Storm Forecast
Valid: Mon 17 Sep 2007 06:00 to Tue 18 Sep 2007 06:00 UTC
Issued: Sun 16 Sep 2007 22:01
Forecaster: GROENEMEIJER

SYNOPSIS

Monday at 06:00 UTC, an intense elongated north-south oriented trough is expected to dig slightly while moving eastward. At the same time, a weak vorticity maximum that was located over Portugal on Sunday is advected northeastward ahead of the trough. A cold front ahead of the trough that is initially located along a line fromnear Hamburg to near Nantes is expected to intensify on Monday afternoon as it starts to accelerate eastward. It is expected near a line from Gdansk via Zurich to Barcelona on Tuesday morning at 06:00 UTC. In the wake of the front an unstable polar air-mass will flow southward, in which scattered weakly electrified storms are expected.

DISCUSSION

France, Switzerland, SW Germany...

In the southwesterly flow ahead of the main trough rapid moisture advection into France, Switzerland and SW Germany and cooling mid-level temperatures should lead to considerable amounts of CAPE to form during the day.

In the warm air advection regime a few mid-level clouds are expected to be present, that may hinder insolation somewhat.

Because convective inhibition should be weak, surface-based convective initiation is expected early in the afternoon. Given that about 15-20 m/s deep-layer shear will be present, the cells will likely become well-organized multicells and a few supercells. These will be capable of producing large hail in places and perhaps a couple of damaging gusts.

It is expected that the cells cluster into a few convective systems that will have a slow net east to northeastward motion while backbuilding to the south.

During the evening, a 20 m/s low-level jet is expected to develop to the west and northwest of the Alps which together with decreasing LCL heights suggests that a chance of a few tornadoes will gradually develop. This should be the case until the storms become elevated and weaken later in the night.

N Italy, Slovenia, NW Croatia...

During the evening, increasing forcing for upward vertical motion will overspread the area from the west. This should lead to an increase of CAPE to values of near 1500 J/kg.
Initiation of slightly elevated and or surface-based storms is expected over central Italy around midnight UTC and later over Slovenia and Croatia. The expected strong deep-layer shear of near 20 m/s and 200-250 m2/s2 of 0-3 km storm-relative helicity will be more than adequate to sustain rotating updrafts even though storms should cluster rapidly. Large hail and damaging winds will be the main threats. Where surface-based convection occurs, low-level shear of 10-15 m/s suggests that a threat of tornadoes will exist as well.

E and NE Spain...

In the morning hours, elevated storms will likely be ongoing over western and central Spain. Despite the weak forcing associated with the passing trough and cold front, capping over the moist Mediterranean air-mass in the east may prove too strong for storms to initiate along the coast. Nevertheless, if storms do develop, they will be able to profit from 1000-2000 J/kg CAPE and over 20 m/s deep-layer shear implying a threat of supercells accompanied by large or very large hail and damaging winds.