Storm Forecast
Valid: Mon 05 Oct 2015 06:00 to Tue 06 Oct 2015 06:00 UTC
Issued: Mon 05 Oct 2015 06:10
Forecaster: VAN DER VELDE

A level 2 was issued for NW Spain and N Portugal mainly for chances of severe convective wind gusts, tornadoes and excessive convective precipitation.
A level 2 was issued for W France mainly for chances of severe convective wind gusts, tornadoes (night).
A level 1 was issued for parts of Iberian Peninsula and France mainly for chances of isolated severe convective wind gusts, tornadoes and marginally large hail.

SYNOPSIS and DISCUSSION

High pressure is present over Scandinavia extending to Italy. The mid level ridge separates a mid level cold pool over Hungary/Romania from an approaching large complex Atlantic low installing over the Celtic Sea. Strong southwesterly flow advects low-level warm air northward over France and the UK with a surface warm front from central UK to Switzerland at 12Z in GFS. The warm front slope is marked by stable lapse rates.

At the same time, a mid level cold front over eastern Spain to western France destabilizes the warm airmass with up to 200 J/kg CAPE mainly in southwestern France with relatively warm equilibrium levels indicated of just -15°C. Forcing may not be strong behind the front and flow over the Pyrenees creates a dry zone just to its north. GFS does predict significant convective precipitation signals over SW/Central France, later eastern France, but these seem related to elevated convection at the mid level cold front. The low level and deep later shear (20 m/s) are strong. Storms rooted in the boundary layer will have chances of tornadoes and severe wind gusts. Weaker moisture, lapse rates and low-level shear are present at 12Z over central Iberian Peninsula (i.e. except the south and north).

Meanwhile, the west coast of the Peninsula is under the main cold front of the low, where 25 m/s wind speed averaged over 1-3 km altitude is present along with strong 0-1 km shear, over 17 m/s and over 200 m²/s² of 0-3 km SREH overlapping with 200-500 J/kg MLCAPE. Significant convective rain sums could accumulate in N Portugal and Galicia. The cold front moves eastwards while CAPE values decrease. If storms maintain, severe convective wind gusts and tornadoes are quite likely, but CAPE is fragile and several WRF models do not produce much signal in precipitation. For this reason level 1 is maintained except northwestern Iberia.

Around 21Z focus will shift to the Bay of Biscay, while it likely becomes elevated and low level shear decreases. At 00Z in GFS-00Z the front comes ashore in southwestern France with 300 J/kg CAPE depicted mainly over sea with adjacent land (Bordeaux area) under 20 m/s (!) 0-1 km shear and 300 m²/s² SREH, supporting significant tornado/gust chances, continuing northward through the night, with mid level lapse rates being steeper than during the day as result of lifting from the main trough.

Another area of interest is southern to eastern Spain, where low-level lift and strong deep layer shear combine with weakly capped, low CAPE. This may yield some cells with rotating characteristics. As low-level shear is marginal to moderate, chances of gusts and tornadoes should not be so high as in the northwest.