Extended Forecast
Valid: Wed 23 Dec 2009 06:00 to Thu 24 Dec 2009 06:00 UTC
Issued: Mon 21 Dec 2009 23:42
Forecaster: TUSCHY

A level 2 was issued for the Strait of Gibraltar and surrounding areas mainly for excessive rainfall.

A level 1 was issued for Portugal and most parts of Spain mainly for severe wind gusts, tornadoes, large hail and to a lesser extent for excessive rainfall.

SYNOPSIS

The progressive weather pattern keeps going for SW and central Europe. Numerous disturbances rotate around a far southward extending are of low geopotential heights over the far NE-Atlantic with the strongest one crossing the Iberian Peninsula during the night. Access to a tropical airmass increases thunderstorm probabilities over most parts of Spain and Portugal, whereas over the rest of Europe, only marginal BL quality precludes deep and electrified convection. Nevertheless, numerous other disturbances cause unsettled conditions over most parts of Europe.

DISCUSSION

... Portugal and Spain ...

This extended outlook was penned to reflect a potential high-impact event for most parts of the Iberian Peninsula.

EZMWF and GFS forecast solutions converged in the main points like strength of the surface depression although some discrepancies regarding the speed of this feature were still present. Both models are also more aggressive compared to the rest of the models, so detailed questions can't be answered yet. However, some more stress was laid on GFS due to its persistence and good handling of the current conditions over the N-Atlantic, verified with a few buoys/ship reports.

A strong, sub-990hPa depression comes onshore over far NW-Spain/N-Portugal during the night. This depression is characterized by an extensive warm sector, which overspreads nearly all of the Iberian Peninsula. The cold front of this depression is forecast to cross Spain from west to east after midnight although it remains questionable, how active this front will be, as pressure already starts to drop west of Portugal with some geopotential height increase over Spain. The front looks like to be most active over N-Portugal and Spain, as some high IPV-air, strong lift and surface cold front overlap. Further south, cold front starts to trail in very high PWAT airmass.

The main uncertainty will be the BL quality as any better instability will be bound to either the impressive moisture tongue over south/central Spain or the moisture pooling along the cold front itself. The warm sector will be strongly sheared, but limited lapse rates at mid-level niveau ought to keep thunderstorm probabilities at the lower-end. The concern arises as the cold front pushes eastwards over N-Portugal and NW-Spain (21 UTC) and north/central Spain (after midnight). If current shear parameters along the cold front verify (25-30m/s 0-3km shear, 20m/s 0-1km speed shear, intense directional shear and roughly 35m/s at 700hPa), a significant severe to damaging wind gust threat and tornado risk would accompany the cold front passage, next to isolated large hail. Another focus is the coast of Portugal and NW-Spain, where even in the postfrontal air mass, onshore flowing moisture and hence some modest CAPE and strong shear overlap, so tornadoes and severe wind gusts are possible all night long. The dynamic nature of this system should suppress any development of a stable stratified, nocturnal layer, so bad timing may be offset by strong LL wind field and abundant LL moisture. A level 2 may be needed, if new model data confirms the current trend of GFS.

A level 2 however was already issued for the excessive rainfall risk around the Strait of Gibraltar. A truely tropical air mass is advected northeastwards towards S-Spain and a prolonged period of lift and onshore flow ought to yield a favorable environment for excessive rainfall. In addition, the cold front starts to train during the night, so rain amounts likely match our criterion. Numerous waves of severe thunderstorms (see paragraph below) and strong rainfall potential increased the confidence enough to go with a level 2 that far out. However, significant modifications of this level may be needed, if probabilities further north may have to be increased.

... S-Portugal and S-Spain during the morning hours ...

Model discrepancies with a gradually decaying disturbance over S-Portugal and S-Spain during the morning hours are still significant, so not much confidence is yet present, but latest GFS data indicates a time-frame between 06 UTC and 15 UTC, where severe thunderstorms with a tornado/severe wind gusts and excessive rainfall risk are possible. Later model data have to verify this trend, before higher probabilities may be issued.

... S-UK ...

An isolated tornado risk could arise over S-UK, as modest onshore flow beneath cold mid-levels causes some weak instability over land. Shear and low LCLs would be adequate for an isolated tornado report. Uncertainties are still too large for any level area.