Storm Forecast
Valid: Sun 06 Nov 2011 06:00 to Mon 07 Nov 2011 06:00 UTC
Issued: Sat 05 Nov 2011 16:37
Forecaster: TUSCHY

A level 2 was issued for far NW-Italy mainly for excessive rainfall amounts.

A level 2 was issued for the Balearic Islands mainly for excessive rainfall amounts.

A level 1 was issued for Central/S-Italy and Sicily mainly for large hail, an isolated tornado and severe wind gusts.

The rest of the level 1 was issued mainly for heavy rainfall amounts.

SYNOPSIS

The major cut-off over the W-Mediterranean remains in place, so the blocking high over low pattern persists for most of Europe. An outbreak of cold air affects far E/NE Europe during the forecast. Apart from the central and western Mediterranean, no augmented thunderstorm probabilities are expected.

A side- note:
Of interest is the LL depression over the W-Mediterranean, which evolved in an highly baroclinic environment during the past 36 h and now moves back to the west. This motion allows the deepening low to move beneath the large-scale upper trough and therefore assists in gradually relaxing shear conditions. UW-CIMMS indicates a decrease of 5 m/s DLS over the W-Mediterranean in the upcoming 24 h, with DLS then in the order of 5-10 m/s. SST ranges from roughly 18 °C in the Gulf of Lion (buoy data reports) to 20-23 °C around the Balearic Islands. Talking about potential (sub) tropical development, those numbers are quite low, but this may be offset by cold mid-levels, which remain just around -20 °C at 500 hPa. Using cyclone phase diagrams as a first guess, those constantly indicate either shallow or deep warm core development, so confidence gradually increases in that scenario (also assisted by other models, which try to develop either an hybrid storm or a true warm-core feature during the following days).

Current thinking is that a gradually transforming low approaches the Balearic Islands during the forecast. Confidence is low that this feature acquires anything more than a shallow warm-core until 06Z, so any strength assessment would propably need to be done by the so called Hebert Poteat technique. In any case, residents along the Balearic Islands have to keep an eye on the potential developmenet of this feature as excessive rainfall becomes a growing risk.

DISCUSSION

...Far NW-Italy, far S-Switzerland and far SE-France ...

Long lasting and regionally devestating excessive rainfall event gradually winds down. Nevertheless, conditions remain supportive for training and slow moving storms from the Ligurian Sea all the way to far NW-Italy (Piemonte and Valle D'Aosta). Models disagree, when exactly the LL wind field starts to back far enough to disrupt the favorable onshore flow (also dependant on how fast the cold front moves to the east, suppressing inflow trajectories from the N-Adriatic Sea ). Right now we expect a slow improvement during the later daytime hours and especially during the night. However, until then, excessive rainfall continues with amounts still exceeding 100 mm/24h (especially in the upslope areas and along the coast). At least the first part of the period should see adequate conditions for DMC along the Ligurian coast, so we decided to include that in our warning criterion before the overall character changes more to a stratiform upslope event. Hence a level 2 was added.

... Rest of Italy, Sicily, Tyrrhenian Sea, Ionian Sea and the Adriatic Sea (including adjacent areas)...

The eastward moving cold front starts to slow down during the forecast, as the main low retrogrades to the west. Upper forcing weakens and frontolysis is in progress. Nevertheless, this front will be the focus for numerous showers and thunderstorms. Especially the E-Adriatic Sea experiences the best set-up for strong onshore flow as 15-20 m/s winds at 850hPa still advect a pretty moist air mass onshore. Slow moving or training storms are possible with heavy to locally excessive rainfall amounts. A level 1 was issued to cover that risk. However, if a backbuilding line may set-up somewehere in the level 1, excessive rainfall amounts (matching a level 2 ) may be possible. Uncertainties with those mesoscale mechanisms and overall deteriorating forcing/shear preclude a level 2 area for now.

Sicily, Central/S-Italy and parts of the Ionian Sea reveal a nice overlap of 500-800 J/kg MLCAPE and 20-25 m/s DLS. Postfrontal well mixed air mass and steep mid-level lapse rates assist in scattered thunderstorms. Well organized multicells are forecast with large hail, isolated tornadoes and heavy rainfall the main threat. An isolated waterspout may be reported along the coasts with somewhat augmented LL CAPE. After sunset, the activity gradually shifts offshore and continues during the night.

... Corsica, Sardinia, the Balearic Islands and S-France (inlcuding all offshore areas in-between)...

As mentioned in the "side-note", a lot depends on the exact evolution of the cyclone. Beside that feature, conditions for repeatedly shower and thunderstorm development are good, given 500 hPa temperatures around -20 °C and SSTs of 18-23 °C. Marginal hail, locally heavy rainfall and gusty winds will be the main hazard despite the following regions:

a) N-Pyrenees and SW-France (especially S-parts of the regions Languedoc-Roussillon and Midi-Pyrénées) may see a good set up for an onshore flow event for a prolonged period, as LL wind field responds to the evolving depression. Excessive rainfall amounts are possible as indicated by a few models. Some weak MUCAPE is present all along the N-Pyrenees, so an isolated, embedded thunderstorm can't be ruled out.

b) The Balearic Islands may see rapidly deteriorating conditions, as the cyclone approaches from the N/NE. If indeed a (shallow) warm-core feature tries to evolve, bands of enhanced convection have to be expected, producing strong to severe wind gusts. Also, directional shear is augmented with LCLs in general below 800 m, so an isolated tornado event remains possible. Heavy to excessive rainfall will be another risk with this feature, especially if the GFS solution comes true, showing a stationary system atop the Balearic Islands. As most models agree with the general motion towards the Balearic Islands and an overall slow down of this feature, we think a level 2 may be needed due to an excessive rainfall risk.