Storm Forecast
Valid: Sun 30 Oct 2011 06:00 to Mon 31 Oct 2011 06:00 UTC
Issued: Sat 29 Oct 2011 21:16
Forecaster: TUSCHY

A level 2 was issued for NE/CNTRL Tunisia mainly for excessive rainfall amounts.

A level 1 surrounds the level 1 mainly for an heavy rainfall risk.

SYNOPSIS

A strong upper low over NE-Algeria continues to move to the east during the forecast and will cross Tunisia until the night hours. The belt of strong westerlies remains displaced well to the north, so high geopotential heights and a cool BL assist in stable conditions over the rest of Europe.

DISCUSSION

... NE-Tunisia, Malta and Sicily ...

A well pronounced upper low, centered over NE Algeria at 21 Z (29th October), gradually shifts eastwards while weakening. Nevertheless, vortex strength throughout the troposphere indicates that this weakening trend won't have any significant effects on our current forecast philosophy. A dryslot currently works its way towards central Tunisia and my affect the highlighted area tomorrow during the daytime hours (creating a sharp precipiation gradient towards the SW/S).

The placement of the cyclonic vortex allows a large inflow fetch of moist/warm air to become established east of Tunisia. Latest data reveals surface dewpoints just shy below 20°C in this area and a broad 10-15 m/s 850 hpa jet is expected to evolve....aligned nearly parallel to the moisture fetch. The path of this depression also favors the set-up of a deformation zone over N-Tunisia, which is already in progress over NE-Algeria [21 Z (29th October)]. Finally, near orthogonal onshore flow allows maximized orographic lifting effects, which may also enhance rainfall amounts. With already ongoing backbuilding storms just offshore of NE-Algeria [21 Z (29th October)], confidence in the development of slow moving /training storms in the level 2 area is high. Models agree well in placing the QPF maximum over central/N-Tunisia with amounts of well above 100 mm/24 h shown in some global model data. Despite the fact that this does not yet justifies a level 2, we think conditions are present for a backbuilding (V-shaped MCS) cluster of storms next to the coast of NE/CNTRL-Tunisia, where local rainfall amounts may easily exceed those values .

We expanded the level 1 area well to the NE, to include parts of Sicily. Although model discrepancies in the precipitation forecast remain high, conditions are also supportive for slow moving storms, which may affect the E-coast of Sicily during the evening/night hours, as the upper low draws near from the west. Heavy to excessive rainfall will be the main risk.