CONVECTIVE FORECAST
VALID 06Z TUE 28/10 - 06Z WED 29/10 2003
ISSUED: 27/10 21:55Z
FORECASTER: HAKLANDER

General thunderstorms are forecast across the NWRN British Isles.

General thunderstorms are forecast across SRN France and the Mediterranean.

SYNOPSIS

Complex upper flow pattern should change significantly during the FCST period. WSW-ly polar jet across Scotland and Central Scandinavia precedes sharpening trough from the Northern Atlantic, which is FCST across Ireland by WED/06Z. Elongated, WSW-ENE oriented closed upper low extending from France towards the Azores breaks up in shorter waves as it merges with approaching Atlantic trough. Farther east...upper ridge across ERN Europe propagates EWD, reaching Romania and Bulgaria by WED/06Z. Across eastern parts of the Black Sea, an ULL is cut off from an upper trough over Western Russia. Across the Mediterranean, a southerly branch of the jet stream can be identified as a strong 100+ kts W-ly flow at 300 hPa.

DISCUSSION

...Western and Central Mediterranean...
Release of latent instability, which is present through a sufficiently deep layer (negative Showalter indices), is expected to cause scattered thunderstorms. Convective activity over the Ionean Sea is not expected until WED, when CAPE is advected into the area at low levels. Multicells seem the most probable convective mode, as MLCAPE is expected to remain below 1000 J/kg and 0-6 km shear of 30-50 knots is forecast. In the vicinity of Sicily, increased deep layer shear and low level WAA could yield enough SREH helicity for (brief) rotation in some storms on WED night. If a supercell manages to form, wind gusts of 50+ knots and a brief tornado cannot be ruled out. However, this scenario is far too conditional to warrant a SLGT risk.

...Southern and Eastern France and Western Alps...
Nearly saturated GFS 12Z forecast soundings with almost neutral lapse rates indicate little resistance for vertical motion across the area. Significant DCVA due to a shortwave upper trough forces upward motion in the course of TUE, along with local orographic forcing. Advection of high theta-e air from the Mediterranean at low levels could result in marginal MLCAPE and some thunderstorm activity.

...Northwestern British Isles...
Weak CAPE in the polar air mass beneath the cold upper air trough could yield the formation of some thunderstorms late TUE evening or during WED night. Convection might be linear, especially along strong surface troughs which are advected into the area. Nonetheless, low-level and deep layer shear seem insufficient to yield severe wind gusts of 50+ knots.

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