Storm Forecast
Valid: Fri 28 Feb 2014 06:00 to Sat 01 Mar 2014 06:00 UTC
Issued: Thu 27 Feb 2014 20:47
Forecaster: TUSCHY

A level 1 was issued for SW France mainly for an isolated tornado and large hail event.

A level 1 was issued for Corsica mainly for excessive rain.

A level 1 was issued for W-CNTRL Italy mainly for an isolated tornado event.

A level 1 was issued for the Balearic Islands mainly for an isolated tornado risk and strong to severe wind gusts.

SYNOPSIS

Most of Europe are covered by a branch of the polar vortex. This configuration causes widespread cool to cold mid-levels. Driven by a 60 m/s mid-layer streak, an impulse dives to the SE and crosses France during the forecast. This feature overspreads the warm SSTs of the W-Mediterranean during the evening and overnight hours. Other than that, weak forcing and/or meager BL moisture result in another quiet day for DMC for most of N/C/E Europe.

Overall BL moisture remains meager at best, excluding the Mediterranean, where moderate air mass modification assists in pockets of seasonable moisture.

DISCUSSION

... France ...

A diffluent but already vertically stacked upper trough approaches NW France during the start of the forecast. This system falls behind a SE-ward racing mid-layer jet to its west and slows down while drifting to the E/SE. Overnight development of another 500hPa vortex is anticipated over the far NW Mediterranean. This scenario likely results in a rapidly SE-ward moving UVV maximum towards the Mediterranean during the daytime hours and leaves behind only weak high-level divergence or even some mid-tropospheric subsidence over W-France. A subtle short-wave rotates to the S/SE over NW France during the afternoon hours and should add some additional lift beside the SE-ward drifting weakening cyclonic vortex.

W-France:

As marine air mass spreads onshore beneath cold mid-layers, 200-500 J/kg weakly capped MLCAPE evolve over SW/W-France. A sharp 0-6 km isoshear gradient exists with mainly 10-15 m/s DLS available. 0-4 km AGL looks most promising for instability build-up, also fostered by 150 J/kg LLCAPE forecast. LL shear in excess of 15 m/s, SRH-1 aoa 100 m^2/s^2 (increasing until 18Z) and LCL heights between 600-1000 m indicate a risk for a few funnels/an isolated short-lived tornado event. Marginal hail (a single low-end large hail event possible) and strong wind gusts are also expected. In general near BL winds strengthen to more than 20 m/s during the evening hours, so even a severe wind gust or two are possible. Thunderstorm probabilities wane rapidly after sunset, although an isolated non-severe thunderstorm event is possible all night long over far SW-France. A level 1 was issued to cover the rather low-end severe risk.

NW-France:

Beneath the center of the vortex, conditions for isolated thunderstorms exist during the daytime hours, before the vortex moves ashore with rapidly decreasing CAPE. Weak shear but conditional BL moisture for some LLCAPE build-up point to an isolated funnel/waterspout risk offshore and along the coast of north-central France. Slow moving storms should add some heavy rainfall to the expected hazards, but the risk remains below a level 1.
Wrap-around occlusion with a 30 m/s 850 hPa speed maximum affects far NW-France during the daytime hours with a constant weakening trend thereafter. The stratification of the mid-troposphere remains too stable for enhanced convection. Severe wind gusts due to the tight gradient occur, but this will not be reflected in our risk scheme.

... W-Mediterranean ...

As the mid/upper impulse enters the W-Mediterranean during the late afternoon, a low/mid-level vortex spins up rapidly just off the S-coast of France. A cold front spreads SE and affects Sicily and the Tyrrhenian Sea during the end of the forecast. The western part of the boundary aligns more parallel to the background flow and slows down over Spain and SW of the Balearic Islands.

Scattered to widespread thunderstorm development is anticipated with most of the activity occuring beneath the weakly sheared base of the evolving upper trough (here: S of France, E/NE of the Balearic Islands). There is not much shear/CAPE and moisture to work with, so expect pulsating storms, which grow upscale into numerous disorganized clusters with heavy rain and gusty winds. Corsica could see excessive rain, both due to prolonged upslope flow (long fetch for western flow) and later-on also due to slow moving thunderstorms (placed beneath the center of the vortex). A rainfall-level 1 was issued.

That level 1 was expanded to the W-coast of central Italy, where an isolated tornado event next to heavy rain are forecast. A weakly organized southbound moving cluster may evolve, affecting the W-coast during the overnight hours. Progressive nature should keep the rainfall risk below "excessive criteria".

The cold front slows down significantly over the Balearic Islands with ongoing model discrepancies how far to the SW it will move. WRF and EZ data point to persistent convergent flow for most of the afternoon and overnight hours even well behind the synoptic boundary. Repeatedly developing showers and thunderstorms are forecast, which could become temporarily organized with strong DLS and some LL shear present. An isolated large hail event with heavy rain will be the main hazard for the Balearic Islands. During the passage of the cold front between 12-15 Z, an isolated tornado/waterspout event can't be ruled out as LL shear and SRH-1 peak during that period.

... Croatia, Bosnia and Herzegovina, N-Serbia and Hungary ...

A weak mid-layer impulse lifts to the NE atop BL mixed layer mixing ratios aoa 5 g/kg. As mid-levels undergo some cooling, mid-layer lapse rates steepen to 7-8 K/km. A few short-lived thunderstorms are forecast with sleet and gusty winds. The activity wanes after sunset. Would not be surprised about an isolated waterspout event along the E-coast of the Adriatic Sea before noon, but the risk is too marginal for a level area.