Storm Forecast
Valid: Fri 14 Sep 2012 06:00 to Sat 15 Sep 2012 06:00 UTC
Issued: Thu 13 Sep 2012 22:04
Forecaster: TUSCHY

A level 2 was issued for S-Montenegro, Albania, Macedonia and Greece mainly for severe wind gusts, large to very large hail, excessive rainfall amounts and tornadoes.

A level 2 was issued for parts of C-Italy mainly for excessive rainfall amounts.

A level 1 surrounds the level 2 areas mainly for strong to severe wind gusts, isolated large hail, heavy rainfall amounts and an isolated tornado risk.

A level 1 was issued for the Tyrrhenian and Adriatic Sea mainly for tornadoes/waterspouts.

SYNOPSIS

A potent upper low moves further to the SE over the C-Mediterranean, which serves as focus for scattered to widespread severe thunderstorm development. A deep cyclonic vortex over Norway/Sweden and attendant sub 975 hPa surface depression result in unsettled conditions over N-Europe with autumn-like conditions expected.

DISCUSSION

... Ionian Sea, S-Montenegro, Albania, Macedonia and Greece ...

Southbound moving cold front becomes more diffuse and quasi-stationary over the SE-Ionian Sea with HIRLAM even showing a temporal northward shift during the night in response to a weak depression, forming along that boundary. With the upper low approaching from the west during the forecast, a prolonged period of strong forcing is expected to spread atop the surface boundary. The main uncertainty will be how the EML affects CI along the boundary with a rapid northward surge of hot 850 hPa temperatures. Model spread in QPF fields atop the S-Ionian Sea is therefore significant, but latest thinking is that conditions become increasingly favorable for offshore CI during the overnight hours.

CAPE fields will be maximized along the coastal area of Montenegro/Albania/Greece with values increasing from 1 to 2 kJ/kg MLCAPE from N to S. Significantly higher CAPE magnitude is expected SW of Greece along the cold front, where moisture pooling beneath EML layer assists in capped 2-3 kJ/kg MLCAPE. Well onshore over Greece and Albania, CAPE gradually lessens to values at or below 1000 J/kg as the BL moisture mixes out.

Kinematics improve rapidly during the forecast, as a gradually weakening 40 m/s mid-level jet overspreads the area of interest with 6 km bulk shear increasing from 15-30 m/s from east to west. A similar trend of increasing LL shear is expected mainly along the coastal areas (up to 15 m/s 0-1 km shear), but enhanced SRH-1 fields are forecast also well onshore.

Scattered to widespread CI is forecast, as the cold front slides to the south along the coastal areas and a mature (V-shaped) MCS is probably already ongoing. Given diffluence aloft and persistent influx of moisture from the south next to the prolonged period of lift, the extensive MCS continues during most of the daytime hours. We also can't exclude prefrontal CI which may preceed the actual cold front. Given aforementioned parameters, supercells with tornadoes, strong to severe wind gusts, large hail (isolated very large hail over S-Greece possible) and excessive rainfall are forecast. Further inland, the overall risk diminishes somewhat due to lowering CAPE and more elevated convection, but any surface based activity will be able to produce an isolated tornado, severe wind gusts and large hail event.

During the night, the cold front also activates further to the south (southwest of Greece). Given strong to extreme thermodynamics and adequate shear, organized multicells/supercells are forecast (probably more isolated in nature). Each storm will be accompanied by all kind of severe.

It has to be emphasized, that this pattern will be supportive of localized training thunderstorm activity with serious and life threatening rainfall amounts, especially in the large level 2 area!

... Rest of the S-Balkan States ...

Uncertainties exist, how much diabatic heating may verify due to thick cirrus clouds moving in from the south (due to DMC activity over Greece and Albania). Therefore, most of the activity will be elevated with isolated large hail, strong wind gusts and heavy rain possible. However, any region with better diabatic heating may be prone to surface based activity, which also increases the tornado risk. Therefore the level 1/2 were expanded far to the N. Also the level 1/2 were expanded NW-wards along the E-coast of the Adriatic Sea, where a great shear/CAPE overlap exists. Shear diminishes during the forecast, with an increase of a waterspout risk during the night.

... C-Mediterranean ...

A favorable set-up for waterspouts exists atop the Tyrrhenian and most of the Adriatic Sea with abundant LL CAPE/vorticity forecast. Main foci for tornado activity will be mesoscale convergence zones and coastal fronts. The risk gradually diminishes after sunset but increases over the E-Adriatic Sea.

Higher resolution models also indicate the chance of significant rainfall amounts due to repeatedly onshore moving showers/thunderstorms along the eastward facing coast of C-Italy, the westward facing coast of SW-Italy and parts of Sicily. Localized serious rainfall amounts and flash flood problems may arise (e.g. WRF still indicates 200-300 l/qm/24h over the regions Marche/Abruzzo [C-Italy]). A confined level 2 was issued for that upslope flow regime.