Storm Forecast
Valid: Tue 20 Oct 2009 06:00 to Wed 21 Oct 2009 06:00 UTC
Issued: Tue 20 Oct 2009 03:57
Forecaster: VAN DER VELDE

A level 1 was issued for Moldova and Ukraine for a chance of tornadoes and excessive precipitation.

A level 1 was issued for southern Portugal and southern Spain for a chance of tornadoes.

A level 1 was issued for eastern Spain for a chance of tornadoes and excessive precipitation.


SYNOPSIS

The cold front of a very large low pressure system, situated between Greenland and Morocco with its center west of Ireland, reaches the Iberian Peninsula and western France. On both sides of the front destabilization takes place. Predicted EL temperatures are initially not so cold over land (-10°C) which could inhibit thunder, but chances are better in the south and east where more moisture is advected onshore.
Another upper low centered over Hungary affects the area around the Black Sea, with northward advection of a warm sector unstable airmass into Moldova and Ukraine.

DISCUSSION

...Moldova and Ukraine...

A shortwave trough swings around the low and forces strong lift over an occlusion area. CAPE gets smaller farther inland while vertical shear values reach 15 m/s and 25 m/s respectively for 0-1 and 0-6 km shear, SREH can go over 300 m²/s². This suggests supercells may be able to form and produce tornadoes. Else, persistent convective rainfall from training storms may locally produce flash flooding.

...southern Iberian peninsula...

Associated with the cold front, a band of slight instability and strong shear (15 m/s LLS, 25 m/s DLS, 250 m²/s² SREH3) at the right entrance region of a jetstreak could produce storms with a chance of tornadoes. Severe gusts are also possible from bow echoes as the cold front pushes quickly SE-ward, rather than drag slowly over a region for long periods of time. Corfidi MCS propagation vectors can reach 25 m/s.

...eastern Spain...

Instability will be present to some extent during the day, but is forecast to increase during the final hours of the forecast period (Wed 03Z-09Z) as the cold front (now with cyclogenesis) comes through. Vertical wind shear, forcing and water content should also increase. Storms will likely develop along the coast and move onshore, where shear is increased (12 m/s LLS, 15 m/s DLS, 150-250 m²/s² SREH) for a part due to friction. A tornado, strong/severe gusts, or isolated marginally large hail are not ruled out. Strong onshore moisture flux and storms contribute to intense rainfall, which could cause local flash flooding, but because the system does not appear to stall over the region a level 2 for precipitation seems not necessary.