Storm Forecast
Valid: Wed 20 Oct 2010 06:00 to Thu 21 Oct 2010 06:00 UTC
Issued: Tue 19 Oct 2010 21:09
Forecaster: TUSCHY

A level 1 was issued for parts of the Aegean Sea and SW-Turkey mainly for an isolated large hail and waterspout event.

SYNOPSIS

A deep cold-core vortex is centered over N-Europe and gives way to the first shot of modified arctic air to parts of Central Europe. This air mass features limited moisture content at LL so onshore, no real thunderstorm risk is forecast, despite the highlighted area. A weakening upper trough still affects parts of SE-Europe but moves away towards the NE, so thunderstorm coverage also decreases. Otherwise, most parts of Europe won't see any organized thunderstorm threat.

DISCUSSION

... Aegean Sea and W-Turkey ...

Weakening upper trough lifts out to the northeast, leaving behind an environment, characterized by rising geopotential heights and a moist BL. This causes thunderstorm peak already during the morning hours and noon, with a constant decrease thereafter due to limited forcing. However, isolated thunderstorms will evolve all day long along the SW coast of Turkey, where coastal convergence balances missing mid-level PVA somewhat. Overall, shear will be modest over SW-Turkey and over the E/SE Aegean Sea with roughly 15m/s DLS. A marginal level 1 was issued for the coastline, as chance remains for an isolated large hail, strong wind gust or waterspout event. Further inland, thunderstorm coverage will decrease quite rapidly, so no level was introduced. Isolated heavy rainfall will be the main hazard with that activity well inland.

... Benelux, parts of Germany/Alps and N-Italy ...

A deep surge of cold air (maritime and hence moist but well mixed at low-levels) occurs during the forecast with temperatures at 500hPa dropping to at or well below -30°C over the area of interest. The main feature of interest seems to be a small-scale, but compact disturbance at mid-levels, arriving over N-Germany roughly around 12Z and moving to the SE thereafter. This results in an increasingly diffluent LL - 700 hPa flow pattern over Germany with a temporarily weakening pressure gradient/wind field. This effect may suppress extensive mixing of the BL for a few hours, keeping a shallow, moist BL in place until the afternoon hours. Also, low-mid level trajectories still emerge out of the North Sea area, so current thinking is that a shallow-moist BL beneath rapidly cooling mid-levels and infiltrating drier mid-level air may all assist in isolated to scattered thunderstorm development, peaking along the coast and decreasing gradually more inland. Also, somewhat better moisture content just north of the Alps may allow an isolated thunderstorm event over S-Germany. Hence a broad low probability thunderstorm area was included. The main hazard with any storm will be sleet/small to isolated large hail and strong to isolated severe wind gusts, especially as 20 m/s 850hPa wind speed maximum moves in from the North Sea. Regarding tornado probabilities, only the Netherlands, NW- and S-Germany may see an isolated event due to better LL moisture and augmented LL shear (the latter ingredient especially over S-Germany, where 12-15m/s LL shear, 150-200 m^2/s^2 SRH-1 and 100-400 J/kg SBCAPE (roughly 50-100 J/kg 0-3 km CAPE) overlap). No level was yet introduced due to the marginal CAPE outputs in model fields and hence pretty short-lived thunderstorms.

BL air mass moisture content mixes out rapidly during the late afternoon/evening hours, as nocturnal stabilization of the BL starts. However, sporadic thunderstorm events keep going all night long from the Netherlands to N-Germany with favorable trajectories and arrival of coldest mid-level air mass.

A very isolated thunderstorm event may also occur over the W-Alps as snow cover is not yet extensive and also over NE-Italy, where better LL moisture keeps SBCAPE around 300 J/kg during the day.