Storm Forecast
Valid: Sat 20 Mar 2010 06:00 to Sun 21 Mar 2010 06:00 UTC
Issued: Sat 20 Mar 2010 06:40
Forecaster: VAN DER VELDE
A level 1 was issued for N Portugal and NW Spain mainly for isolated excessive precipitation.
SYNOPSIS
In a persistent southwesterly flow over western Europe, near-neutral, moist profiles are advected.
Over the western part of the Iberian Peninsula somewhat higher instability and dynamic forcing are present.
DISCUSSION
...Portugal and NW Spain...
In the advected airmass conditions for convection are becoming better, with higher/colder ELs in the afternoon. Orographic lifting as well as coastal convergence in the N Portugal/Galicia area could enhance precipitation sums there.
During morning and afternoon, vertical wind shear and helicity are sufficiently strong to mention a chance of a tornado. 25 m/s DLS, 10-15 m/s LLS and 125 mē/sē SREH3 should be present before 15Z, with very low cloud bases.
...N France, Benelux, W Germany...
The GFS model predicts consistently very slight CAPE over the this area, but in different positions every time. Dynamic forcing should also be nearly neutral, and there appear no features in 10m winds in GFS. It is possible for a very isolated thunder to occur, which may be elevated in the warm air advection regime. 00Z Brest sounding shows potential instability which could be turned into convection with some lift. In principle low level shear and helicity are decent enough for an isolated tornado if a stronger cell manages to develop, though deep layer shear is not great at 15 m/s. There is more doubt than there is confidence in this scenario.
