Storm Forecast
Valid: Tue 03 Sep 2013 06:00 to Wed 04 Sep 2013 06:00 UTC
Issued: Mon 02 Sep 2013 14:44
Forecaster: PISTOTNIK

A level 1 was issued for parts of Poland, Belarus and the Ukraine for severe wind gusts and tornadoes.

SYNOPSIS

A pronounced low-pressure system moves southeastward over Belarus and the Ukraine, with a jet streak that curves around its Western and Southern flank. Further upstream, a West-East aligned frontal zone establishes over the Atlantic to the North of the British Isles.
Much of Western, Central and Southern Europe will be under anticyclonic influence with seasonable temperatures, whereas a surge of cool air spreads southward in Eastern Europe. Poor lapse rates throughout Europe bring about a mostly calm day convection-wise.

DISCUSSION

... East-Central and Eastern Europe ...

The most interesting convective environment develops in the periphery of the Ukrainian low.
Limited area models show a sharp, "thread-like" cold front which moves southward across Eastern Germany (Saxonia), the Czech Republic, Slovakia, Eastern Austria and Hungary in the morning to afternoon hours. As it outruns the region of synoptic-scale forcing, this convective line will hardly become deep enough to produce any lightning (except maybe for Western Romania later in the afternoon, where the front will loose its sharp structure but the diurnal timing is better). With marginal CAPE and 0-3 km shear around 15 m/s, sub-severe wind gusts can be expected with its passage.
Behind a postfrontal belt where strong subsidence (which becomes manifest in a striking drop of the dew point from ~14°C to ~5°C) creates sunny and dry conditions, a tongue of low-level moisture from the Baltic Sea wraps around the low again with more distance to the cold front. Cold upper levels, rising motions ahead of passing vorticity lobes and some insolation may create neutral to slightly unstable profiles again. Scattered showers and thunderstorms are expected to form over Belarus, Eastern Poland and the Western Ukraine. The convection near the core of the upper-level low will remain unorganized. However, towards the West deep-layer shear quickly increases to 20-25 m/s. Much of this shear will be concentrated at lower levels - especially the values for 0-1 km shear (>15 m/s) and 0-1 km storm-relative helicity (>200 m^2/s^2) are really outstanding! - and can therefore easily be consumed by the low-topped convection, which can organize into small multi- and supercells or short bowing lines. A level 1 is issued for a risk of severe wind gusts and tornadoes. A possible limiting factor is too little insolation in the tongue of maritime air.

... Black Sea ...

Late in the forecast period, the cold front and strong QG forcing will also overspread the warm water of the Black Sea, where the buildup of some CAPE is forecast. If deep convection will develop early enough to still benefit from a departing belt of enhanced vertical wind shear and if it will make landfall in Northern Turkey before the end of the forecast period (both of which is doubtful), heavy rain and severe wind gusts are not ruled out. However, this is too conditional for a level 1.

... Mediterranean region ...

With the retreat of the steepest lapse rates and the best low-level moisture to the South, CAPE magnitude over the Mediterranean Sea continues to decrease. A gradual re-warming of the upper levels and the lack of any QG lift will also reinforce the cap. Isolated to scattered thunderstorms will likely be confined to parts of Northern Africa, Sicily and Sardegna, where daytime heating over land may be strong enough to still break the cap. Towards the South, deep-layer shear rises to 15 m/s beneath remnants of a subtropical jet streak, which may be sufficient for some multicellular organization with a marginal hail and wind risk.

... England ...

A small but fairly pronounced PV anomaly will travel eastward along the warm side of the Atlantic frontal zone. Its lift might help to create marginally unstable profiles in an environment of 15 m/s over the East Midlands towards evening. Isolated, probably elevated thunderstorms are not ruled out.