CONVECTIVE FORECAST
VALID 06Z SUN 27/07 - 06Z MON 28/07 2003
ISSUED: 27/07 03:47Z
FORECASTER: GROENEMEIJER
+ + + CORRECTED + + +

There is a moderate risk of severe thunderstorms forecast across southwestern, central and northeastern Germany, the western Czech Republic and northwestern Poland

There is a slight risk of severe thunderstorms forecast across an area surrounding the moderate risk area including a large part of Germany, eastern France, the Alps, western Poland and southeastern Sweden

General thunderstorms are forecast across much of western continental Europe, southern and central Scandinavia and parts of eastern Europe and northern Turkey

SYNOPSIS

An upper trough over the eastern Atlantic is gradually moving eastward into Europe. Downstream...a strong theta-e advection regime is in place over western Europe. A waving frontal zone is expected to be located from the Benelux countries over western France into Spain at the beginning of the forecast period. One wave will likely be moving slowly northeastward from the northern Benelux. A second wave will be developing ahead of the upper trough over southern France and move northeastward. Extent of development of the wave is uncertain as the numerical models differ quite strongly. AFWA MM5 00Z has a relatively strong development of the wave and a slow progression speed, whereas GFS 18Z has a more rapid progression and a later development of this wave. AFWA MM5 18Z had much less development.

DISCUSSION

...France, Switzerland and southwestern, central and eastern parts of Germany, Poland and the Czech Republic....
Within the warm advection regime east of the cold front, a few elevated storms will be ongoing over south-central France and later over eastern France. Per model forecast a pre-frontal convergence line is expected to form over eastern France during the morning hours, which will move eastward into Switzerland and southern Germany during the late afternoon or early evening. At that time...low-level moisture advection, solar heating and large-scale ascent will have destabilised the air-mass ahead of the line. Initiation of surface-based convection is expected over southwestern Germany, Switzerland and extreme eastern France. Forecast soundings indicate that on the order of 1000 J/kg MLCAPE50 should be available to the convection. Given forecast deep-layer (0-6km) shear in the 40 - 55 kt range, expect that some storms will become supercells. Strong forcing suggests that storms will rapidly increase in coverage and merge into linear convective systems. The storms will bring a risk of large hail and damaging gusts over extreme eastern France, Switzerland and southwestern Germany. The low level flow pattern will not be particularly favourable for tornadogenesis in this area but one or two tornadoes cannot be excluded. During the evening...the MCSs will move northeastward into central and northeastern parts of Germany, the western Czech Republic and northwestern Poland. Expect the risk of damaging gusts to gradually increase as low-level shear increases which may result in the development of bow-echoes. In addition to the aforementioned threats...very large precipitation amounts may accumulate over the central and northeastern parts of Germany causing flooding problems in places. If the aggressive scenario of AFWA MM5 00Z verifies, strong low level shear and a large damaging wind threat will develop there...so that an upgrade to HIGH risk may be needed.

...Bavaria and Austria...
Extent of convective development over Bavaria and Austria is quite uncertain since he amount of moisture and forcing that will be available there are unclear. Current thinking is that scattered high-based storms will form over northern Austria and Bavaria during late afternoon and evening that pose a threat of strong downdrafts/severe gusts and locally some large hail.

...southeastern Sweden...
Uncertainty with respect to the development of the frontal wave implies that the forecast for southeastern Sweden is unclear as well. It seems likely that the wave and the associated convective activity will be located near southeastern Sweden at the end of the forecast period and that some 100s of J/kg CAPE will be available in its warm sector. Strong low-level shear seems likely so that storms will be capable of producing strong gusts and perhaps pose a threat of a weak tornado and some large hail as well. This threat is conditional, but the potential impact of the storms will warrant a slight risk.

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