Storm Forecast
Valid: Fri 08 Dec 2006 06:00 to Sat 09 Dec 2006 06:00 UTC
Issued: Thu 07 Dec 2006 23:20
Forecaster: DAHL
SYNOPSIS
Intense large-scale Atlantic upper trough will slowly continue to amplify while propgressing eastwards ... with this amplification mainly being a result of vorticity maxima rotating about the periphery of the main trough. This upper system is associated with an extensive surface-pressure system covering all except the SE parts of Europe and the NE Atlantic. Feature of interest will be a frontal wave which is already apparent in satellite imagery approaching the Gulf of Biscay, as of Thursday evening. This feature will become separated, with one part moving into the NW Mediterranean where is is expected to develop into a small and rather intense cyclone, and another part lifting across central Europe into central Scandinavia as frontal wave. A weak southward departing upper low will affect the S-central Mediterranean Sea.
DISCUSSION
... France ... Benelux ... Germany ...
It seems that severe TSTM activity along the cold frontal boundary on Thursday was associated to a certain mesoscale structure of the cold front involving special positioning of an upper/mid level dry intrusion. Indications are that this configuration is falling apart as the upper trough lifts northeastwards. The frontal wave which is expected to move from N France across the Benelux States and N Germany into the Baltic Sea on Friday does not seem to be likely to develop this special cold-frontal structure and it is anticipated that no convection will develop near or along this boundary. GFS suggests very weak CAPE over Iberia and France but LI's greater than 4 to 6 K, so neither forced nor deep free convection is anticipated at the moment. However, the mesoscale evolution of the frontal structure remains somewhat uncertain and should be monitored. As soon as convection develops, shear would be more than sufficient for severe evolution, including tornadoes.
... NW Mediterranean ...
GFS 12Z run struggles to develop deep CAPE ahead of the cold front of the SFC low moving into the N Mediterranean. However, scattered TSTMS are expected, especially late Friday evening and night. Potential should exist for severe wind gusts, marginally severe hail and possibly a few tornadoes. Though depth of convection is somewhat uncertain, a level-one threat appears marginally warranted.
... Ionian Sea ... S Aegean Sea ...
Scattered TSTMS may persist over the Ionian Sea in weakly capped air mass in the vicinity of the upper low. An isolated waterspout or two may occur with this activity given rather strong low-level buoyancy but allover severe threat should be quite low owing to weak CAPE/shear.
... Gulf of Biscay ... British Isles ... North Sea ... Norwegian Sea ...
Post-frontal cellular convection should spread across the Gulf of Biscay and the E Atlantic, which may affect western France and the British Isles. Though being imbedded in rather strong flow the effective shear may be rather low. Also, shear profiles will likely be rather straight-line and potential for supercells should be rather low. However, it seems that convection may become linearly-organized over the Gulf of Biscay and W France as suggested by GFS frontogenesis/temperature advection fields. This configuration may also result in backed SFC winds ahead of this feature, which may enhance the severe weather threat with this activity, but currently allover severe threat appears to be too low for a categorical risk.
