Storm Forecast
Valid: Fri 24 Feb 2017 06:00 to Sat 25 Feb 2017 06:00 UTC
Issued: Thu 23 Feb 2017 14:43
Forecaster: TUSCHY

A level 1 was issued for parts of the W-Ukraine and far S-Belarus mainly for severe wind gusts and an isolated tornado threat.

A level 1 was issued for parts of the Adriatic and S-Tyrrhenian Sea mainly for an isolated severe wind gust, tornado and large hail threat. An excessive rainfall risk evolves along the E-coast of the CNTRL-Adriatic Sea beyond 00 UTC.

A level 1 was issued east of Corsica mainly for a few waterspout reports.

SYNOPSIS

A pronounced cold-core vortex is situated over Finland. This vortex is surrounded by two major impulses. One lifts to the northeast as a sharpening mid-level wave, which exits NE Poland during the start and affects far W-Russia during the end of the forecast. Further to the west, phasing of another impulse along the SW periphery of the vortex occurs with a gradually filling and opening cut-off over the Iberian Peninsula/ far W-Mediterranean. Combined with the supportive impacts of the Alps, another cut-off develops over NW Italy during the overnight hours. Meanwhile, broad ridging builds east over NW Europe and results in quiet conditions (regarding convection) during the forecast period.

Within the lower troposphere, a strong surface depression is tied to the NE-ward racing mid-level wave. This depression is expected to fill by roughly 10 hPa during the 24h period. Another vortex evolves beneath the strengthening cut-off over Italy and has an impact for most of the CNTRL parts of the Mediterranean. A surface high pressure area over France builds east during the forecast, whereas Ireland, UK and Scotland already feel the influence of another major depression near Iceland.


DISCUSSION

... Far S-Belarus and NW Ukraine ...

A narrowing warm sector due to a rapidly eastbound racing cold front should see sufficient BL moisture for some low-end CAPE build-up within the level 1 area. Meager mid-level lapse rates however preclude more robust instability, which limits the overall severe risk. A few showers/isolated thunderstorms may evolve around noon and during the afternoon. Favorable kinematics (e.g. 30 m/s 0-3 km shear and 10-15 m/s LL shear next to enhanced SRH-1) may support severe wind gusts even with weak convection. The mesoscale dictates the final tornado risk as not much diabatic heating or local moisture pooling is needed to increase CAPE and updraft strength for somewhat longer-lived thunderstorms. Right now we think convection stays too short-lived for anything more than a very isolated tornado event. Convection weakens rapidly during late afternoon/early evening as BL cools and as the moisture plume continues to advect east.

... CNTRL Adriatic Sea, SE-Tyrrhenian Sea and east of Corsica ...

The strengthening cut-off over the CNTRL Mediterranean continues to pull an healthy EML to the north, which interacts with the moist Mediterranean boundary layer air mass. Resulting pockets with modest MLCAPE of 400-800 J/kg and abundant forcing support scattered thunderstorms within the highlighted area. 0-6 km bulk shear in excess of 20 m/s assists in multicells or isolated supercells with large hail and strong to severe wind gusts. Beyond 00 UTC, the excessive rainfall risk increases over parts of S-Croatia, Bosnia and Herzegovina and Montenegro, as the offshore fetch increases all the way to the W-Ionian Sea. However, progressive and rather ill defined LLJ should keep final rainfall amounts below the level 2 threshold (e.g. 24h amounts of GFS/EZ between 50-100 l/qm with only isolated spots of more than 100 l/qm due to orographic enhancement). Although LLCAPE build-up remains rather weak throughout the forecasting period, robust LL shear in excess of 15 m/s and SRH-1 of more than 150 m^2/s^2 may support an isolated tornado event along the coastal regions of the level 1 area.
More robust LLCAPE may support an isolated waterspout risk mainly east of Corsica.

Along the northern periphery of the evolving surface depression, strong moisture advection beneath cooling mid- and upper-levels results in a weak MUCAPE plume all the way to N-CNTRL Italy, where localized thunder snow is possible.