Storm Forecast
Valid: Fri 19 Mar 2010 06:00 to Sat 20 Mar 2010 06:00 UTC
Issued: Thu 18 Mar 2010 21:26
Forecaster: TUSCHY

SYNOPSIS

The north Atlantic pattern features a train of large depressions, moving off the SE coast of the US to the east-northeast. The depression of interest moved from roughly 35°N/55°W to 40°N/30°W during the past 48h. A plume of tropical air, situated at 50°-70°W/20°-30°N was tapped and this air was ingested into a rapidly developing warm conveyor belt along the depression's southeastern fringe. Latest TPW anomalies hint on a robust plume of modified tropical air, which now characterizes the large warm sector, entering the forecast area from the SW. The proceeding warm front / N-Portugal/NW-Spain at 06 UTC, English Channel during the evening hours / is well organized and advances rapidly to the northeast and so does the aformentioned warm sector. At the same time, another high-level streak approaches our neutral tilted upper trough from the west over the N-Atlantic, causing an increasingly positive tilt during the end of the forecast period. This scenario also supports decreasing surface pressure west of Portugal, gradually advancing eastward and approaching N-Portugal/NW-Spain during the end of the forecast.

Geopotential heights increase over S/SE-Europe with stable conditions. Further north, a more zonal flow pattern evolves and fosters a gradual removal of the persistent dome of very cold, continental air. Convective-wise, this pattern change accommodates no thunderstorm risk, as CAPE is nearly non-existent.

DISCUSSION

... N/W-Portugal and NW-Spain ...

First round of somewhat deeper/more persistent convection arises along the interface of a NE-ward moving upper streamer and the west side of the warm conveyor belt, but also at the overlap of enhanced IPV and the NE-ward moving warm front further north, mainly offshore. Global models like GFS develop some elevated CAPE and isolated thunderstorms are possible from N-Portugal/NW-Spain up to N-Bay of Biscay. Latest WV loop does not support any serious thunderstorm risk and therefore no 15%-area was introduced for that activity and that time-frame, as overall conditions won't change significantly during the following 36h (warm front even tends to weaken somewhat while entering the Bay of Biscay thereafter).

Then (15-21UTC), the focus for isolated activity shifts to N-Portugal and NW-Spain but again, no robust convection threat is anticipated. Brisk SW-erly flow does not feature any significant UVV maxima and the air mass remains warm/moist throughout the low/mid troposphere. Weak lapse rates are the result and only low-end SBCAPE values are forecast. Main spot for isolated storms arises along the coast, where coastal convergence and increasing PBL friction enhance LL lift, which may support sporadic activity. Again, this is a low-end thunderstorm probability at best and we went with a 15-% contour. Strong gusts are the main threat.

Finally, the positive tilted trough draws near and results in a thunderstorm increase just offshore from N-Portugal/NW-Spain at 03 UTZ onwards (although variations in time still exist in the model pool). This time frame may be the one with the most action as isolated to scattered thunderstorms develop along the surface low pressure channel, moving eastwards and affecting the area during the morning hours. Again, CAPE is the limiting factor for more vigorous activity as shear at all levels would support some organization. Isolated severe wind gusts will accompany that activity, but limited coverage and overall weak nature of those storms preclude a level area for now. The tornado risk has to be evaluated in following model outputs as current indications remain borderline for an isolated tornado activity with good LL shear, but weak LL CAPE at best.

Despite the history of the air mass and probably repeated showers/thunderstorms in the 15-% thunderstorm area throughout the day, no serious rainfall risk is forecast with fast moving and disorganized storms.

It has to be emphasized, that a sporadic and short-lived thunderstorm event is not out of the realm of possibility for a vast area over W-Europe with marginal MUCAPE release, including the Bay of Biscay, the W-English Channel and most parts of the W/N-Iberian Peninsula. However, no real focus is seen, where enhanced thunderstorm activity would be a distinct possibility.