Storm Forecast
Valid: Tue 17 Oct 2006 06:00 to Wed 18 Oct 2006 06:00 UTC
Issued: Tue 17 Oct 2006 11:25
Forecaster: VAN DER VELDE

SYNOPSIS

The charts show three low pressure areas associated with deep convective activity:

1. Large Atlantic low, peripheral disturbance over Ireland, frontal zone over Spain, western France and southwestern British Isles. Wave in frontal zone could enhance convection over Gulf of Biscay during evening. Central convective airmass affecting the west coast of Portugal and Spain after midnight.

2. Highly baroclinic situation associated with a low surface pressure system near an cutting off upper trough near Crete and Turkey.

3. Black Sea low / upper cold pool, stabilizing.

DISCUSSION

...southwestern Turkey area...

Unstable airmass continues to give active convective systems, while the low intensifies with warm air flowing into the area from the south and cooler northern flow into the low inforcing a cold front over Crete during the period. Convection is expected to focus in this convergence zone, with little capping and LCL heights below 1000 m.

With the main low pressure core south of Crete, 10m winds will back to southeasterly directions, remaining strong at 15 m/s into the area, while midlevel flow is strong from the southwest (>20 m/s at 700 hPa in GFS).
Resulting: DLS over 25 m/s, LLS over 13 m/s and 0-3 km SREH amounting to more than 200 or even 300 m2/s2.

In these conditions long-lived MCSses can develop, as well as (tornadic) supercells, with also a threat of large hail and large scale severe gusts. The south coast of Turkey may become affected by flash floods due to long period of convective rains.

...Portugal, Spain, western France...

Front can feature embedded convection, which seems to be mostly of elevated nature given that practically no CAPE is present in GFS over central Spain. This limits the risk somewhat of storms bearing large hail and tornadoes, something not unthinkable in present kinematic conditions. Surface-based convection is more likely over Portugal and southwestern Spain, where moist Atlantic airmass can intrude the land.

DLS around 20-25 m/s, LLS ranging 10-15 m/s, and SREH over 200 m2/s2 in principle create favourable conditions for supercells (large hail), also tornadoes, with the LLS and LCL heights of below 500m supporting this threat.

The wave affecting the Biscay area is under lesser but still moderate shear conditions, so an isolated severe event there isn't ruled out.

Along the east coast of Spain Mediterranean moisture will converge later in the period, possible giving rise to surface-based convection, which as well may become severe given widespread favourable shear conditions.

...Ireland...

Cannot exclude the possibility of isolated severe weather as a patch of instability passes with the trough, with over land shear enhanced to moderate-strong values. Brief rotation may develop in cells and augment the chance of large hail, and a marginal chance of a tornado, as LLS > 10 m/s and low LCLs.