Storm Forecast
Valid: Fri 14 Mar 2008 06:00 to Sat 15 Mar 2008 06:00 UTC
Issued: Thu 13 Mar 2008 18:16
Forecaster: TUSCHY

SYNOPSIS

The forecast area is cleft into a more progressive pattern over the western half of Europe and a pool of low geopotential heights over eastern / northern Europe with stable conditions during the next 24 hours.
Regarding thunderstorm potential the eastward moving depression attracts our attention as this feature reaches the Bay of Biscay during the morning hours of the 15th. Phase diagrams indicate a transient evolution of a shallow warm core structure, which could be an allusion for an overall enhanced convective activity around the depression's center but conditional moisture content of the airmass and weak instability release around the center should restrict the thunderstorm coverage.

DISCUSSION

... North Portugal and NW Spain ...

Placement and strength of the approaching upper-level trough is still somewhat uncertain, but is crucial regarding placement of strongest WAA axis / strongest capping. During the past few days, a general trend was detected that models want to advect the warm and dry N-African airmass from Morocco to SW France during the forecast period with readings at 850hPa of 15°C over extreme SE-Spain and 10°C all the way up to SW-France. Strongest WAA is confined to a region above 800hPa, resulting in a rapidly strengthening cap and as boundary layer airmass is dry and stays dry with readings in the single digits, no convection can be anticipated over most parts of Spain and S-Portugal.
Environment gets more conducive for a few thunderstorms over N-Portugal and NW Spain after about 21Z as strong cyclonically-curved flow overspreads the area from the west. At the surface, dewpoints around 10°C will be present and as forcing gets stronger and mid-levels cool down, a few isolated thunderstorms could develop as early as 21Z. Thunderstorm activity should increase during the night hours, as cold front approaches those regions from the west around midnight. It is somewhat uncertain how many thunderstorms can manage to develop in the prefrontal airmass as mid-levels are very dry and thermodynamics are marginal at best.
The best chance for a few severe thunderstorms would be a time-frame between 21Z and 03Z over NW Spain and N-Portugal as shear would be supportive for rotating storms and a level-1 could become necessary if confidence in thunderstorm initiation and higher coverage ahead of the cold front will grow.

Otherwise, best shear and instability stay separated and marginal hail and gusty winds will be the main threat.

A few thunderstorms will move off the coast of northern Spain / develop over the Bay of Biscay and enhanced speed shear / left exit region of a 700hPa jet over the SE-corner of the Bay could support a few strong to severe thunderstorms with a severe wind gust risk.