CONVECTIVE FORECAST
VALID 06Z WED 26/11 - 06Z THU 27/11 2003
ISSUED: 26/11 05:54Z
FORECASTER: GROENEMEIJER

There is a slight risk of severe thunderstorms forecast across southern Italy and the southern Adriatic region including parts of the extreme western Balkans.

General thunderstorms are forecast across much of the central Mediterranean and across southern and western parts of the British Isles as well as western France and parts of northern Spain.

SYNOPSIS

A longwave trough is deepening over western Europe. A surface low is located slightly to the northwest of Ireland moving northeastward. A quite unstable polar air-mass is expected to work its way eastward behind a cold front reaching western continental Europe before the end of the forecast period. A trough with a small embedded closed low is located over the Thyrrenean sea, lifting northward and merging with the westerly flow to the north. A moderately strong (60kt @ 500hPa) mid/upper jet streak is moving northward over Italy.

DISCUSSION

...southern Italy and the southern Adriatic region including parts of the extreme western Balkans....
Soundings show that at 00Z some modest latent instability is present in the air-mass to the east of a north-south oriented cold front, that is located over the Thyrrenean sea and Sicily at the beginning of the forecast period. Some storms have developed in this air-mass near Sicily and storms may develop at some other places. Quite strong low-level shear... up to 30 - 40 kt 0-1 km shear ... is forecast by numerical models as well as elevated low-level storm relative helicity values. In combination with low LCL heights and negligible convective inhibition, as was measured in Brindisi (LIBR) at 00Z, this suggests some rotating updrafts/supercells may well occur, that could produce a few tornadoes, severe wind gusts and large hail.

Apart from these, non-supercell convection may also cause wind gusts exceeding severe levels as vertical exchange of of horizontal momentum in storms will likely be adequate to transport the 40-50kt winds forecast at 925 hPa down to the ground.

The storms will likely be quite scattered and, given that latent instability is quite modest, only some of them will produce severe weather, so that a risk level higher than slight risk does not seem necessary.

The storms will likely affect Sicily and extreme southern Italy during the morning hours and then work their way northward during the afternoon. Storms occuring near and over Greece as well as those over the northern Adriatic will have a smaller chance of becoming severe as the shear profiles are not as strong. Therefore these areas have been excluded from the categorical risk.

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