Storm Forecast
Valid: Tue 29 Dec 2009 06:00 to Wed 30 Dec 2009 06:00 UTC
Issued: Tue 29 Dec 2009 05:21
Forecaster: VAN DER VELDE

A level 2 was issued for SW Spain and S Portugal mainly for excessive convective precipitation.
A level 1 was issued for Spain and Portugal, and W France mainly for a chance of severe convective wind gusts and tornadoes.


SYNOPSIS

A long warm front belonging to an Atlantic low moves northward and will (at surface level) become stationary over southern England, Netherlands and central Germany by Wed 06Z, according to GFS18Z. The front marks a strong contrast in temperature regimes. Sub-zero temperatures to the north, temperatures above 10°C to the south. On the warm side, over NW France and eastern Atlantic, CAPE is present. Convective clouds developed already during the night. Another strip of significant CAPE associated with a cold front will affect the SW part of the Iberian Peninsula for mainly the first half of the period.

DISCUSSION

...SW Iberian Peninsula...

Destabilization is forecast so that by the time the cold front reaches Portugal several hundred J/kg CAPE should be present. At the same time, EL temperatures are forecast to be relatively warm and the large PW content combined with high saturation (PW-sat. deficit below 300 hPa of 35 kg/m²) indicate an environment of high precipitation efficiency. Storms may organize along a regenerating line which at times trains slowly through the level 2 area, which has the highest chance of excessive precipitation. But storm cell speed itself should be fast in the strong flow, more than 25 m/s, and 1-3 km AGL mean winds can reach up to 75 kts (>35 m/s) over central Spain. So, severe gusts are possible, possibly from bow echo line segments, wherever convection can occur - GFS suggests even central Spain has at least a small amount of CAPE at 15Z. In the same area, DLS can reach very high values around 40 m/s 0-6 km, SREH 300-400 m²/s² and LLS 15-20 m/s. An isolated tornado may occur.

...NW Iberian Peninsula...

Around 00Z a new disturbance with unstable air will come onshore, backed by a mid level PV anomaly. Strong deep layer shear of 30-45 m/s with a steep gradient will overlap the unstable air. LLS is >17 m/s. Tornadoes and severe gusts could occur.

...W France...

This region is highlighted because unstable air advected onshore till 18Z or so is crossed by good deep layer shear (20-35 m/s). SREH is mildly enhanced to 100-200 m²/s² and low level shear above marginal >10 m/s. An isolated tornado may occur.

...S England...

In the slope of the warm front, strong SREH of 450 m²/s² is forecast, with weak instability indications immediately to the south. It is not unthinkable that some convection may develop in this zone. While most veering is below the sloping fontal plane, the sounding of Brest 00Z indicates also decent shear above it. However, most likely an elevated cell is not going to do much in the way of severe weather.