Storm Forecast
Valid: Sun 11 Nov 2012 06:00 to Mon 12 Nov 2012 06:00 UTC
Issued: Sun 11 Nov 2012 06:55
Forecaster: VAN DER VELDE
A level 2 was issued for west-central Italy mainly for excessive convective rainfall, chance of tornadoes and large hail.
A level 2 was issued for Slovenia and Croatia mainly for excessive convective rainfall.
A level 1 was issued for eastern Spain mainly for excessive convective rainfall and (spout type) tornadoes.
SYNOPSIS
A large upper trough is cutting off over the Iberian Peninsula. Cold air has invaded the western Mediterranean Sea and the cold front remains virtually stationary today over western/central Italy to Slovenia, with slight retrograde motion on Monday morning, as cyclogenesis is in progress over the southern part of the western Mediterranean Sea with easterly winds picking up.
DISCUSSION
...eastern Spain....
The Balearic Sea hosts a trough filled with modest CAPE which will advect onshore and trigger storms in the coastal convergence zone, which may remain offshore but drag inland near Valencia. Locally deep layer shear is predicted to be 20 m/s but low level shear is not strong and storm motion will be slow. This creates mainly a excessive precipitation threat where storms affect land. Low level buoyancy should be large, favorable for waterspout type tornadoes in convergence lines.
...western/central Italy to Slovenia/Croatia...
With the jet over the cold front 20-30 m/s deep layer shear is created, while strong low-level flow forces moist, unstable air over elevated terrain. GFS and finer scale WRF models predict a very narrow convergence zone along the front where storms will trigger and backbuild continuously, riding the west side of the mountains, both in Italy and the Adriatic north shores. This creates very significant potential for excessive rain, (flash)floods and perhaps landslides. CAPE is much larger (around 1000 J/kg) in the Tyrrhenian Sea than the Adriatic Sea, though. The strong low-level flow also creates large vertical wind shear in the lowest kilometer, even over sea 15 m/s 0-1 km shear, so it is not just caused by friction. SREH over 300 mē/sē supports this. GFS model hodographs show incredibly veering winds in the lowest kilometer all along the cold front. Helped by the deep shear, supercells are the main convective mode, large hail is likely in Italy, and one or more tornadoes (potentially strong ones, F2 ) are possible or even likely. Isolated severe wind gusts are possible as well. Further away from the coast precipitation from convective anvils and orographic lifting will be the primary threat (hail/tornado/gust threats reduced). One remark: GFS indicates weak capping (small LFC-LCL height difference) for western Italy until the evening. Convection may become elevated after 18Z or so. It might reduce tornado threat somewhat, but not rain threat.
