Storm Forecast
Valid: Sat 23 Feb 2013 06:00 to Sun 24 Feb 2013 06:00 UTC
Issued: Sat 23 Feb 2013 00:55
Forecaster: PISTOTNIK

A level 1 was issued for the Western shores of Central Italy, Croatia, Bosnia and Montenegro mainly for excessive convective precipitation.

A level 1 was issued for a belt from the Western Mediterranean via the Tyrrhenian Sea and Southern Italy into the Southern Adriatic Sea for severe wind gusts, large hail and an isolated tornado.

SYNOPSIS

Pronounced blocking pattern is continuing across Europe as a body of very cold air and low 500 hPa geopotential heights slips southward across France into the Western Mediterranean region. Its stimulus for low-level cyclogenesis finally bundles the widespread, messy cyclonic activity across the Mediterranean region into a dominant 992 hPa low-pressure system, which slowly moves northeastward from Sardegna towards Northern Italy during the forecast period. While dry and cool air is advected southward over the Iberian Peninsula and into the Western Mediterranean at its rear side, a broad and intense Southwesterly to Southerly warm air advection regime develops further downstream into the Eastern Mediterranean, Central and Eastern Europe. An upper-level jet streak rounds the dominant steering center, pushing deeply into Northwestern Africa before curving northward again over Italy and the Western Balkans.
Meanwhile, a belt of high pressure stretches from Russia via Scandinavia to the British Isles, resulting in a continuing advection of cold and dry air from the East over much of continental Europe.

DISCUSSION

While the large-scale weather pattern seems straightforward, low inter- and intra-model consistency and tricky mesoscale details make the forecast quite challenging.

... Central Italy, Western Balkans...

Forecast models agree that warm air advection and synoptic-scale lift will result in the build-up of low instability (CAPE on the order of 200-500 J/kg) in the warm sector over much of the Central Mediterranean region in the morning to afternoon hours and later on over the Adriatic Sea. Forecast areas of instability at least partly overlap with enhanced deep-layer shear and storm-relative helicity, the more the further South and East it gets. However, chances are that quite some time of forced ascent will be needed to enable convective initiation in the warm air advection regime. Hence, the first storms will probably form too late to benefit from the decent vertical wind profiles.
In any case, the main threat is excessive precipitation when embedded convection moves onshore along West-facing coasts, where orographic lift adds to the synoptic-scale lift support. An isolated event of severe wind gusts, large hail or a tornado is not ruled out in case of more discrete cells and/or an initiation timely enough to ingest a stronger helical inflow. Highest coverage of thunderstorms is expected along the Italian West coast in the afternoon to evening hours and in Western Montenegro, Bosnia and Croatia in the evening to night hours.
Due to the complex structure of the surface low, it is unclear how far North the tongue of rich Adriatic moisture, and hence the excessive rain risk, will extend. Current thinking is that the release of marginal elevated CAPE and/or symmetric instability may bring embedded convective debris as far North and inland as Eastern Croatia and even Slovenia. Even though the excessive rain risk per se should quickly decrease after the cut-off of the rich low-level moisture, it is worth mentioning that the combination of snowmelt and (sub-severe) rain might still result in localized flooding in inland areas.

... Tyrrhenian Sea, Southern Italy, Southern Adriatic Sea ...

The most convincing trigger mechanism arrives with the trailing cold front, which will leisurely move southeastward into the belt of strong deep-layer shear (20-25 m/s) underneath the jet streak between Algeria and Sardegna in the morning hours. Isolated to scattered thunderstorms are expected with a chance to organize into multicells or even supercells, moving eastward and possibly reaching Southern Italy until midnight. Severe wind gusts and (to a lesser degree) large hail and an isolated tornado are possible.
Ahead of the front, a tongue of a Saharan elevated mixed layer moves northeastward until it nicely overlaps with the moist Southerly low-level jet over the Northern Ionian and Southern Adriatic Sea towards the end of the forecast period, resulting in decent but also strongly capped CAPE (500-1000 J/kg). Despite some passing patches of vorticity, convective initiation within the current forecast period is doubtful and it was decided to stick with a level 1, even though the combination of moderate CAPE, strong vertical wind shear (20-25 m/s @ 0-6 km, 10-15 m/s @ 0-1 km) and enhanced storm-relative helicity would be sufficient for supercellular organization with all kinds of severe weather.

... Western Mediterranean ...

Postfrontal convection may be enhanced over Corsica and the Ligurian Sea in the afternoon and evening, as another short-wave trough emerges from the back side of the steering upper-level low and taps into some better low-level moisture in the wake of the Western Alps. Marginally severe wind gusts are possible in Corsica.
Otherwise, no severe weather is expected.