Storm Forecast
Valid: Sat 24 Jun 2006 06:00 to Sun 25 Jun 2006 06:00 UTC
Issued: Fri 23 Jun 2006 21:59
Forecaster: GROENEMEIJER

SYNOPSIS

Saturday at 06:00 UTC... at 500 hPa a shortwave trough over eastern Finland is rapidly moving eastward. To the southwest... a SWly flow is present over much of western Europe downstream of a closed low north of Scotland and a sharp shortwave trough extending along the Portugese west coast. Warm air advection takes place over much of France, the Low Countries and Germany.

DISCUSSION

Spain, southwestern and central France...

Ahead of the Portugese trough, DCVA-related forcing upward motion is expected over Crtl Spain in addition to frontogenetical forcing over Nrn Spain. As a result of adiabatic cooling of mid-levels in association with the upward motion, scattered elevated storms are likely ongoing at the beginning of the period.
Per GFS surface-based instability corresponding with 500-1000 J/kg MLCAPE50 will develop. The lift should help erode the cap, too, so that surface-based convective storms are expected to form over Nrn and Ctrl Spain and SWrn France during the early afternoon.
Deep-layer shear on the order of 15-20 m/s suggests that multicells and a few supercells will form. These will likely bring isolated large hail, while the steep lapse rates suggest that evaporational cooling will be strong enough for a few severe downbursts to form.
As storms cluster, one or two large MCS's can be expected and that may bring more widespread strong and severe wind gusts, especially over Srn France during the evening, where southerly low-level winds should increase in repsonse to pressure falls over SWrn France. A significant threat of the storms appears to be high precipitation, which may cause flash floods in some places.
During the evening and overnight, additional -increasingly elevated- storms are expected to form over central and northern France that may bring a couple of strong gusts and some small hail as well, but their primary threat will likely be flash flooding.