Storm Forecast
Valid: Mon 22 Feb 2010 06:00 to Tue 23 Feb 2010 06:00 UTC
Issued: Sun 21 Feb 2010 21:37
Forecaster: TUSCHY

A level 1 was issued for Portugal and parts of Spain mainly for severe wind gusts, large hail and tornadoes.

A level 1 was issued for SW-France mainly for an isolated tornado.

SYNOPSIS

An active frontal zone is displaced well to the south, affecting the Iberian Peninsula and the Mediterranean. This results in further precipitation events for areas, which already received enough rain. No extreme rain event is forecast during the forecast period. Scattered showers and thunderstorms affect mainly the Iberian Peninsula and the west-central Mediterranean. A cold and stable air mass still covers N-/E-Europe, which precludes deep convection.

As a side-note:

An outstanding and potential dangerous depression is in full progress during that forecast at roughly 48N/19W. A depression (probably a Shapiro-Keyser-type) becomes quasi-stationary over that region with forecast surface pressure dropping down to 964hPa (in repesct of latest OPC discussion) and outstanding signals of an intense warm seclusion process. Phase diagrams support that idea and the feature acquires shallow warm core characteristica and even a deep and symmetric warm core structure during the end of the forecast period. A good connection to a modified subtropical air mass causes a warm/moist back-bent warm front/occlusion to wrap around the center with a gradually intensifying hypergradient along its western fringe. Following the core structure, strongest wind evolve in the lowest 1-3km AGL, especially as the back-bent front wraps around the center. GFS sticks to that scenario for the past runs with winds at 850hPa increasing to 40-45m/s, which ought to yield surface wind gusts well in excess of 35-40 m/s just to the SW/S of the depression's center (42°-50°N/15°-25°W). We would not be surprised to see an eye-like feature, as convection consolidates around the center as it drifts eastwards.

DISCUSSION

... Portugal and Spain ...

The progressive weather pattern keeps going through the forecast as phased subtropical/polarfront jet resides over that area. Numerous disturbances, embedded in this westerly flow, cause enough lift for a prolonged period with showers/convection, as a moist, maritime air mass seeps in. An intense vorticity lobe crosses Portugal/Spain from the west (09-15 UTC) with the first round of enhanced convection. Showers/thunderstorms occur well inland, before BL ML mixing ratios decay. Up to 500J/kg SBCAPE are a reasonable guess with higher values over Portugal. 20m/s DLS is adequate for storm organisation, but hodographs remain straight, supportive for rapidly splitting storms. Large hail and severe wind gusts are well possible. The tornado risk is conditional but not zero. Directional shear in the lowest 1km is weak at that time, so we're not sure that the environment is supportive for organized LL mesocyclones. Nevertheless, an isolated tornado can occur.

After sunset, overall set-up remains the same with more showers/thunderstorms, but directional shear improves markedly. Therefore, the tornado risk increases somewhat after dark. Nocturnal boundary layer stabilization may be delayed or even suppressed by good mixing, so the inflow layer in thunderstorms remains low and probably surface based. Next to the tornado risk, severe wind gusts and large hail (mainly over S-Portugal/SW-Spain) remain the hazard with thunderstorms.

... Corsica and Sardinia ...

After midnight, thunderstorm probabilities increase with an approaching disturbance from the west. Modest shear and low-end CAPE preclude more organized storms, but multicells could still produce some marginal hail and gusty winds.

... SW France [15-20 UTC] ...

A short-wave enters the extreme S-Bay of Biscay around noon and moves to the east-northeast thereafter. Some moisture comes ashore during peak time heating, so low-end SBCAPE are forecast. Shear is modest in the lowest 3km and 6km, but strong in the BL with 15m/s speed shear and 250 plus m^2/s^2 directional shear. An isolated tornado is possible despite limited CAPE build-up (especially in the lowest 3km). The risk decreases rapidly after sunset.