Extended Forecast
Valid: Tue 09 Sep 2008 06:00 to Wed 10 Sep 2008 06:00 UTC
Issued: Sun 07 Sep 2008 19:06
Forecaster: TUSCHY

SYNOPSIS

A branch of low geopotential heights SW of Iceland sprawls SE-ward. Along this baroclinic zone, numerous disturbances rapidly gain strength, while circling around the southern periphery of this feature. One of those depressions is Ex-hurricane Hanna and its input of very warm/moist extropical air should lead to rapid intensification. Models in general have not the best handling with such features and further seesaw changes of placement/strength of this depression are likely. Changes downstream of this system also have an influence on the European weather, especially for NW Europe. Right now, an active trough should affect NW Europe during the forecast period, while strong ridging results in hot and stable conditions over the Mediterranean. An intense baroclinic zone sets up over NE Europe as cold air spreads south/southeastwards.

DISCUSSION

... NW/N-France, SE-UK ...

Downstream of an approaching upper trough axis, models agree on a well evolved Spanish plume, characterized by a warm and humid airmass, spreading north/northeastwards. This airmass advects northwards behind a warm front, which lifts to the N/NE during the day, reaching N-France / SE-UK during the midday hours. Atop of the tongue of moist BL air, mid-level lapse rates acquire adequate strength to support at least modest instability release with 500-1000 J/kg MLCAPE, fanning out/weakening during the early night hours over SE-UK. DLS in excess of 25m/s builds eastwards over the instability axis during the afternoon/evening hours. Dependant on the final placement and strength of the surface trough, a broad area with LL shear of 10-15m/s and SRH-1 values of 100-200m^2/s^2 could establish with a tornado / large hail risk. A level area was issued and later outlooks may refine the area with new model data.

... Spain and SW/Central-France ...

Upper pattern is somewhat diffuse as trough over the Gulf of Biscay constantly weakens/retreats northwards. Hence it is unclear, how much forcing will cross the extensive area of interest although the more to the west the better the chances to be under one of numerous short-waves, traveling NE-wards. Hence, global models change their hot spots regarding initiation more or less all the time although areas with mountainous terrain seem to be in favor. Another point of uncertainty will be degree of diabatic heating and hence mixing of BL during the day, which could have an impact on surface dewpoints/degree of instability release. Better BL moisture will be over E-Spain, but limited forcing makes initiation more questionable so for now only a broad thunderstorm area was included, but regional upgrades may become necessary as 15-20m/s DLS , some veering with height and steep mid-level lapse rates hint on a large hail risk.

... Highlighted area over extreme W-Russia, mainly S/SW of Moscow ...

Ahead of a slowly eastward propagating cold front, conditions become supportive for a tongue of enhanced instability to build ahead of this front during the midday / early afternoon hours. GFS has strong LL CAPE release ahead of this boundary and if SBCAPE values in excess of 1000 J/kg verify, an isolated tornado risk would exist.