Storm Forecast
Valid: Tue 04 Sep 2012 06:00 to Wed 05 Sep 2012 06:00 UTC
Issued: Mon 03 Sep 2012 23:06
Forecaster: TUSCHY

A level 2 was issued for parts of Corsica, Sardinia and W-Italy mainly for excessive rainfall amounts.

A level 1 surrounds the level 2 area mainly for heavy rain and a few waterspout events.

A level 1 was issued for parts of C-Mediterranean mainly for isolated large hail, heavy rain and an isolated waterspout event.

SYNOPSIS

A weakening cut-off low moves slowly east and affects most of the W/C Mediterranean during the forecast. A progressive pattern still affects N Europe with unsettled conditions. Ridging keeps weather stable over W/C and SE Europe.

Side note:

Since yesterday, a well structured cold-core low looped cyclonically atop Corsica and is currently situated roughly at 40°30' N and 8-9° E, featuring an E-W elongated center. Also, mid-tropospheric vortex built downwards and 19Z ASCAT scan featured a 15-20 kt surface depression just to the NW of Sardinia which moved east/southeastwards since then. A 30-35 kt surface wind maximum along its northern/western fringe is not correlated with the organizing surface circulation but exists rather due to a channeled gradient flow. Just before landfall over N-Sardinia, DMC increased in strength and organization next to the center but decreased markedly after landfall. An increase in DMC was noted along the northern/northeastern fringe due to increasing LL convergence.

During the start of the forecast period, the depression's center should emerge off of Sardinia into the N-Tyrrhenian Sea and therefore further to the north than most models indicated 24 h ago. However, due to the asymmetric and elongated circulation center and movement atop the rough orography of Sardinia, uncertainties still exist if the center may reform a bit more to the south. Anyhow, this motion brings the depression's center atop an anticyclonic eddy with SSTs of 27-28 °C / positive SST anomalies of 2-3 K and good heat content. As the cyclonic vortex is not yet vertically stacked, some slow eastward motion is still expected with a more or less quasi-stationary motion over the far NE Tyrrhenian Sea during the end of the forecast. At that time, SSTs decrease by roughly 3K. Despite a gradual warming trend of the mid-/upper level vortex and an increase of the geopotential heights as a response of the more zonal flow pattern over N-Europe, background environment remains supportive of a temporarily confined core modification. DLS is already weak and continues to weaken with some weak upper divergence forecast.

A lot depends on the final track of the depression, but confidence increases that a subtropical cyclone may evolve between Sardinia and C-Italy during the forecast. Still a few high resolution models indicate moderate to rapid intensification and is was noted that overall model tendency approached the shallow warm core phase in FSU phase diagrams. Surrounding sounding data also confirm a decrease of the dry slot strength and ongoing moistening, so overall environmental conditions seem supportive for strengthening/organization. An intensity forecast is hard to create due to

a) proximity to the islands/Italy
b) potential infiltration of more stable stratified, rain cooled air from N-C Italy during the forecast
c) uncertainty how strong/persisting convergence and DMC east of Corsica may affect the development of the depression's center and attendant banding structure

Especially in case of the depression's motion atop the warm eddy, we would not rule out a ST 2.5 classification, which would equal 35-40 kt . In any case, the feature will be closely monitored during the following 24 hours.


DISCUSSION

... Corsica, Sardinia and W-coast of C-Italy ...

A lot depends on where the depression wil be placed and how strong it will be. The main interest will be a persistent convergence zone to the north of the center, which points towards Corsica. A prolonged period of onshore moving showers and thunderstorms are forecast, producing excessive rainfall amounts. A similar risk exists over W-Italy. Beside local rainfall amounts in excess of 100 mm/24 h, a few waterspouts are well possible along any stronger convergence zone.

... Adriatic Sea and rest of Italy ...

Numerous showers and thunderstorms evolve in the area of influence of the main mid/upper vortex, despite its overall weakening trend. Compared to yesterday's deep layer flow strength, a considerably weakening trend is forecast, which may limit overall probabilities for V-shaped and training DMC activity (as observed yesterday). Nevertheless, especially N Italy may again see heavy rain due to repeatedly onshore moving showers/thunderstorms. Despite the decreasing probabilities, we can't rule out isolated V-shaped convection and hence a broad level 1 was issued mainly for heavy rain. An isolated waterspout event is possible along the coasts and along any convergence zone.

... N-Ukraine and extreme S-Russia ...

A strong short wave affects those areas during the daytime hours with attendant cold front shifting SE-wards. Prefrontal air mass features moderate BL moisture content with surface dewpoints in the mid tens. This may assist in isolated DMC within a 20 m/s DLS environment and therefore an isolated organized multicell event with large hail and strong wind gusts is possible. However, overall meager CAPE profiles preclude a level 1. Further to the north, a few thunderstorms evolve beneath the base of the upper wave, where mid-levels remain cold. Marginal hail and strong wind gusts will be the main hazard. Storm coverage and intensity decreases after sunset.