Forecast Update
Valid: Tue 26 Jan 2010 16:00 to Wed 27 Jan 2010 06:00 UTC
Issued: Tue 26 Jan 2010 16:28
Forecaster: TUSCHY

A level 1 was issued for parts of Sardinia, Sicily and surrounding areas mainly for large hail, excessive rainfall and tornadoes (maximized over Sicily).

A level 1 was issued for the area east of the Strait of Gibraltar and far NW-Algeria mainly for excessive rainfall.

SYNOPSIS

The intense polar vortex over eastern Europe remains in place, so no deep convection is forecast over all of Europe, despite the south-central Mediterranean.

DISCUSSION

... Sardinia, Tyrrhenian Sea, Sicily and S-Italy ...

Models diverge in their solutions, where to eject the developing surface depression over the Mediterranean, east of Tunisia. GFS and EZ agree quite well in timing, strenght and path, similar to GEM. The rest of the models refuses to show a clear center and prefer to develop a broad channel of low pressure over Tunisia eastwards. However, as local mesoscale model runs also tend to agree with GFS/EZ and GEM, we decided to weight their solutions more significantly compared to the rest. No surprise that models have difficulties in such an area with sparse data coverage, but also due to the complex nature of this developing depression, highlighted by numerous cyclonic vortices over E/NE-Algeria and Tunisia, seen in synop data and HVIS/WV images.

From now on, convection will be on the increase over the area of interest, already visualized by a rapidly expanding shield of cooling cloud tops over N-Tunisia and eastwards. This region with moderate isentropic lift continues to spread northwards with moderate rain, mainly stratiform in nature. There exist two spots, where deep convection plays a significant role:

First, the northward pushing warm front, which is just about to affect Sicily. The surface front itself will cross Sicily roughly during the midnight hours and an increasingly warm/moist warm sector affects the island. At the same time, lapse rates at mid-levels steepen significantly as EML covers the warm sector, which pushes SBCAPE values in excess of 1000 J/kg. Best forcing only graces the immediate postfrontal air mass, where good SRH-1/LL speed shear and instability overlap. However, 850hPa winds of 20m/s suppress any development of a more stable nocturnal PBL, so sheared/unstable surface parcels may be ingested by thunderstorms. The final thunderstorm coverage is a bit uncertain and may remain more conditional, especially along the warm front itself, but at least an isolated storm may acquire rotation with a tornado and large hail risk. The threat spreads eastwards as the center of the surface depression turns eastwards. Isolated high rainfall amounts are possible, but marginal dynamics and no focused LLJ keep the overall risk low. In addition, surface trajectories indicate a direct path from Tunisia, where dew points are in the lower singles at best. The fetch offshore may not be large enough for a deep, moist layer to evolve.

The second area is found over extreme NE-Algeria, N-Tunisia and Sardinia. The overall wind field north of the developing depression relaxes and reveals an highly diffluent pattern, where a deformation zone sets up. This area is placed northwest of the developing warm front and hence in a more stable stratified environment. However, some offshore CAPE is present due to the overall cold troposphere and isolated, embedded storms are possible. The main concern right now is an area SE of Sardinia, where strong LL convergence is present. If thunderstorms manage to evolve, a NW-SE oriented cluster of training storms is possible, which could cause excessive rainfall over Sardinia. Despite the favorable set-up, this scenario remains a nowcast problem, but we think a level 1 for excessive rainfall will cover that event for now.

... Western Mediterranean ...

Active day with showers and thunderstorms is about to find its end, as the tongue of better LL moisture vanishes/comes ashore over N-Morocco and Algeria during the evening and night hours. A band of training showers/thunderstorms SE of Spain persists, but warming tops offshore and increasing convection in the inflow region over far N-Algeria hint on a gradual decreasing rainfall risk over far SE Spain. The cluster may affect the region east of the Strait of Gibraltar for the following hours and a level was issued for that place but also for current thunderstorm activity over NW-Algeria, which looks quite healthy. The main risk with the thunderstorms will be locally heavy rain, gusty winds and marginal hail.