Forecast Update
Valid: Mon 12 Jan 2009 19:00 to Tue 13 Jan 2009 06:00 UTC
Issued: Mon 12 Jan 2009 18:44
Forecaster: TUSCHY

SYNOPSIS

Please refer to the outlook, issued at Sun 11 Jan 2009 18:07 UTC.

DISCUSSION

...Tunisia, Malta and Sicily...

Latest IR/WV loop indicates that intense low-/mid-level jet has set-up over NW Libya as shield of Cs/Ci spreads north/northeastwards in WAA regime. Well anticipated by GFS is the intrusion of dry, lower stratospheric air over SE-Tunisia and extreme NW Libya, which builds northwards betimes. The surface pressure over S/SE-Tunisia was falling with readings around 1006-1008hPa (15Z) and synop. data hints on a developing, broad surface vortex over east-central Algeria/S-Tunisia and NW Libya, which was forecast by most of the models. Run-to-run consistency about the final track of the depression still remains somewhat uncertain. Most of the models now have a path along the E-coast of Tunisia to the north during the night hours or along a gradual eastward moving and constantly strengthening moisture convergenze zone, which is forecast to run from extreme NE-Tunisia towards the SE, separating dry air to ist south/west from warm/moist air to its east. Latest high resolution Met-9 images support this scenario with rapidly eroding St-fields just off the coast of SE-Tunisia/NW-Libya.

Upper levels look favorable as diffluent left-exit region approaches from the south and a compact vorticity maximum enters the area also from the south after midnight. Focus for initiation should be the aforementioned convergence zone with only minor movement towards the northeast.

The well defined warm sector just ahead of this convergence zone is upgraded to a level-2 as conditions are very favorable for severe thunderstorms. 20-25m/s at 850hPa and about 40m/s at 500hPa with SRH-1 running from 100-250 m^2/s^2, LCLs below 500m and a moist 0-2km inflow layer are present and tornadoes/strong to severe wind gusts and large hail are well possible. Initiation in the level-2 area may occur between 03-06Z.

... Coastal areas of NW Spain ...

A sharp upper trough draws near from the west with an intense vorticity coupled present in 12Z runs. Modified IR images show a patch of enhanced convection just along the tip of the trough with pulsating, weak cells. Current thinking is that increasing convergence along the coast, a tongue of higher mixing ratio at the surface pointing to this region and a rapid drop of mid-level temperatures could result in an isolated, short lived thunderstorm event and hence a small general thunderstorm area was introduced. Marginal hail and strong wind gusts accompany showers/thunderstorms.

GFS06Z also had a favorable environment for isolated thunderstorms just west of Wales (00Z onwards), but 12Z already backed off and latest satellite images show only a few isolated convective cells SW of Ireland. No thunderstorm area was introduced, but still a very isolated thunderstorm could occur just off the coast of SW-UK until 06Z.