dimanche 21 septembre 2014

European Storm Forecast Experiment

Storm Forecast
Storm Forecast
Valid: Sun 21 Sep 2014 06:00 to Mon 22 Sep 2014 06:00 UTC
Issued: Sat 20 Sep 2014 23:02
Forecaster: PISTOTNIK
A level 1 was issued for N Italy, S Austria, Slovenia, Croatia, Bosnia-Hercegovina, Montenegro, Serbia and W Bulgaria for excessive precipitation, large hail and severe wind gusts.

A level 1 was issued for E France, Switzerland, S Germany, the W Czech Republic and W Austria mainly for excessive precipitation.

A level 1 was issued for S Portugal and S Spain mainly for large hail.


An upper-level trough rapidly amplifies from Scandinavia into central Europe. It induces a strong surface cyclogenesis over the Baltic Sea and various weak ones in the triangle between Poland, Northern Italy and Romania. A tongue of polar air erupts southward into central Europe on their rear flank and replaces the unseasonably warm and moist air of the past few days.
An anticyclone is placed over the British Isles, to its south flanked by a stationary cut-off low off the Portuguese coast. A strong zonal flow (up to 30 m/s at 500 hPa) stretches from northwestern Africa into the central and eastern Mediterranean region.


... southern Alps and central Mediterranean into the Balkans ...

Still plenty of CAPE (700 - 2200 J/kg) was present across the central Mediterranean region according to the Saturday 12 UTC soundings. However, apart from Trapani (3100 J/kg), not the extreme values of the previous days were reached any more. Low-level cooling with increasing westerly winds will likely continue to quietly reduce CAPE and reinforce the capping inversion on Sunday. Central and southern Italy will therefore only see isolated, struggling storms or no activity at all, similar to the previous days.
A higher coverage of storms is expected from the southern Alpine region into the Balkans, where CAPE will be lower (probably below 1000 J/kg) but only weakly capped, and where a series of travelling vorticity maxima facilitate convective initiation. Especially over orographic features of the Balkans, quite many storms may form with a maximum in the afternoon and evening hours. Coverage in the southern Alpine and northern Adriatic region is somewhat more uncertain, but the arrival of the main trough may keep storms going well into the night there.
With still plenty of moisture and a possibility of repeated rounds of convection, excessive rainfall is again the main hazard. In addition, deep-layer shear between 15 and 25 m/s and regionally enhanced helicity may also promote better storm organization with a risk of large hail and severe wind gusts. Distinct upper-level cooling overnight could support the formation of one or two tornadoes (either supercellular or "spout-type") with offshore convection over the Gulf of Genoa and the northern Adriatic Sea late in the forecast period.

... France into Poland and east-central Europe ...

Ahead of the cold front, the same warm and moist air will again allow CAPE up to 1000 J/kg under 10 m/s deep-layer shear. Scattered to widespread, mostly disorganized thunderstorms will form. Due to slow storm motion and precipitable water around 30 mm, once more a few flash floods are possible.
Vertical wind shear will gradually increase from the northwest with the amplification of the main upper-level trough, but pronounced cold air advection and associated subsidence in the range of the southward slipping cold front will probably kill any deep convection before it can attain any better organization.

... Denmark, S Scandinavia, Baltic Sea ...

Showers and low-topped thunderstorms under limited CAPE and mostly weak shear in the polar air mass should not pose any severe weather risk.

... Iberia ...

Warm air advection and positive vorticity advection provide increasing lift ahead of the cut-off low. Rich Atlantic moisture will be advected well inland with southwesterly winds, and CAPE is expected to build to ~500 J/kg inland and probably to higher values near the east coast. Scattered to widespread, diurnally driven storms are expected.
Deep-layer shear increases from 10 m/s in the northwest to 20 m/s in the southeast, with slightly veering profiles. This may be sufficient for multicells and a few supercells with a primary risk of large hail. In case of supercells, severe wind gusts and an isolated tornado are not ruled out, either.
Activity may propagate offshore and spread towards the Balearic Islands overnight with an ongoing severe weather risk.

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