Storm Forecast
Valid: Thu 22 Oct 2009 06:00 to Fri 23 Oct 2009 06:00 UTC
Issued: Wed 21 Oct 2009 23:52
Forecaster: TUSCHY

A level 2 was issued for parts of the western Mediterranean, the Tyrrhenian Sea, W/SW Italy and parts of Sicily mainly for excessive rain, tornadoes, severe wind gusts and large hail.

A level 2 was issued for parts of the E/NE coast of the Adriatic Sea mainly for excessive rainfall.

A level 1 was issued for the surrounding areas, including the western and the central Mediterranean.

SYNOPSIS

A highly negative tilted trough affects western and central Europe and induces a strong surface depression over the western Mediterranean. Another impulse further northeast affects the central Mediterranean with unsettled conditions. The rest of Europe remains cool and stable.

DISCUSSION

... Italy, Adriatic Sea and parts of the Ionian Sea...

Eastward traveling vorticity lobe slows down and fades away during the forecast as another impulse to the west digs southeastwards, increasing geopotential heights over the area of interest. Lowest surface pressure is confined to N-Italy and the Alpine region, which places the Adriatic Sea and adjacent areas under a strong southerly wind field. Maximized moisture transport, orographic lift, increasing parcel layer depth and good moisture quality of the inflow airmass (10-12 g/kg averaged mixing ratio) indicate the risk for probably numerous more or less structured backbuilding MCSs (the most vigorous one over NE-Italy, SW-Slovenia and W-Croatia). A level-2 for rain was issued mainly in those areas, where persistent orographic lift raised the confidence.
Next to the rain risk, an isolated tornado along the coast is possible as LL shear increases rapidly onshore.

Further south, over S-Italy and parts of the northern Ioanian Sea, thunderstorms keep going all day long with abundant MLCAPE and up to 15m/s DLS, so multicells and isolated supercells with large hail, strong to severe wind gusts and locally flash flood producing rain are forecast. In addition, some directional shear in the lowest 1000 m AGL and low LCLs point to an isolated tornado threat. The daytime thunderstorm risk will give way to the second and more severe round of storms during the night, so more details on that area later in the outlook.

A third area of interest is central Italy during the morning hours until roughly noon. Strong LL shear and some CAPE release indicate an augmented tornado risk. The final degree of tornado risk depends on the strength of the surface wave, but GFS insists on up to 20m/s LL shear with 250 m^2/s^2 SRH1. LCLs below 500m combined with such high kinematic values even point to the possibility for strong tornadoes, if the PBL remains buoyant/uncapped. Increasing cloud depth and moist/strong onshore flow make this a likely scenario. The risk diminishes rapidly until 12 UTC.

... West/central Mediterranean ...

All models agree more or less in the development of a vigorous depression west of Sardinia with a central pressure around 995hPa although ECMWF is a bit slower compared to the rest of the model pool. A strong cold front reaches the western Mediterranean during the morning hours and continues eastwards, probably in form of an organized MCS. A 20-30m/s jet at 850hPa overspreads the unstable warm sector of the developing depression and assists in the rapid eastward propagation of the cold front. The main risk with this activity will likely be severe-damaging wind gusts, probably widespread in nature. This risk spreads eastwards, crossing Sardinia between 12-15 UTC and thereafter affecting the Tyrrhenian Sea.

An even more explosive environment develops during the late afternoon hours onwards east of N-Tunisia, Sicily and the S-Tyrrhenian Sea if indeed another surface wave evolves and rides NE-ward along the eastward racing cold front as GFS proposes. The main attention will be the immediate prefrontal airmass, with abundant MLCAPE release in excess of 1000 J/kg. Dependant on the strength of the backed surface wind field (and hence the strength of the wave), LL shear (speed and directional) next to LCL heights of 500-800m print a very favorable environment for tornadoes (strong events are possible). Severe wind gusts and large to very large hail are all forecast, especially if a more discrete cell develops ahead of the cold front despite more capped conditions. Flash flooding is probably maximized over Sicily and SW-Italy.

A 30-35m/s jet evolves along the western fringe of the strong depression just west of Sardinia. Latest model data keep deep convection confined to the center of the depression and therefore displaced to the strongest shear. The airmass is well mixed with no capping inversion forecast below 700 hPa, so downward mixing is likely with damaging wind gusts. ECMWF and GFS both agree in the development of this jet just to the NE of the Balearic Islands.

... Bay of Biscay and northwards ...

A weak shear/moderate CAPE environment evolves in that area beneath eastward propagating, cold mid/upper trough. Scattered showers/thunderstorms are forecast with decreasing trend in coverage and intensity later the day. Strong wind gusts and marginal hail are the main risk.