Storm Forecast
Valid: Mon 12 Jan 2009 06:00 to Tue 13 Jan 2009 06:00 UTC
Issued: Sun 11 Jan 2009 18:07
Forecaster: TUSCHY

SYNOPSIS

An extensive channel with low pressure runs from Iceland to NW-Norway beneath strong west/southwesterlies. The airmass is only marginal unstable but a few lightning strikes can occur along the west-central coast of Norway. No thunderstorm area will be introduced. A broad upper trough also approaches W-Europe during the evening and night hours. The airmass is weakly capped, moist and global models show moderate instability release/deep convection. However, there are no signals of deep convergence over the far E-Atlantic and the cold airmass at mid-levels gets warmer during the night hours, so no thunderstorm area was issued although a very isolated thunderstorm could occur. A strong upper low over N-Africa shifts to the northeast and affects the central Mediterranean. NE-Europe remains cold and stable.

DISCUSSION

..Tunisia,Malta and Sicily...

The potent upper low finally lifts northeastwards and approaches the highlighted area from the SW at 21Z onwards. At the same time, a surface depression consolidates over W-Libya/S-Tunisia and favorable placement in the left exit region of a 60m/s high-level jet maximum and a strengthening N-S aligned baroclinic zone just east of Tunisia support a constant strengthening trend during the morning hours, as the depression lifts north. Global models agree well in timing and track which is not self-evident in such a region with limited data coverage.GFS/GME remain the most aggressive models.

However this evolving surface depression is just a part of a much broader surface vortex, covering Tunisia and E-Algeria. The most likely scenario with that constellation will be persistent moisture advection to the west, affecting N/NE Tunisia and the coastal areas of NE Algeria. As the upper low draws near during the evening hours and upper divergence increases, scattered thunderstorms develop over those areas and this activity persists during the night. DLS of roughly 15m/s and 200-500J/kg MLCAPE don't assist in widespread orgainzed thunderstorms although enhanced directional shear fosters an isolated large hail and strong wind gust risk.

00Z onwards, the thunderstorm activity will increase significantly over E-Tunisia and later-on also over/south of Sicily as cold mid-levels arrive. The instability is on a rapid increase and a tongue of 1000 J/kg MLCAPE evolves east of Tunisia/south of Sicily and a cluster of thunderstorms will develop over E-Tunisia and just offshore. Further east (south of Malta), initiation is not expected due to the long time, the continental airmass needs to ingest moisture over the 17-18°C warm Mediterranean. So the final focus for thunderstorms will be the brisk northward moving warm front, affecting areas north of Malta after midnight. Surface based convection between Malta and Sicily is possible in a strongly sheared environment with SRH-1 well above 200 m^2/s^2 along and south of the warm front. It is still not clear how widespread initiation will be, but a level-2 may be issued during the day for this particular area. Tornadoes, severe wind gusts and large hail can accompany those storms and the risk will spread north/northwestwards beyond the forecast period.