Storm Forecast

Storm Forecast
Valid: Mon 29 Oct 2012 06:00 to Tue 30 Oct 2012 06:00 UTC
Issued: Sun 28 Oct 2012 22:49
Forecaster: TUSCHY

A level 2 was issued for Crete, parts of Greece, W-Turkey and the Aegean Sea mainly for heavy to excessive rainfall amounts, large hail, strong to severe wind gusts and an isolated tornado event.

A level 1 was issued for parts of Bulgaria, Romania and Moldova mainly for a large hail, strong to severe wind gust and tornado risk. Isolated heavy rain also occurs.

A level 1 was issued for the offshore and coastal areas of the Tyrrhenian and Adriatic Sea mainly for waterspouts and heavy rain.

SYNOPSIS

A decent polar vortex is situated over far N-Europe with numerous branches affecting all of Europe. One deep trough affects SE Europe and drives a cold front to the SE while another trough approaches the Bay of Biscay and the Iberian Peninsula during the night. Most areas north of the Mediterranean feel the influence of a cold and dry air mass, so any DMC chances remain confined to offshore areas like the North Sea.

DISCUSSION

... Greece, Aegean Sea, Bulgaria, parts of Romania/Moldova, the Black Sea and W-Turkey ...

A leisurely eastward moving cold front dictates the severe risk for that area. This front will be roughly placed from S-Greece to W-Bulgaria around noon and over the W-Black Sea and W-Turkey around midnight. Strong forcing and a 30 m/s mid-level jet all cross that boundary and cause scattered to widespread and organized CI of DMC.

The prefrontal air mass is characterized by rich BL moisture beneath a NE-ward pointing EML plume, so widespread 1000-2000 J/kg MLCAPE are expected. A 15-20 m/s LLJ advects that plume of unstable air far to the N so even the W-Black Sea and adjacent areas might see MLCAPE in excess of 800 J/kg.

With the approach of the mid-level streak, 20-25 m/s DLS overspreads the cold front and the immediate prefrontal warm sector with similar magnitude in the lowest 3 km AGL. Of interest is a broad area with enhanced LL speed and directional shear, which extends from the W coast of Turkey all the way to E-Moldova. Not only the veering wind profile along the actual cold front, but also a weak LL depression over Bulgaria insert that much LL shear with 15 m/s speed shear and 200 m^2/s^2 SRH-1. LCLs ahead of the cold front drop to below 1000 m and CI might occur upstream of strongest cap.

The main negative point to discuss will be the near parallel alignement of forcing and shear vectors to the cold front, so discrete initiation won't be the dominant storm mode. However, especially during the initiation stage, shear is supportive of supercell and organized multicell development. Thunderstorms will be capable of producing large hail, severe wind gusts and heavy rain during the initiation stage with a transition more to a rain and wind risk after clustering. Storms, which ride along the cold front or evolve just ahead of the cold front will have a good chance to produce a few tornadoes. The highest risk of more discrete storms with aforementioned hazards (inlcuding an isolated tornado risk) will run from E-Bulgaria/Romania/Moldova to the W-Black Sea area. We expanded the level 1 with a low probability lightning area far west over Bulgaria and Romania to account for isolated overnight activity with some MUCAPE and strong shear present. Despite elevated nature, I do not want to exlude and isolated large hail risk during the night.

Further south, over N/NE Greece and along the W coast of Turkey, repeatedly onshore moving thunderstorms likely produce heavy to excessive rainfall amounts. Tail-end storms and any more discrete thunderstorm can also produce all kind of severe, including an isolated tornado. The cold front remains progressive and so is the thunderstorm activity. However, a few spots might see rainfall amounts in excess of 100 mm/24h, so a level 2 was issued for those areas. This level 2 also covers S/SE Greece and Crete, where strong shear/extreme CAPE create a favorable environment for a few severe storms with an all kind severe risk. This level 2 was issued with lower confidence for Crete, E Greece and the C-Aegean Sea due to numerous influencing mechanism (e.g. influx of drier air from the south or faster cold front motion than forecast and hence a lower risk for E-Greece). Confidence however is high for N/NE Greece and W-Turkey especially due to the rain risk.

... Tyrrhenian and the Adriatic Sea ...

An unusually intense and deep cold-core vortex is situated over C-Italy and affects the Tyrrhenian and Adriatic Sea with very cold mid-levels ( 500 hPa temperatures just shy above -30C). With SSTs of 19-24 C (increasing from north to south), abundant LL CAPE is forecast in an environment with enhanced LL and background vorticity. Despite a slow weakening trend of that vortex, even the 10m wind field reflects an healthy cyclonic circulation with areas of enhanced LL convergence. Those convergence zones or other mesoscale boundaries like outflow boundaries will be the foci for waterspout activity. Beside that, slow moving storms will also produce heavy rain and isolated large hail and therefore a level 1 was added for a broad area.

... Far NW Spain and Portugal ...

Another sharp upper trough affects the area during the overnight hours with an increase of DMC probabilities. Beside a sporadic strong wind gust and marginal hail hazard, no severe risk is anticipated with that activity.

... Coasts of Benelux and NW Germany ...

A fetch of enhanced CAPE evolves beneath a weak SW-NE tilted mid-level trough. This should increase the probabilities for deeper convection somewhat and an isolated waterspout might occur. The risk decreases after sunset when brisk NW-erly flow pushes a cold front to the SE and brings a more unstable airmass ashore.

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