Storm Forecast
Valid: Sat 09 Oct 2010 06:00 to Sun 10 Oct 2010 06:00 UTC
Issued: Sat 09 Oct 2010 05:59
Forecaster: VAN DER VELDE
A level 2 was issued for northeastern Spain and southern France mainly for excessive convective rain, but also for isolated severe wind gusts, tornadoes, or marginally large hail.
A level 1 was issued for the NW Black Sea for waterspouts.
A level 1 was issued for Sicily and Malta mainly for large hail and severe wind gusts.
SYNOPSIS
High pressure centered between Iceland and Baltic countries dominates northern Europe, while a large low pressure area shifts from the Atlantic Ocean to the Iberian Peninsula and western Mediterranean Sea. A cold front moves slowly across Spain to the east. Unstable airmasses are present over the ocean as well as over eastern Spain and western Mediterranean Sea. The cold front will meet a supply of increasingly warm moist air (12-14 g/kg) in the boundary layer as it moves into eastern Spain. GFS predicts cyclogenesis over northeastern Spain around 18Z, and strong lifting will destabilize the airmass and cause an outbreak of thunderstorms and adjacent rain which can reside over Catalonia and southern France for a long time.
Another low drifts across the Black Sea towards Romania.
DISCUSSION
...northeastern Spain and far southern France...
A significant convective rain episode starts at approximately 18Z-21Z as the cold front pushes into increasingly unstable Mediterranean airmass along the coast. GFS predicts very strong deep convergence along the front as it passes over eastern Spain with MLCAPE around 300 J/kg over land to more than 1000 J/kg over sea. Over much of NE Spain, 0-3 km storm-relative helicity values will range between 100 and 250 mē/sē which could support rotating updrafts (supercells). 20 m/s deep layer shear and locally 12 m/s 0-1 km shear with LCL heights of only 500-800 m and more than 150 J/kg 0-3 km CAPE along the coast support also tornadoes, although GFS-predicted hodographs become smaller and more circular around 21Z. Decreasing storm motion will favor cells and clusters which can remain long in the same small area along the coastal convergence zone and dump tens of millimeters and probably more than 100 mm in several locations. Until Sunday 12Z, multiple models predict large rainfall sums: COSMO 100-130 mm/12h over NE-Spain/S France and east of the Balearic Islands, GME 140 mm/12h, ECMWF 100 mm/12h and GFS 60 mm/12h over S-France. LAMMA-WRF18Z and Meteoblue NMM predict actually a slower arrival of the front, only at 03Z, which would push the largest rain threat towards the next forecast period.Large rain sums are also forecast by several of those models deeper into the Ebro valley.
Adjacent areas are included in level 1 for the same threats.
The 00Z run of GFS appears to show a deeper overlap between instability and low-mid level shear over central Spain. This would favor tornadoes and large hail.
...Sicily, Malta...
GFS indicates uncapped conditions on the north edge of a CAPE region which is under strong deep layer shear and 0-3 km SREH values of 300 mē/sē, but with low LCL heights. Supercells can grow in this region with a chance of large hail. Tornadoes not likely by lack of low-level shear.
