CONVECTIVE FORECAST
VALID 06Z SAT 16/08 - 06Z SUN 17/08 2003
ISSUED: 15/08 21:58Z
FORECASTER: DAHL

There is a slight risk of severe thunderstorms forecast across Italy.

General thunderstorms are forecast across south and east Europe.

SYNOPSIS

Meandering zonal upper flow has established across northern parts of central Europe ... with an eastward-moving embedded synoptic-scale upper trough over the NE portions of Europe. Rather weak ... slowly eastward-progressing southern-stream trough will affect Iberia during the FCST period. Quiescent conditions prevailing across S and SE parts of the forecast domain. Main low-level baroclinic zone is curving from N France eastwards across the Alpine regions ... N Romania into W Russia ... where it will make some eastward progress ahead of the NE European upper trough.

DISCUSSION

...Italy into central France...
Region of main interest is the central Mediterranean ... especially Italy. 12Z soundings and latest SFC obs indicate impressive thermodynamic setup ... with low-level dewpoints well in excess of 20°C and afternoon temps in the low to mid 30s ... and a pronounced EML atop the BL ... yielding MLCAPEs calculated with virtual temperature in excess of 3500 J/kg (ref LIRE 12Z ascent). Subsidence associated with subtle mid/upper ridging appears to maintain the cap ... preventing the moisture to be mixed out. Large-scale wind field will not be too impressive ... and about 20 to 30 knots shear between SFC and 500 hPa should be realized tomorrow. Low-level flow/shear may locally be rather strong owing to differential heating and chanalling both associated with the complex terrain over Italy.

Allover scenario is quite strongly depending on how intense the ridging will be ... with the cap potentially being too strong in the face of very weak dynamic forcing for upward motion. GFS and GME 12Z runs have weakened the abovementioned ridging somewhat ... and current thinking is that isolated TSTMS will initiate late in the day in upslope flow regime over Italy. Also ... weak/small-scale vorticity maxima are indicated by the models ... which may also be supportive to initiation.

If storms form ... thermodynamic and kinematic environment will be very supportive to damaging winds and large hail. If low-level wind field augmentation is taken into account ... there appears to be some threat of supercellular storms. These will pose the threat for very large hail ... damaging wind gusts and possibly a tornado or two. ATTM TSTMS are expected to be fairly isolated ... and a SLGT risk appears to suffice ... if storms turn out to be more numerous ... an upgrade will be required.

Within WAA regime developing over S and central France during Saturday night ... elevated TSTMS may develop. Some hail and wind threat is accompanying these storms as the EML is advected northwards as well ... providing steep lapse-rates and dry midlevels.

...Balkan...
Airmass over the Balkan States appears to be somewhat drier ... and thermodynamic profiles should not be sufficient for an organized severe TSTM threat given the marginal shear. Weak vort max will likely promote widespread convective activity though ... and a few severe hail/wind events are likely. Later obs have to be awaited to specify severe threat.

...Iberia...
Downburst threat exists primarily over northern parts of Iberia ... where weakly unstable inverted-V type profiles are predominant. Storms should develop towards the afternoon with the advent of the upper trough ... and could evolve into short lines/bow echoes as shear is anticipated to increase ahead of the vort max ... possibly into the 30 to 40 knots range in the lowest 6 km.

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