Storm Forecast

Storm Forecast
Valid: Sat 25 Aug 2012 06:00 to Sun 26 Aug 2012 06:00 UTC
Issued: Fri 24 Aug 2012 21:57
Forecaster: TUSCHY

A level 1 was issued for parts of the Ukraine and Belarus mainly for large hail and severe wind gusts.

A level 1 was issued for parts of Switzerland and the W-Alps mainly for heavy rain. Isolated large hail may occur over far S-Bavaria and far S/SE Austria.

A level 1 was issued for N-Italy and SE France mainly for large to very large hail, strong to severe wind gusts and heavy rain.

A level 1 was added for extreme N-Germany and SW Denmark mainly for heavy rain.

SYNOPSIS

Embedded in a belt of agile westerlies, a strong mid/upper wave moves in from the west .... from the Bay of Biscay (start of the forecast period) towards W-Germany (end of the forecast period). At 500 hPa, no pronounced ridge signature is seen over Europe despite a warm anomaly over SE Europe, where stable and hot conditions prevail. Cooler air infiltrates towards N-Europe, where unsettled conditions occur.

The northern fringe of the Mediterranean marks the boundary between hot and muggy conditions to the south and cooler/drier air to the north. This boundary features an elonagted cold front, which runs from Belarus all the way to Portugal (with a wavy structure along the Alps and gradually dissolving over the Iberian Peninsula).

DISCUSSION

... Parts of the Ukraine and far S-Belarus ...

A warm sector stretches over the highlighted area. Attendant air mass is well mixed with a daytime driven lowering of surface dewpoints. BL moisture is maximized along the wave (cold front over W/NW Ukraine and warm front over N-Ukraine), where moisture pooling boosts surface dewpoints into the upper tens. An EML is situated atop the Balkan States all the way into the Ukraine. Overlap of the EML atop the aforementioned boundary layer moisture features roughly 800 J/kg MLCAPE in the warm sector and up to 1.5 kJ/kg MLCAPE along the boundaries. Not much background forcing is forecast, so CI will be bound to surface temperatures, exceeding the convective temperature threshold and the orography/boundaries. Hence, isolated CI is forecast in the warm sector with scattered CI along the boundaries itself. 20 m/s DLS and similar values in the lowest 3 km assist in well structured multicells/isolated supercells (due to locally augmented SRH-3) with large hail and strong to severe wind gusts. An isolated bowing segment with an enhanced severe wind risk is possible but weak forcing keeps this risk rather low for now.

... Alps ...

Despite serious looking QPF fields over the Alps, instability will be the limiting ingredient for this forecast. Again, surface cold front squeezes along the N-Alpine crest with maximized BL moisture just north the Alps and in the Alpine valleys. Compared to yesterday, no Alpine EML plume is expected to fan out to the NE (towards S-Bavaria (Germany)). Nevertheless, maximized BL moisture may result in a narrow tongue of 500-800 J/kg MLCAPE just north of the Alps. During the daytime hours, large-scale PVA approaches from the west as the mid/upper level wave approaches. This forcing overtakes the surface boundary, which is aligned along the Alpine crest and models agree fairly well in scattered to widespread CI from west to east. Current thinking is that a large cluster of showers/straiform rain and embedded thunderstorms evolves with a gradual movement to the east. Heavy rain will be the main hazard although any isolated thunderstorm ahead of the cluster is able to produce isolated large hail as well due to 25 m/s DLS. We expanded a marginal level 1 to the north of the Alps, to cover the isolated heavy rain/hail risk.

During the night, a growing cluster expands to the NE and affects the Czech Republic with mainly heavy rain. At the same time, CI is forecast south of the Alps, as cooler mid-levels spread atop the warm/moist air mass over the N-Alps. With surface dewpoints up to 20 C, 1.5 - 2 kJ/kg MLCAPE are likely. The set-up favors enhanced directional shear (seen in SRH-3 data) next to 25 m/s DLS, so a few well organized multicells and isolated supercells are likely. Large to very large hail and strong to isolated severe wind gusts will be the main hazard. Given strength of forcing, kinematic and CAPE, an isolated extreme hail event can't be ruled out. However, confidence is not yet adequate for an upgrade to a level 2. Betimes, storms grow upscale into numerous storm clusters with heavy rain the main risk.

A similar hazard exists over SE France with isolated large hail and strong to severe wind gusts during the afternoon/evening hours.

... France, Benelux and parts of Denmark ...

A well mixed maritime air mass from the Atlantic and moderate mid-level lapse rates due to cooler air at 500 hPa cause widespread low-end SBCAPE build-up in the order of 300-600 J/kg. A short time frame exists during the late afternoon hours, where LL winds increase and some CAPE still exists. A few better organized showers/thunderstorms are possible with marginal hail and strong wind gusts. Despite low LCLs (especially over N-France and Benelux), directional shear remains too weak to including an isolated tornado risk for now. After sunset, LL shear rapidly increases over N-France, but CAPE vanishes, so thunderstorm probabilities decay after sunset. However, isolated storms continue offshore over the S-North Sea and the E-English Channel. Storms, which move ashore along the coasts of Belgium/the Netherlands, NW Germany and W-Denmark may become temporarily better organized, as LL shear increases onshore. Main concern however is a potential heavy rainfall risk during the second part of the night with organized shower streets building over the extreme SE North Sea towards SW Denmark. A level 1 for rain was issued. This risk continues beyond the forecast range and expands S-wards.

... UK ...

With 500 hPa temperatures dropping to at or below -20 C atop a moist maritime BL air mass assists in moderate SBCAPE of roughly 800 J/kg during peak heating. This amount of CAPE is enough to support a few strong and probably isolated severe thunderstorms with marginal (isolated large) hail and heavy rain. However it is tough to pinpoint the area of highest activity, as CI will be dictated by mesoscale features. Hence, no level will be added for now. Storms decrease in coverage and intensity after sunset.

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