CONVECTIVE FORECAST
VALID 06Z WED 31/03 - 06Z THU 01/04 2004
ISSUED: 30/03 20:07Z
FORECASTER: GATZEN

There is a slight risk of severe thunderstorms forecast across southern central Mediterranean

General thunderstorms are forecast across northern central France

SYNOPSIS

Central European ridge and troughs over northern Iberian Peninsula and the Black Sea are present. As European High ridges northward into Scandinavia, intense troughs will amplify over western and eastern Europe. Iberian Peninsula trough will be disrupted as the northern part will be pushed northward while the southern vort-max and coupled jet streak will move eastward across the southern Mediterranean.

DISCUSSION

...Southern central Mediterranean ...
Models suggest that a strong African jet streak will travel over the souhtern central Mediterranean at the southern flank of advancing vort max, leading to strong synoptic forcing. Latest satellite images show a well-developed warm conveyor belt south of the jet streak. During the forecast period, cyclogenesis is expected over the southern central Mediterranean, leading to warm air advection over the central Mediterranean. As the surface low moves northward later on, a cold front will reach the affected region from the west. There is no doubt that synoptic forcing and vertical wind shear will be favorable for severe convective development, with low-level easterly winds, southwesterly winds @ 850 hPa and 60+ m/s wind speed @ 300 hPa. However, CAPE will be limited over most of the affected region. GFS model output suggests that CAPE will form in the range of a Theta-E axis from Tunisia to Sardinia that spreads ESEward into Sicily later on. Latest sounding over Cagliari shows that mixed mid-level layers are present and low-level moisture increases underneath the cap. GFS suggests surface dewpoints near 15 C by tomorrow afternoon, what seems to be a reasonable scenario. For this reason, we expect CAPE reaching several hundreds J/kg over the affected region. As the jet streak approaches ... strong synoptic forcing will act on the warm airmass and thunderstorms should form from Tunisia to Sardinia. Strong low-level and deep layer shear is expected and thunderstorms should organize into linear multicells and embedded mesocyclones, moving northeastward. Large hail and strong wind gusts should be the main severe threat. Tornadoes may be possible especially where low-level capped airmass will be very moist, leading to low LCL heights. However ... instability will be marginal over the northern portions of the affected region, and it is still questionable if this scenario comes true. Later on ... synoptic forcing should increase to the southeast, and thunderstorms are expected east and along the approaching cold front. As low-level wind shear will decrease gradually, risk for mesocyclones and associated large hail/tornado threat will be significantly lower. However ... a MCS may form over or south of Sicily, and strong wind gusts and intense precipitation are expected.

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