Storm Forecast
Valid: Sun 05 Aug 2007 06:00 to Mon 06 Aug 2007 06:00 UTC
Issued: Sun 05 Aug 2007 11:25
Forecaster: VAN DER VELDE

SYNOPSIS

A large mid level omega ridge of high pressure spans the area from eastern Spain to its center over the Baltic Sea. An old upper low with unstable airmass drifts around over western Russia. A large cold pool associated with a cut-off upper low continues to create unstable conditions over the Balkan into Turkey.
Flow over the Iberian Peninsula has become cyclonic due to the approach of an upper trough.
A band of unstable air, partly elevated, associated with the warm sector of a frontal wave may cause some thunder over parts of the British Isles.


DISCUSSION

...Iberian Peninsula...

Very steep lapse rates from the African-Iberian elevated mixed layer create unstable conditions with the advection of low level moisture (Td observed of 21-25 degrees in southern Spain) and the cooling effects of QG lift on mid levels. 00Z soundings show deep inverted-V profiles, suggesting a threat of dry microburst (...and enhanced threat of wildfires due to lightning combined with evaporation of rain before reaching ground).
GFS forecasts 500-1500 J/kg MLCAPE and reasonable but not strong shear conditions (<15 m/s deep layer shear, <150 m2/s2 SREH, <8 m/s low level shear). This will likely result in multicell storms capable of producing severe gusts and large hail. During the evening they may group into a mesoscale convective system (MCS), most likely over central northern Spain to Pyrenees. Southwestern France may also see some convection/MCS during the late night/early morning hours. As there is no clear linear forcing and thermal gradients, and relatively weak flow, the threat of gusts with these systems will be low, but precipitation amounts may reach several tens of millimeters.

...United Kingdom...

A band of strong 0-1 km shear and some backed flow near the surface front is forecast by GFS, where also some modest CAPE is predicted. Together with the very low LCLs forecast, a storm that manages to organise itself well may produce a tornado.

...E Greece, S Bulgaria, NW Turkey...

SREH 200-300 m2/s2 and 1-8 km shear of 20-30 m/s (15-25 m/s 0-6 km) around the upper low with backed surface flow occur in an area of weak CAPE (<500 J/kg mostly). Due to continuous convection, profiles should be rather moist with weak lapse rates and weak CIN over western parts of the area. An isolated storm may profit from better instability and shear and develop supercellular characteristics, enhancing chances of large hail and gusts. Where low level shear is enhanced to 10-15 m/s also due to complex terrain over the southern Balkan, a tornado may belong to the possibilities as well.