Storm Forecast
Valid: Fri 28 Sep 2012 06:00 to Sat 29 Sep 2012 06:00 UTC
Issued: Thu 27 Sep 2012 22:11
Forecaster: TUSCHY
A level 2 was issued for parts of SE/E/NE Spain, the Balearic Islands and parts of the W-Mediterranean mainly due to excessive rainfall, large to very large hail, strong to severe wind gusts and a few tornado events.
A level 1 surrounds the level 1 with similar risks but with minor intensity.
SYNOPSIS
A more zonal flow pattern evolves over N-Europe with strengthening ridging over SE Europe. A strong cut-off over the Iberian Peninsula moves slowly east/northeastwards and brings heavy rain to parts of SW-Europe.
DISCUSSION
... Spain and the Balearic Islands ...
A concentrated outbreak of severe thunderstorms is forecast over SE/E Spain. Life-threatening flash flooding is likely !
An eastward moving cut-off low enters the W-Mediterranean during the overnight hours. The mid-level vortex also strengthens at lowers levels with a diffuse LL cyclonic vortex forecast over Spain and the far W-Mediterranean. Beneath very favorable high-level diffluence, the intensifying LL depression starts to advect a very moist and unstable air mass towards Spain. The main forcing mechanism will be the intrusion of a maritime Atlantic air mass/cold front from the west, which moves slowly to the E and later on to the NE.
An extensive cluster of slow moving storms is probably already ongoing during the start of the forecast period with GFS 18Z showing a very deep confluence zone setting up over south central Spain (probably as a result of lowering heights/pressure to the east). This cluster gradually shifts along the south coast to the east (until noon) and to the northeast (thereafter). Given strengthening flow in the lowest 1-2 km AGL (even outside of areas with potential convective feedback problem of the model), conditions will be highly supportive of back-building/training storms along the S/SE/E coasts of Spain, producing amongst others excessive rainfall. With global models pointing to roughly 100-150 l/qm in 24 h, some higher resolved models add local peaks in excess of 300 l/qm in 24 h, which is in the realm of possibility. Onshore training line of storms can evolve everywhere within the level 2 area, but the most favorable conditions probably evolve along the SE/E coast around noon and during the afternoon hours, as the aforementioned LL vortex intensifies with 850 hPa winds exceeding 20 m/s.
A combination of existing drought conditions, near orthogonal onshore flow towards the complex topography and an only leisurely northward lifting front result in excessive / life-threatening / rainfall amounts with an high-end flash flood risk!
Another focus will be the conducive environment for well organized multicells/supercells, especially along the coasts and offshore. Models diverge, how far east storms are able to build into a strongly capped environment, but we would not be surprised to see storms affecting the Balearic Islands, too. LL shear increases rapidly, both along the northward sliding front and along any evolving LL depression next to 20 m/s 3 km and 6 km bulk shear and strong directional shear. All kind of severe is possible with that activity, including a few tornado events, large hail (an isolated very large hail event possible), strong to severe wind gusts and excessive rainfall amounts.
The level 2 and 1 areas were both extended far inland, as very heavy rain may spread far west, given persistent and strong influx of moisture and striking QPF model signals. Lightning activity however weakens with DMC probabilities decreasing onshore due to rapidly waning CAPE. Some spotty activity however is possible well inland due to some MUCAPE and that's why we also added C-Spain to the lightning area.
During the end of this outlook and ongoing into the day 2 period, the risk of excessive rain gradually increases over far NE Spain and S-France.
... Rest of the thunderstorm areas ...
Limited CAPE precludes anything organized elsewhere with only low probability lightning areas issued for that activities.
