Storm Forecast

Storm Forecast
Valid: Wed 03 Jul 2013 06:00 to Thu 04 Jul 2013 06:00 UTC
Issued: Tue 02 Jul 2013 21:37
Forecaster: TUSCHY

A level 1 was issued for N-Algeria and N-Tunisia mainly for a few large hail events.

A level 1 was issued for E-Moldova and S-Ukraine mainly for heavy rainfall amounts and isolated large hail.

A level 1 was issued for parts of N-Finland mainly for isolated large hail.

A level 1 was issued for parts of Germany, Austria, the Czech Republic and Poland mainly for a few large hail and strong wind gust events next to heavy rainfall amounts.


No pattern change is forecast over E/S Europe with weak height gradients still present. Areas between Switzerland and the Black Sea see roughly the same 500 hPa heights, so the elongated cyclonic vortex over Romania won't move a lot during the forecast. In fact there is no real thermal print left with that feature despite a distinct cyclonic wind field. Over NW/W Europe a significant rise in 500 hPa heights is expected due to deep WAA. Along the eastern fringe of that ridge, a sharp upper trough crosses Benelux and enters Germany before it rapidly dissolves. This trough features an exceptionally sharp shear/temperature gradient above 500 hPa with a 50 m/s high-level jet causing rapid amplification to the south.
A potent mid/upper level wave exits the Baltic Sea and affects N-Finland during the forecast.


... C/E Germany, Austria, the Czech Republic and W-Poland ...

An highly diffluent upper trough enters the forecast area from the west. Anticipated intense amplification and weakening at the same time complicates the forecast due to mixed shear/CAPE signals ... especially in respect of overlap.

For initiation purpose it is hard to find a focus. Leisurely eastward moving surface fronts become trapped in the increasingly diffluent mid/upper level wind field but also undergo a decrease in forward speed due to constant backing of the wind field and hence an increasingly more parallel alignement of the background flow to the fronts. It looks like increasing winds downstream of the trough also increase upper divergence especially from Austria to W-Poland, so a broad area will see favorable conditions for initiation. This fact is backed by weak capping and numerous mid-level impulses which eject from the base of the evolving/sharpening upper trough and move to the NE. Current thinking is that a mess of showers/isolated thunderstorm from the night before enters C-Germany and strengthens roughly east of a line Hamburg-Lake of Constance. Nothing severe is expected with that activity.

Beyond 12Z, more robust convection is expected to fire over NE/E/SE Germany into N-Austria and W-Czech Republic, where maximized BL moisture (along the synoptic front) beneath steeper mid-level lapse rates assists in a belt of 500 to locally 1000 J/kg MLCAPE. Forecast soundings reveal typical veering structures through the mid-troposphere due to the ongoing weak WAA regime. No jet maximum is dectected (despite a weak 3 km jet over NE-Germany) but still some weak mid-level curvature in hodographs is seen, so expect a few strong to temporarily severe thunderstorms with an isolated large hail and strong wind gust threat. Given type/history of the air mass with effective PWs in excess of 30 mm and slow storm motions, heavy rain and local flash flooding will be a possibility.
Of interest is that there are a few mesoscale models, which show good 3 km helicity and strong mid-level shear over E-Germany into W-Poland which might support even supercellular structures e.g. within a corridor Dresden-Wroclaw to the north. In that case, the hail risk increases.

A storm cluster might evolve with the thunderstorm activity over extreme SE Bavaria, which enters far N-Austria thereafter. Some model runs in the past showed healthy looking pressure rises during/after the passage of the storms and we don't want to rule out a temporarily better organized line of thunderstorms, capable of producing a swath of strong to isolated severe wind gusts. Lack of any substantial forcing and rapid decay of CAPE beyond sunset limit that risk both in space and time.

Another area with enhanced probabilities for a loosely organized thunderstorm cluster could evolve over NE-Germany as some better forcing lifts north during the afternoon/evening hours. Before upscale growth, aforementioned hazards are anticipated. Thereafter the heavy rainfall risk increases before storms move offshore during the evening hours.

... E-France to C-Germany ...

Placed beneath the base of the upper trough, a weak shear/modest CAPE environment supports scattered to widespread showers and thunderstorms, which slowly decay after sunset. Nothing severe is anticipated although marginal hail and strong wind gusts can accompany stronger thunderstorms.

... N-Finland and N-Sweden ...

For N-Finland: a potent mid-level wave crosses the area from the south and induces widespread lift for scattered thunderstorm initiation. Forecast soundings show high chances for surface based initiation between 9-18 Z with MLCAPE of 300-600, locally up to 800 J/kg. Winds below 3 km remain weak but strengthen substantially above. Hence the main concern will be the elongated and straight hodographs within the level 1 area. A strong vorticity lobe might support the line-up of thunderstorms (e.g. numerous short line segments). A solid MCS seems unlikely right now due to a lack of significant convergence in the lower troposphere. Nevertheless, concentrated swaths of strong to isolated severe wind gusts might occur next to an isolated large hail event. After sunset, storms weaken and remain sub-severe, but conditions still support isolated CI all night long over extreme N-Finland.

For N-Sweden scattered showers and thunderstorms develop beneath the base of the impulse. Weak shear but MLCAPE of 300-600 J/kg assist in a few strong storms with marginal hail and gusty winds. The thunderstorms rapidly decay after sunset.

... N-Algeria and N-Tunisia ...

A similar set-up compared to yesterday is forecast with less dynamics and weaker shear. Isolated thunderstorms develop during the evening and overnight hours with a large hail risk given 15 m/s DLS and 500-800 J/kg elevated CAPE overlap. A few strong downbursts accompany strongest storms. A level 1 should cover that risk.

... E-Moldova and adjacent areas ...

A level 1 for slow moving thunderstorms was issued. Onshore flow advects moisture a bit inland, where up to 1000 J/kg MLCAPE are forecast. Shear remains weak, so the main risk will be isolated large hail during the initiation stage, changing more to an heavy rainfall risk during upscale growth. Thunderstorms re-develop all night long, so flash flooding becomes a problem.

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