Storm Forecast
Valid: Tue 15 Aug 2006 06:00 to Wed 16 Aug 2006 06:00 UTC
Issued: Tue 15 Aug 2006 10:13
Forecaster: VAN DER VELDE

SYNOPSIS

A surface low pressure area and collocated upper cold pool affect southern Scandinavia.
Advection of warm, moist air over eastern Spain and southern France ahead of an upper trough destabilizes the airmass causing numerous thunderstorms in a deeply sheared environment. Over Russia and the Ukraine, thunderstorms will initiate within the theta-e plume, while an upper shortwave and jet create lift and vertical shear.


DISCUSSION

...Russia, Ukraine...

Strong instability is present in some Russian 00Z soundings, with steep low-midlevel lapse rates and veering wind profiles. All along the front storms may initiate, that given 15-20 m/s deep layer shear, moderate storm-relative helicity (up to 200 m2/s2) and moderate (5-8 m/s) 0-1 km shear may become supercells with large to very large hail, severe outflow winds, and possibly an isolated tornado, especially given LCL heights (<1000 m). A few MCS storms with bow echoes and damaging winds are anticipated.
The upper shortwave will affect the area towards/after midnight with increased deep layer shear and lift, possibly in a second round of storms.

...northeastern Spain, southern France, northwestern Italy...

Ahead of upper trough the low level flow returned to southerly directions, bringing warm moist air northward into the level 1 area. As midlevel and upper winds at the nose of the trough peak to over 15 m/s at 700 hPa, 20 m/s at 500 hPa, and 40 m/s at 300 hPa, with light easterly to southerly surface winds, strongly veering wind profiles are created with strong deep layer shear (around 20-25 m/s) and storm-relative helicity (250 m2/s2 in level 2 area). Low-level shear will be moderate to strong through the level 1 area. Expect 500-1200 J/kg MLCAPE to be present. In this environment, storms may develop rotating updrafts, and these supercells pose a threat of (very) large hail, severe gusts, and given moderate low-level shear and low (<1000 m) LCL heights, also a few tornadoes.

The organisation of storms will be greatest in northeastern Spain, where GFS lays a strong boundary between moist and dry air, with widespread, probably less organised storms over southern France, and Italy into the night/morning. Upslope flow of moist air in southern France may create a continuous period of thunderstorms, with copious amounts of precipitation leading to local flash floods. Due to strong shear environment, large hail and perhaps a tornado/waterspout or two are also possible.

...Denmark, southern Sweden, Baltic Sea coasts...

With unstable airmass swinging around in the low, and steep low level lapse rates created by the difference of cool air temperature and warm Baltic Sea/Kattegat surface temperature, several waterspouts will be possible, some capable of doing damage. 00Z Copenhagen and Schlesswig indicate fairly classic profiles for spouts, with weak surface winds. Indication of strong low-level shear and veering with height in the Copenhagen sounding may imply that although primary tornadogenesis is by stretching of pre-existing vertical vorticity, strong updrafts may also tilt locally available horizontal vorticity into the vertical and create stronger spouts (tornadoes). Convection over wind shift lines should be monitored closely.