Storm Forecast
Valid: Sat 15 Mar 2008 06:00 to Sun 16 Mar 2008 06:00 UTC
Issued: Fri 14 Mar 2008 18:53
Forecaster: DAHL

SYNOPSIS

Feature of main interest this period will be an Atantic upper trough which is simulated to move from the Gulf of Biscay to NE France while evolving into a cut-off low. Attendant SFC low will likewise move into NE France/the Benelux countries. The cold front trailing from this low should cross France during the afternoon hours when build-up of weak CAPE is expected in the pre-frontal air mass. A warm front is extending eastwards from the SFC low, constituting the main low-level baroclinic zone over Europe, and will extend from the N UK across the North Sea southeastwards towards Romania/the Black Sea region at Saturday 12Z per latest model guidance.

DISCUSSION

... France ...

Evolution of the thermodynamic fields in the pre-frontal air mass over France is somewhat uncertain. GFS advects 6 to 8 K/km lapse rates from Iberia across France while low-level moist advection from the Atlantic is anticipated. Given mid-/upper-level cooling associated with the approaching upper trough and diabatic SFC heating, this should result in weak CAPE along and ahead of the cold front.

Kinematic profiles are expected to become sufficiently strong for severe evolution. Deep-layer shear of 20 to 25 m/s should develop across the warm-sector air mass in addition to low-level shear simulated to be just slightly below 10 m/s, and 0-3 km SRH in excess of 150 J/kg.

Rather strong large-scale forcing due to DCVA will exist along the front, and should aid in initiating deep convection. Given that the deep-layer flow should have a rather large component normal to the low-level boundary, cells may tend to remain rather isolated, which increases the chance for persistent mesocyclonic storms.

Thinking is that storms will initiate or dramatically increase in coverage in the afternoon hours, some of which should evolve into well-organized multicellular and mesocyclonic storms, posing a threat for severe wind gusts and large hail. Also, low-level shear profiles and configuration of low-level forcing/deep layer winds suggest that supercells may indeed become the dominant convective mode, in addition to hail and severe wind gusts posing a treat for isolated tornadoes.

After sundown storms should rapidly diminish, though weak/elevated debris may advect as far eastwards as SW Germany by Sunday early morning.