...West-Iberian Coastal regions...
Cellular convection ... capable of producing scattered CG's will likely persist off the Iberian W coast beneath the upper cold pool. Some of the TSTMS may affect coastal regions of Portugal and NW Spain. Organized severe TSTM threat will be quite low with this activity owing to weak shear/instability.
#