Storm Forecast
Valid: Tue 06 Oct 2009 06:00 to Wed 07 Oct 2009 06:00 UTC
Issued: Tue 06 Oct 2009 05:26
Forecaster: VAN DER VELDE
A level 1 was issued for England and NW France mainly for the chance of an isolated tornado.
A level 1 was issued for W Iberian Peninsula for chances of wind gusts, large hail, excessive precipitation and tornado.
SYNOPSIS
A low pressue area near the Faroe Islands moves eastward. The increasing thermal gradient due to colliding warm subtropical airmass advected by a steady SWly flow and a polar maritime airmass should inforce the cold front over Ireland and central UK during the period. The occluded front will drag over western Norway causing intense precipitation. Slight instability will be present over England and NW France, and marginal signals track downstream over the southern North Sea, Benelux, N Germany and Denmark, perhaps partially elevated in nature.
Another low pressure system arrives from the Azores to Portugal later during the period and is filled with an unstable airmass.
DISCUSSION
...S Ireland, England and NW France...
GFS predicted widespread convective precipitation over the eastern Atlantic for yesterday, however almost nothing occurred in reality. Lack of significant quasi-geostrophic forcing or even subsidence may have prevented this, and predicted LFC-LCL differences, which were quite substantial, could have played a role as well. With this in mind, a look at GFS reveals some CAPE over the southern half of the UK and NW France predicted mostly for 9Z-15Z, and now LFC-LCL height difference should be small and initialization easier, while QG forcing is better. Other models also predict a band of rain passing during this period. The cold front may provide additional forcing over Northern England, at the margin of CAPE.
Since CAPE and dynamics seem quite marginal (not very baroclinic situation and absence of the jet), it looks like a low end situation, however there is 15 m/s DLS, 150 mē/sē SREH and >12 m/s LLS predicted, with low LCLs, which in principle could yield cells with rotating characteristics and possibly an isolated tornado. Strong but probably not severe gusts are possible.
If convection is able to develop at the front, it could train parallel to it and augment precipitation sums locally.
...western Iberian Peninsula...
Larger instability (300-1000 J/kg CAPE, and potential instability) arrives to the Iberian Atlantic coast in the late evening and night, in combination with QG forcing, a deep moist airmass (effective PW around 40 kg/mē) and reasonably strong shear conditions: 15 m/s LLS, 150 mē/sē SREH and 25 m/s DLS. This suggests potential for supercells with large hail, severe gusts and an isolated tornado, as well as local flash floods. Corfidi MCS vectors hint at fast moving systems (>25 m/s) with chance of severe gusts. The best forcing appears to be in the northern part of the level 1.
