Storm Forecast
Valid: Fri 09 Mar 2012 06:00 to Sat 10 Mar 2012 06:00 UTC
Issued: Thu 08 Mar 2012 19:31
Forecaster: TUSCHY

A level 2 was issued for Malta and surrounding areas mainly for heavy to excessive rainfall, tornadoes -a strong event possible-, severe wind gusts and large hail.

A level 1 surrounds the level 2 mainly for excessive rainfall, an isolated tornado/severe wind gust and large hail event.

SYNOPSIS

A spacious high pressure area at the surface over W/CNTRL Europe and mid/high-level ridging, building eastwards over CNTRL Europe result in stable and quiet conditions (DMC-wise). A progressive pattern is still present over far N-Europe as zonal flow remains in place. No real forcing embedded in this flow next to scarce moisture influx from the S result in modest convective activity over the Norwegian Sea. An isolated short-lived thunderstorm event is possible mainly over the N-Norwegian Sea but limited coverage precludes a thunderstorm area. Strong to severe wind gusts accompany that activity. A compact upper low/surface depression affect the S-Mediterranean (see side-note) whereas a cyclonic vortex remains anchored over far E-Europe, assisting in CAA over that area.

Side-note:

8th March 12Z model data and WV imagery all show a deep trough over the CNTRL Mediterranean and Algeria with a strong piece of energy dropping southwards (resulting in widespread dust storm events from W-Sahara to Mali to Nigeria). Both, strength and placement of this trough seem to be on track, so confidence in existing model consistency is quite high. A blend of persistent ECMWF/NCEP deterministic/probabilistic data was used for that forecast.

A constantly strengthening/deepening baroclinic zone evolves along the boundary of E-Algeria,Tunisia and W-Libya with all models showing a rapid vortex spin-up along that boundary. Some discrepancies exist when this feature moves out of Libya/Tunisia onto the warm waters of the S-Mediterranean but latest thinking is that a time-frame of 21-00Z seems reasonable (despite experience of past/similar events with delayed timing of depressions moving out of N-Africa). SSTs of 15-17 °C and 500 hPa temperature dropping below -20 °C yield a favorable environment for more robust destabilization offshore. Also the surface depression is expected to move offshore in its occlusion stage already wrapping some LL warm air around the center. A 2-3 days forecast (beyond current forecast responsibility) indicates a slow motion to the NE and E and last phase diagrams also show a low chance for a gradual modification of the center (ref. to a shallow warm core structure). Main inhibiting factor will be utterly dry continental air (latest METAR readings upstream show dewpoints in the low-end single digits over N-Africa), which advects offshore. Also, the offshore fetch is restricted due to the proximity of the depression to land and hence some time is needed for DMC build-up next to the center. All models show only marginal intensification independent of their resolution, but there is at least a low-end chance that a subtropical cyclone evolves over/south of the S-Ionian Sea, before environmental conditions become slightly worse on Sunday. This area will be monitored during following model runs.

DISCUSSION

...Malta and Sicily 18-21 Z onwards ...

As described above, conditions become supportive for a strong cyclogenesis over E-Tunisia/W-Libya, which emerges into the Mediterranean around 21Z. The focus for DMC will be the occlusion, which gradually shifts northwards and affects the area of interest after ~ 18Z. Conditions within the occlusion become increasingly favorable for a backward building MCS event as 20-25 m/s winds at 850 hPa fuel thunderstorms with adequate moisture....in fact GFS 12Z forecast soundings from Malta reveal wind speeds of 30 m/s at 900 hPa. South of Sicily a favorable overlap of 500-800 J/kg MLCAPE and intense shear (0-1km speed shear of 20 m/s and SRH-1 of 400 J/kg) exist between 18-00 Z, before shear relaxes thereafter as the occlusion gradually works its way to the north and affects Sicily until 06Z.
Current thinking is that a well organized W-E aligned MCS affects areas west of Malta all the way to N-Tunisia with heavy to excessive rainfall. In Malta and further to the east, probabilities for more discrete storms/tail-end charlie events increase due to some influx of drier air from the south. Embedded in aforementioned environmental conditions, severe to damaging wind gusts and tornadoes - an isolated strong one - are forecast. Large hail also accompanies stronger storms. After 00Z the main risk changes more to a heavy rainfall risk, also affecting Sicily until 06Z. Still an isolated tornado/hail event remain possible. A level 2 was added due to the risk of a few significant events. Also the level 1 was expanded far to the west (outside the thunderstorm areas due to repeatedly onshore moving showers/thunderstorms with high rainfall amounts).

A few thunderstorms may occur further to the north /e.g. S-Adriatic Sea or Ionian Sea /, but the overall coverage remains limited and the activity ends during the day. Nothing severe is forecast although an isolated spout event can't be ruled out given still marginal favorable set-up.