Storm Forecast
Valid: Fri 21 Sep 2007 06:00 to Sat 22 Sep 2007 06:00 UTC
Issued: Thu 20 Sep 2007 19:56
Forecaster: TUSCHY

SYNOPSIS

50hPa temperature maps of the past few days show a pool of warmer air crossing southern Greenland and Iceland during the past 1-2 days. This results in a strengthening polar front jet with up to 150kt at 300hPa just south of Iceland and is accountable for a rapidly eastward shifting upper-level trough. The wavelength of this feature is forecast to shorten significantly during the next 24 hours and split, while affecting northwestern and northern Europe. Despite this weakening trend it should help an upper-level trough over the Strait of Gibraltar to start an eastward movement during the forecast period and should overtake this opening upper-level feature until early Saturday.
A broad upper-level trough over eastern Europe will slowly lift towards the E/NE, but weak geopotential height gradients will keep the forward propagation quite low.
Otherwise numerous short wave troughs will cross Ireland, Scotland, Norway and Sweden from the west .

There are no significant LL boundaries present over most parts of Europe, which could focus any convective development.

DISCUSSION

...Gulf of Cadiz, the Strait of Gibraltar,eastern Portugal, most parts of Spain, the Alboran Channel, coastal areas of Morocco, Algeria and the Balearic Islands ...

Although run-to-run consistency was not good, the current model pool agrees quite well on the evolution and thunderstorm coverage.
At upper-levels a upper-level trough will shift eastwards, while weakening. As a result of a mitigating geopotential height gradient, jet streaks also weaken and the overall tilt of this system should stay neutral.

The main uncertainty was if and how strong the Sirocco could evolve over parts of northern Algeria with offshore - advection of a dry and hot desert airmass.
Models now want to bring an open wave over the Strait of Gibraltar and have no longer a closed circulation over the western Alboran Channel. The new AFWA MM5 run indicates 20-30kt easterly winds ( a possibly levante event ) over this area, which should feed moisture westwards and should suppress any offshore flow from the Algerian mainland . Yesterday, the Atlas mountains were a borderline between 18-23°C dewpoints to the north and 5-10°C to the south ( although yesterday's storms should have weakened the strong gradient somewhat ).

Thunderstorms over SW / central and northern Spain and Portugal will continue during the morning hours and should go on during the daytime with a gradual decrease from the west. As 850 temperature stay quite warm and mid-levels steepen somewhat, we should see weakly capped 500 - 1000 J/kg MLCAPE and topography / strong forcing will subserve for scattered thunderstorms. Shear will be too weak for any enhanced severe weather threat and main risk will be strong gusts and marginal hail in stronger pulse storms.

A low-end level-1 over norther Spain was issued mainly due to strong 0-3km CAPE release and numerous short-lived spin-ups look likely with the focus along numerous convergence zones, forecast by GFS.

More robust instability release ( ~ 1kJ/kg MLCAPE ), DLS of 15-25 m/s and at least somewhat enhanced LL shear will be present south of the Sierra Nevada Mountains during the morning hours. SRH-3km values rapidly increase towards the south and a few supercells with mainly a large hail / severe wind gust threat can be expected although low LCLs also support an isolated tornado risk.

The thunderstorm coverage gets more complicated over the Strait of Gibraltar and the Alboran Channel during the rest of the day. Strong forcing, a cooling lower troposphere, the aforementioned good environment at low-levels and abundant mid-level moisture in GFS are enough points to go on and bump up the probabilities over those areas. We don't want to ignore a persistent QPF minimum during the past few model runs north of the Morocco-Algerian border and lowered probabilities in this area.
Overall kinematic and thermodynamic parameters indicate a favorable environment for rapid storm organisation. There is an enhanced severe wind gust risk along the coastal areas of Algeria and in addition increasing large hail probabilities further inland, where SRH values increase substantially and lapse rates steepen.
LCLs are not too high and there are hints on locally enhanced LL shear, which could support an isolated tornado report, but overall threat seems quite low ATM.

Another maximum of storms should develop somewhere west of the Balearic Islands during the evening and night hours. Shear could be enough for multicells around the Balearic Islands, but will be too weak further towards the north for any storm organisation.

...Aegean Sea and areas towards the south...

Latest synop reports around the Aegean Sea indicate an ongoing Etesian event in conjunction with a weakening and southward moving cold front. Dew points behind this front are in the mid-tens and in the lower twenties ahead of it.
During the morning hours, ongoing storms over Greece and western Turkey should undergo a gradual weakening trend as BL gets drier. Further towards the south, storms should be confined along the stalling front somewhere west and south of Crete. Shear stays too weak for any storm organisation.

...Norway, Sweden and Finland....

Numerous short-wave troughs rotate over those regions from the W/SW and each of them could locally support a short-lived and low covered thunderstorm event.
We went with a blend of best thermodynamic profilers, possible topographic influence and still quite warm SSTs ( 10-14°C ) and highlighted the regions, which seem to have the best thunderstorm chances.
Low LCLs and enhanced LL shear also indicate an augmented risk for tornadic thunderstorms. Expected risk and major uncertainties of thunderstorm coverage won't justify a broad level-1 right now.
Warm EL temperature also support mainly shallow convection, which won't take profit of significantly enhanced DLS beside the small level-1 area, where deeper convection could occur during the night hours and where latest analysis point to the highest threat of convectively induced severe wind gusts and an isolated tornado.