Storm Forecast
Valid: Sat 29 Sep 2007 06:00 to Sun 30 Sep 2007 06:00 UTC
Issued: Fri 28 Sep 2007 20:32
Forecaster: DAHL

SYNOPSIS

Intense upper low over the central portions of Europe will slowly move into southern Scandinavia during the period while upstream Atlantic trough digs southeastwards, reaching southwestern Europe early on Sunday. Backing low-level flow ahead of this feature is expected to promote advection of steep mid-level lapse rates across the Mediterranean, which should result in ample CAPE given around 10 g/kg 0-1 km mean mixing ratios.

DISCUSSION

... NW Mediterranean ... S France ...

It seems that some severe threat will evolve especially over the northwestern portions of the Mediterranean during the day. Maximum MLCAPE should be on the order of 1500 J/kg, with deep shear increasing to 15 to 20 m/s towards the evening.
Over southern France, this will be coupled with strong frontogenetic forcing along the northern edge of the warm/moist air late in the day/evening, which should be supportive of upscale growth of the convection into one or more MCSs. Expect severe wind gusts and large hail with this activity. Low-level shear profiles are simulated to be strongly supportive of tornadic supercells, if storms are able to tap boundary-layer air, which is somewhat uncertain at the moment. Farther south, convection will likely be less well-organized, but it should still be capable of isolated severe thunderstorms.

... Sicily ...

Models hint at southern Italy as focus for potentially severe convective evolution late Sunday night. CAPE will likely be quite high, as will be the deep-layer shear magnitude ... and severe multicells as well as supercells may occur. Main threats should be severe straight-line winds as well as large hail.

... E Europe ...

Weak residual CAPE should be present within a narrow plume over eastern Europe, east of the central-European upper low. GFS shows some overlap of 20 m/s deep shear and CAPE, so that a few of the storms may briefly become organized. Some of Friday's ascents from this air mass show nearly saturated boundary layers and steep low-level lapse rates (but minimal allover CAPE). Saturday is likely to lack low-level shear, but if Friday's low-level thermodynamic profiles can be sustained through the period, a weak threat for mainly non-mesocyclonic tornadoes will exist. However, main threats will be hail and outflow winds, which may briefly attain severe thresholds. Allover threat should be too low for a categorical risk.