Storm Forecast

Storm Forecast
Valid: Sun 28 Jul 2013 10:00 to Mon 29 Jul 2013 06:00 UTC
Issued: Sun 28 Jul 2013 10:27
Forecaster: VAN DER VELDE

A level 2 was issued for parts of Germany, Switzerland, Czech Republic, and Poland mainly for severe wind gusts and large hail.

A level 2 was issued for southern France mainly for large hail and excessive convective precipitation.

A level 1 was issued for parts of central Europe, southern Scandinavia and the western Mediterranean area mainly for large hail and severe wind gusts.


SYNOPSIS

A plume of strong conditionally unstable mid level lapse rates has been advected from the Sahara into western and now central Europe by a deep southwestern flow at the east flank of a broad Atlantic mid level trough and the western half of a central European mid level ridge. A cold front lies stationary over central/eastern France, central Germany, bending westward into an occlusion over southern Sweden and Norway. A warm front stretches from southern Sweden southeastward toward the Ukraine.
Most of 00Z soundings in the eastern Germany show nearly dry adiabatic temperature profiles under 600 hPa and roughly neutral above. As a result, places with surface dewpoints over 17C should have CAPE with CIN removed in the late afternoon by diurnal warming. Over Poland and southern France CIN is already minimal by 12Z. The zone between southern France and central Germany is rather saturated and CAPE is consumed by storms as much as is regenerated by lapse rate recovery from quasi-geostrophic and frontal lifting processes. The same seems the case over southern Scandinavia. Low level convergence that should trigger convection is found along the fronts in the afternoon, should be strongest near the occlusion point over northwestern Poland, which remains stationary in the GFS model till 06Z the next day.
After 18Z, a shortwave mid level trough enters northeastern Spain producing strong QG lifting. It will then affect southeastern France during the night. It can likely destabilize and trigger convection also over sea near Corsica.


DISCUSSION

...Germany, Poland area...

Surface mixing ratios of 13-15 g/kg are currently reported along the cold front and warm front. More than 1000 J/kg of MLCAPE should be able to form. Southeastern Germany is an exception with a dry notch of 9-11 g/kg which should inhibit convective initiation. A backed flow (easterly 10m winds) is in place north of the Alps. This should provide good storm-relative helicity, model-predicted values increase to more than 250 m/s and 30 m/s deep layer shear along the German section of the cold front by 15Z. Supercells with (very) large hail are therefore quite likely, but will have to compete against linear clusters, which align with the front. Since the shear vector is rather parallel to the front, it will be mostly inactive in producing severe wind gusts, but isolated bowing sections with severe gusts are possible. Ahead of the front to the east are very deep boundary layers (2-2.5 km LCL) which support wind and hail threats.
The front likely activates later than a system in the north at the occlusion which has roughly the same severe weather threats.

...France...

The northern section of the front here is saturated and lacking CAPE, and existing storms will train and destroy downwind storm potential. But isolated excessive rain is possible. Southern France is more favorable. Northern Mediterranean moisture is flowing back to the north side of the Pyrenees and is continually supplied to the frontally triggered storms over southeastern France, which can backbuild for a long time and produce excessive rain as well as large hail. There, low-level shear is also enhanced to over 10 m/s which in combination with LCLs <1000m support an isolated tornado event.


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