Storm Forecast

Storm Forecast
Valid: Tue 23 Aug 2011 06:00 to Wed 24 Aug 2011 06:00 UTC
Issued: Tue 23 Aug 2011 01:18
Forecaster: DAHL

A level 2 was issued across N France ... Benelux ... W Germany for damaging winds, large hail, excessive rainfall, and tornadoes.

A level 1 was issued across N Spain ... the Alpine regions ... Germany ... Benelux mainly for large hail, damaging wind gusts, and excessive rainfall.

A level 1 was issued across France mainly for large hail, damaging wind gusts, excessive rainfall, and tornadoes.

A level 1 was issued across the N Ukraine and S Belarus mainly for large hail, damaging wind gusts, and excessive rainfall.


A deep upper long-wave trough is setting up over the eastern Atlantic, with several short-wave troughs crossing western Europe at its eastern periphery. One of these impulses will maintain a wave low along the main low-level baroclinic zone, which should stretch from western Iberia across southern England into the North Sea early on Tuesday. East of this front, a moist and rather unstable air mass is present. This air mass along with the frontal wave will likely be the focus for another episode of potentially severe convection on Tuesday.


... N France ... Benelux ... W Germany ...

Instability: It seems that instability will be a little weaker than in the same air mass on Monday, given CAPE consumption by the extensive convection over France and Benelux on Monday night, as well as lingering convective debris on Tuesday, which should limit insolation. Still, rather moist low levels and at least modest diabatic SFC heating should allow for MLCAPEs in excess of 1000 J/kg over western Germany and Benelux. Farther east, where the air mass should be unaffected by previous convection, somewhat higher MLCAPE may accumulate, possibly up to 2000 J/kg.

Shear: GFS suggests a broad region including much of France, Germany, and Benelux of 15 m/s DLS, and a somewhat localized region of 20 m/s over W Germany and Benelux. However, the best deep-shear and instability will become increasingly decorrelated as the day and evening progress. Much of the shear should be confined to the lowest 3 km, though the shear close to the ground (0-1 km) is somewhat marginal with 7-12 m/s.

Lift: Large-scale forcing for ascent will be provided mainly by a short-wave trough that will cross the region rather early in the day. As a result, convective development should ensue rather early, probably around local noon. Convective initiation will likely take place along outflow boundaries laid out by Monday nights convection, near orographic features, and/or along the synoptic-scale cold and warm fronts.
Storm mode: The reasonably strong shear profiles suggest convective modes ranging from well-organized multicells to supercells and bow echoes, with all modes potentially coexisting, although there will be the usual tendency for initially isolated cells to merge into a linear MCS given the deep-shear being largely parallel to the SFC cold front. The primary threats with this activity will be severe wind gusts, excessive rain, as well as large hail. A tornado or two are possible with isolated cells, especially if storms interact with outflow boundaries or orogenic circulations.
A small chance exists that storms form along the warm front and briefly tap air N of the front, before becoming elevated. At this stage, the storms would ingest strongly-sheared low-level air and would have increased potential for tornadoes. However, this scenario is rather uncertain.

Overall, the convective threat justifies a level-2 risk. The activity should begin rather early in the day and may last into the late evening hours.

... N Spain ... France ... Alpine regions ... S Germany ...

Additional storms may develop elsewhere in the air mass, mainly along orographic features and along outflow boundaries (France). The severe threat will be due mainly to large hail, severe winds, and excessive rainfall. Especially over France, low-level shear in the 7-12 m/s range suggests that a brief tornado cannot be discounted either.

... N Ukraine ... S Belarus ...

A few marginally severe hail/wind/rainfall events are possible in a rather strongly sheared air mass over the Ukraine and the Belarus. Storms should diminish with diurnal heating given weak/no large-scale forcing for ascent over the area.

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