Storm Forecast
Valid: Sat 08 Aug 2009 06:00 to Sun 09 Aug 2009 06:00 UTC
Issued: Fri 07 Aug 2009 21:58
Forecaster: GROENEMEIJER

A level 1 was issued for eastern Spain mainly for large hail and severe winds.

A level 1 was issued for the Pyrenees mainly for excessive precipitation, and to a lesser extent for large hail and severe winds...

SYNOPSIS

Saturday at 06:00 UTC... a broad ridge still stretches from Scandinavia to the central Mediterranean. Within the upstream longwave trough, a shortwave trough is sharpening as it moves eastward. This system, which becomes a cut-off low, moves into Iberia during the forecast period, where it becomes more or less stationary.

DISCUSSION

Eastern Spain...

The approaching trough is expected to cause height falls that strengthen the thermal cyclone across the Iberian Peninsula. This increases the onshore and upslope flow across the eastern half of the Peninsula, at the same time that mid-troposphereic temperatures drop. This will result in significant instability developing during the day, with about 1000-2000 J/kg expected across much of the level 1 area.

Storm initiation should not prove problematic, and scattered storms are expected during the afternoon. A negative factor for severe weather development is the relatively weak vertical wind shear, with mostly below 15 m/s of bulk shear expected in the 0-6 km layer. Still, a couple of well-organized multicell storms are forecast.
Given the expected steep low to mid-level lapse rates, a distinct threat of large hail will exist with the storms. Additionally, the deep dry boundary layer will provide a large potential for evaporative cooling and hence, a few downbursts may occur.

Pyrenees...

The scenario as laid out above, with afternoon storms developing and posing some threat of large hail and potential downbursts applies to the Pyrenees as well, although CAPE and shear should be a bit weaker than further south. But additionally, the GFS, in contrast to ECMWF, suggests that convection will organize into an MCS across northern Spain Saturday evening. Such a system, being slow-moving due to weak mid-level winds and being fed with a moist flow from the southeast, would create a potential for very high rainfall accumulations in the Pyrenees. To acknowledge this, the level 1 has also been issued for this area.