Storm Forecast
Valid: Fri 11 Oct 2013 06:00 to Sat 12 Oct 2013 06:00 UTC
Issued: Thu 10 Oct 2013 14:25
Forecaster: TUSCHY

A level 1 was issued for coastal areas of NE-Algeria and N-Tunisia, Sicily, S-Italy, the Ionian Sea and parts of NW-Greece and Albania mainly for excessive rain, large hail and strong to severe wind gusts. An isolated tornado event is possible.

A level 1 was issued for the NE-Adriatic Sea and adjacent areas mainly for excessive rain. An isolated tornado event and severe wind gusts are also possible (mainly along the coast).

SYNOPSIS

A strong cut-off, centered over N-France during the start of the forecast, moves south a bit and becomes quasi-stationary over SE/E France during the end of the forecast. To its north, a substantial height increase is noted and a persistent high-over-low blocking pattern evolves.
Further east, a progressive upper trough pushes a strong cold front towards NW-Russia with 850 hPa temperatures dropping to -10 °C behind that boundary.
Cool mid-levels cover the Mediterranean to the south of the major cut-off. Isolated to scattered DMC is anticipated over parts of the central Mediterranean .

DISCUSSION

... NE-Adriatic Sea, far NE Italy, NW Croatia and parts of Slovenia ...

Yesterday's lee cyclogenesis spun up also in parts of the lower troposphere which resulted in a deepening vortex and persistent moisture advection towards the NE coast of the Adriatic Sea (surface dewpoints in the mid to upper tens). During the start of the forecast, this gradually weakening vortex is about to cross the N-Adriatic Sea, exiting the area of interest to the NE while dissolving. The time frame of main concern will be until noon, when the highlighted area still resides ahead of the vortex. A combination of still increaseing S/SE-erly inflow and intense forcing (combination of a E-/NE-ward swinging mid-level trough axis and the aforementioned vortex) result in repeated thunderstorm development towards the NE Adriatic Sea. A temporarily back-building cluster in the level 1 area is possible with excessive rain the main risk. However, tail-end storms will see a good LL shear (directional and speed) and CAPE overlap for an isolated tornado and severe wind gust event. This hazard however should remain confined to the coastal areas. The inland level 1 was mainly issued for excessive rain and flash flood problems. Until 12Z, an eastward surging cold front should cut the cluster off of the moist inflow from the Adriatic Sea with a diminishing severe risk.

... E-Germany, SW-Poland, the Czech Republic, E-Austria, W-Slovakia and W-Hungary ...

Aforementioned intense forcing and connection to a moist plume of Mediterranean air over the Adriatic Sea assist in a surge of very moist air to the north ahead of the eastward moving frontal system. Strong differental temperature advection combined with that moisture and a northeast/northward moving dry slot should be enough for a S-N aligned prefrontal tongue with modest elevated CAPE/MUCAPE to extend from E-Germany/SW Poland all the way to W-Hungary. Deep layer shear will be impressive, but convective nature (elevated) should keep a severe risk very low. Limited (sounding) data for Europe due to the ongoing shutdown in the US however inserts some low-end uncertainty, if isolated, near surface based convection could still evolve ... especially when some clearing along the apex of the dry slot occurs (e.g. over the Czech Republic). In case of that type of convection, an isolated tornado event will be possible due to favorable wind profiles. The main risk however will be heavy rain.
During the night, the focus for isolated elevated convection will shift towards SW Poland. No severe risk is forecast.

... Coastal areas of NE-Algeria and N-Tunisia, Sicily, S-Italy, the Ionian Sea and parts of NW-Greece and Albania ...

A wavy quasi-stationary boundary extends from just south of Sardinia to S-Italy to the S-Adriatic Sea. BL moisture will be maximized in the proximity of that boundary. Also a surge of very moist air towards the Ionian Sea is expected. Placed beneath an elevated mixed layer, MLCAPE in excess of 1000 J/kg is forecast along and south of the boundary. DLS remains steady aoa 20 m/s so organized DMC is likely. The main inhibiting point for a more roubst severe event will be missing forcing, as no serious impulse is forecast to emerge out of a newly formed mid-level trough axis (S-France). Ongoing NVA should keep CI limited, which remains bound to either the orography or to the main boundary/local outflow boundaries. Any storm which manages to evolve will be strong to severe with a large hail, excessive rainfall and severe wind gust risk. An isolated tornado event can't be ruled out especially along the front itself, where enhanced LLCAPE and some LL shear overlap.
Once again, Albania will be the focus for repeatedly onshore moving thunderstorms with a favorable access to a very moist air mass over the Ionian Sea. However, BL winds weaken somewhat with no clear LLJ signals present, so no level 2 was added. However, excessive rain could cause flash flood problems and large hail/severe wind gusts could cause local damage.
Expect numerous smaller-scale thunderstorm clusters to affect the level 1 area until 06Z.