Storm Forecast
Valid: Mon 27 Aug 2007 10:00 to Tue 28 Aug 2007 06:00 UTC
Issued: Mon 27 Aug 2007 10:04
Forecaster: TUSCHY

SYNOPSIS

A branch of an ill defined polar vortex continues to strengthen and starts to build southward, covering most parts of N-Europe. This will herald cool and unsettled conditions over Norway, Sweden and Finland.
The upper-level low just west of Portugal strengthens the WAA over the western Mediterranean, supporting hot and mostly dry conditions.
A broad upper-level trough over SE Europe helps to spark numerous thunderstorms...otherwise very warm and dry conditions prevail over the eastern Mediterranean.

DISCUSSION

...Coastal areas of NW / N Spain and SW France ...

Extremely hot and dry air downstream of the upper-level low pressure area just west of Portugal pours northward, covering most parts of Spain, western Mediterranean and extreme southern France. Temperatures at 850hPa are forecast to reach 20 - 25°C and hence a stout cap will suppress convection.

However there are areas, where at least a marginal chance for some development exist.
Dewpoints along the NW / N coast os Spain are in the upper tens and reach 20°C over SW France. An impressive elevated mixed layer is present ( please refer to the 00Z sounding ascent of Bordeaux) and mid-level lapse rates are forecast to continue to steepen. The model pool is quite inconsistent regarding convective activity, but we will introduce at least a chance for an isolated storm, given some mid-level forcing during the night hours.
Dry air between 800 and 900 hPa could partially mix down, as diabatic heating continues, but dewpoinst will still be in the upper tens and combined with very steep mid-level lapse rates, up to 1.5 kJ / kg instability can be expected. Elevated nature of developing storms, 20-25m/s DLS and enhanced SRH3 values would be supportive for an enhanced large hail threat, mainly along the W/SW coast of France. An upgrade may become necessary, if storms indeed develop.

... Bulgaria and western Turkey...

A broad upper-level trough will influence the area of interest. Latest sounding ascents of Istanbul indicate a constantly moistening atmosphere and up to 1800 J/kg MLCAPE at 00Z. Scattered thunderstorms are forecast to develop in a weakly sheared environment but 15m/s DLS and marginal enhanced SRH values would be still supportive for an somewhat more organized thunderstorm threat with an attendant large hail risk. We placed the highest probabilities along the strongest shear/instability axis.

...Denmark,Baltic Sea and S-Sweden...

An intense short-wave trough will pass the forecast area from the WNW during the following 18h. Strongest instability signals can be found mainly offshore, as cold mid-level air overspreads the region . Good timing of the passage of the trough axis, low LCLs and some weak signals in the helicity charts could support weak LL mesocyclones with an attendant short-lived tornado report ( mainly over eastern Denmark and S-Sweden), but threat is way too marginal for issuing higher probabilities. Otherwise shear is too weak for any organized convection.