Storm Forecast
Valid: Thu 12 Apr 2007 10:00 to Fri 13 Apr 2007 06:00 UTC
Issued: Thu 12 Apr 2007 10:04
Forecaster: GATZEN

SYNOPSIS

Synoptic feature of main concern is a cut-off low over eastern Iberian Peninsula, forming the western part of European omega flow. At the eastern flank of this cut-off, strong upper jet streak spreads northward over western Mediterranean into southern France.

DISCUSSION

Western Mediterranean

Thunderstorms have formed in the range of the Balearic Islands. Latest ascends indicate an elevated mixed layer originating from the Atlas mountains that has been advected into western Mediterranean during the last night. This air mass is rather warm and vertical profiles remain stable over most of the region as boundary layer is cool. However, due to quite moist low levels in the range of a frontal boundary near the eastern coast of the Iberian Peninsula, some instability has developed. QG forcing at the cyclonic flank of mentioned upper jet streak as well as mid-level height falls in the range of slowly eastward propagating cut-off likely supported actual convective development.

Today, QG forcing will likely go on as upper jet streak continues to propagate north- and eastward. Uncertainties exist about the development of instability. Latest model guidance shows that relatively dry low-level air mass spreads eastward into the region south of the Balearic Islands behind a cold front, and vertical profiles will rapidly become stable. To the north and east, no significant changes are expected, as low-level air mass remains quite moist and mid-level height falls will continue. Although mid-level lapse rates will become neutral to slightly unstable due to current convection, slight instability is forecast to spread northward during the day. Given persistent QG forcing, thunderstorms are expected to spread into north-western Mediterranean during the day.

Given strong vertical wind shear (20 m/s DLS) underneath the jet streak, some organized thunderstorms are forecast. Strong to severe wind gusts are expected to be the most significant thread, but isolated large hail is also not excluded in the range of mesocyclones that are also forecast due to favorable veering of vertical profiles. The chance for tornadoes is forecast to be weak as low-level buoyancy is poor.

Convective activity is forecast to weaken in the evening hours as upper jet streak moves into France. However, another jet streak that follows from the Atlas mountains is expected over south-western Mediterranean late in the period, and convective activity will likely restart south-east of the Balearic Islands in the range of the surface frontal boundary. Although most thunderstorms will likely be elevated over the cold low levels, some cells may root to the boundary-layer. Those cells will have a potential to become severe given favorable veering profiles. Large hail and severe wind gusts are expected to be the most significant severe thread, again.

Central France

At the northern flank of Mediterranean low-pressure system, easterly winds are present over France. Strong insolation and rather cold mid-levels will likely create steep low-level lapse rates during the day, and instability is expected to form. Thunderstorms are forecast during the day, especially as QG forcing will likely strengthen in the range of approaching upper jet streak. Although ingredients for organized convection are forecast to be weak, with 10 to 15 m/s DLS and weak CAPE/forcing, isolated cells may become organized today. As a consequence, isolated hail or severe wind gusts are not completely ruled out, but thread seems to be too low for a level 1.