Storm Forecast
Valid: Fri 02 Mar 2012 06:00 to Sat 03 Mar 2012 06:00 UTC
Issued: Thu 01 Mar 2012 20:38
Forecaster: TUSCHY

SYNOPSIS

The omega pattern slowly breaks down as ridging over CNTRL Europe starts a gradual weakening trend. A faint upper trough affects the Iberian Peninsula during the forecast and results in an active day with showers and thunderstorms. An isolated short-lived thunderstorm event may occur along the coast of far N-Norway due to the overlap of steep mid-level lapse rates and a marine boundary layer. Expected overage and short window of activity precludes a 15-% thunderstorm area.
An isolated thunderstorm event may occur over far west Ireland during the end of the forecast period. Again, no thunderstorm area was issued due to the limited coverage of storms.

For the rest of the forecast area, conditions remain too hostile for deep moist convection.

DISCUSSION

... Spain, SW France and offshore areas east of Spain ...

The stage is set again for daytime driven showers and thunderstorms. Last WV imagery places a weak upper trough just west of the Iberian Peninsula with last frames of the HIVIS channel indicating moisture advection towards Spain (increasing depth of Cu fields atop the Gulf of Cadiz). MIMIC-TPW history does not indicate any significant pool of moisture available SW of Spain, so expect a moderate marine air mass to spread from SW to NE over Spain. This may push surface dewpoints up a couple of degrees, enough for some modest destabilization. 500 hPa temperatures remain cold beneath the trough with readings at or below -25 °C. Diabatic heating and forecast steep lapse rates will be adequate for surface based initiation, mainly bound to the complex topography (as seen yesterday). As the upper low moves in, all layers in the lower to mid troposphere see an increase in moisture, which limits DCAPE compared to the past 24 hours. Also, shear will be weak to non existent and therefore, short-lived pulsating storms with sleet/marginal hail and strong wind gusts will be the primary risk. Locally heavy rainfall may occur due to very slow storm motion. Thunderstorms gradually decay after sunset. A 50-% thunderstorm area covers mainly the complex terrain roughly from the Sierra Nevada all the way to the Picos de Europa and eastwards.

Thunderstorm probabilities were expanded far to the NE, including SW France. A tongue of steep mid-level lapse rates spreads northwards during the night and overspreads marginal BL moisture over SW-France. Up to 200 J/kg MUCAPE are possible and hence a sporadic short-lived thunderstorm event beneath the cold upper trough can't be ruled out. Nothing severe is anticipated.

The 15-% thunderstorm area was also expanded well offshore with increasing destabilization as the upper trough approaches. It is not yet clear why models have so much CIN in their outputs. Modified soundings show near zero CIN and at least isolated thunderstorm activity during the night hours is likely.

In the wake of the upper trough, weak ridging covers Portugal and keeps thunderstorm probabilities limited, although a short event can't be ruled out. Also an isolated thunderstorm risk arises west of Portugal after midnight as a cold front approaches from the west. Prefrontal moisture advection may assist in a concentrated tongue of low-end CAPE with a sporadic thunderstorm event. The thunderstorm risk in both areas will be too marginal for a thunderstorm area.