Storm Forecast
Valid: Fri 12 Feb 2010 06:00 to Sat 13 Feb 2010 06:00 UTC
Issued: Thu 11 Feb 2010 21:06
Forecaster: TUSCHY

A level 2 was issued for W-Turkey mainly for tornadoes, damaging wind gusts and large hail.

A level 1 was issued, surrounding the level 2 mainly for severe wind gusts, large hail and high rain amounts.

A level 1 was issued for most parts of Greece mainly for tornadoes, severe wind gusts, large hail and high rain amounts.

A level 1 was issued for the Tyrrhenian Sea, parts of S-Italy an S-Adriatic/N-Ionian Sea mainly for tornadoes.

A level 1 was issued for NW-Morocco mainly for excessive rainfall.

SYNOPSIS

An intense vortex works its way to the east over the central Mediterranean. A broad swath with showers and thunderstorms is expected beneath this feature and along the SE-ward racing cold front.

Another impulse affects the extreme SW-part of Europe and enters the western Mediterranean during the morning hours. The rest of Europe remains covered by a cold/stable continental air mass.

DISCUSSION

A well structured cold-core low over the west-central Mediterranean advances rapidly to the east and is about to exit the S-Adriatic Sea during the end of the forecast. In its sphere, various severe weather risks exist, which are listed below.

... West-central Mediterranean, Tyrrhenian Sea and S-Adriatic/N-Ionian Sea ...

The thermal structure of this feature is very cold to say at least, as 500hPa readings drop to -30°C to -36°C in its center. The lower levels are also quite cold (700hPa roughly -15°C and 850hPa shy above -5°C), but modification of this layer is well underway, as the air mass crosses the warm Mediterranean. The origin of this air mass is polar-continental, so low mixing ratios are the main concern. We expect numerous showers and thunderstorms with the main activity placed beneath the coldest air. LL CAPE is augmented and despite ongoing modification of the BL air mass, LL lapse rates remain supportive for waterspout reports. Local convergence zone / outflow boundaries from decaying storms and coastal convergence zones will be the primary focus for spout development. We went with a level 1 for those areas, where conditions seem to be the most supportive, e.g. beneath the center of the deep vortex. Isolated large hail is is also possible. Thunderstorms end from west to east but the activity will continue until 06Z over parts of the Adriatic Sea/Ionian Sea.

... Greece ...

At least 2 rounds of thunderstorms are forecast. The first one already starts during the morning hours with deteriorating conditions thereafter. An active cold front races eastwards, crossing the Ionian Sea from west to east until 18 UTC with GFS hinting on a very moist prefrontal theta-e tongue. We are a bit surprised to see such robust moisture values in model data, as time-frame for air mass recovery is not that long. SST above 15°C and strength of the baroclinic zone however could boost moisture values up to that magnitude (good moisture pooling). In any case, thunderstorms rapidly develop along this front and the environment is prime for a large cluster of showers/thunderstorms to affect Greece from NW to SE. Impressive upper divergence and placement in the left exit of a sharply curved cyclonic jet favor this scenario. There will be a decent tornado risk mainly along the W/SW coast as LL shear (directional and speed) ramps up. This will be especially true for any more discrete cell, developing ahead of the cold front itself. Severe wind gusts and large hail accompany the thunderstorms. For the flash flood risk, the final extend remains a bit tricky. No focused LL jet evolves, cold front advances southeastwards quite rapidly and the amount of LL moisture is also a bit uncertain. However, clustering storms and intense forcing ought to yield a good environment for heavy rain, spreading inland (northeastwards) with an isolated flash flood risk.

Another round of showers/storms is expected after midnight, as the main vortex draws near from the west. Shear at all levels relaxes, but an isolated waterspout report is well possible.

... NE Greece, Aegean Sea, W-Turkey and E-Bulgaria ...

A strengthening surface depression moves into W-Bulgaria and Romania later the night and causes a warm front to push rapidly to the north, crossing the Aegean Sea until 18Z and reaching SE-Romania around midnight. The east/southeastward moving cold front over NE-Greece/Aegean Sea will be the focus for scattered showers/thunderstorms, as the front moves into the warm/unstable and moist warm sector. Placed beneath the left exit of a powerful mid-/upper-level jet and keeping the east/northeastward racing forcing in mind, initiation will be quite widespread and a cluster of showers/storms is forecast over the N/NE Aegean Sea, building SE-wards along the W-Turkey coast. A concern is the extreme shear environment, which evolves as the deepening surface depression approaches. 850hPa winds increase to 25-35m/s and overspread the SE-ward moving cold front. We're not yet convinced that the environment assists in the development of more discrete, prefrontal storms. The current idea is a rapidly NE-ward expanding cluster of storms along the front itself. This would cause an intense moisture advection into those storms, so heavy rain, severe wind gusts, large hail and isolated tornadoes would be the main risk. However, any more discrete cell would also pose a risk for damaging wind gusts and a strong tornado event, especially in the level 2 area. We will monitor this scenario for further modifications. A low probability thunderstorm area was issued far to the north/northeast, as weak MUCAPE signals and good forcing can be seen all the way up to SE-Romania and NW-Turkey.

... NW-Morocco ...

A weak upper impulse approaches from the NW and causes an increased shower/thunderstorm activity during the late afternoon/evening hours. Connected to a very moist subtropical air mass, convection evolves in an high PWAT environment, so heavy rainfall will be a risk and a level 1 was issued. Thunderstorms decrease in strength and coverage after midnight.