Storm Forecast
Valid: Mon 27 Jul 2009 06:00 to Tue 28 Jul 2009 06:00 UTC
Issued: Sun 26 Jul 2009 21:42
Forecaster: GROENEMEIJER

A level 2 was issued for east-central France, extreme southwestern Germany and western and northern Switzerland.

A level 1 was issued for Eastern Spain, and for southern, eastern and central France, parts of Switzerland, southern Germany, and western Austria, mainly for severe wind gusts and large hail.


SYNOPSIS

Monday at 0600 UTC, a closed mid/upper-level low is located over the Belorussian/Russian border. It is embedded within a trough stretching from northern Scandinavia to the Central Russian Upland. It is filled with a polar air-mass in which diurnally-driven convection is forecast. An upstream ridge from southern Scandinavia to northern Italy is expected to progess eastward. Between the ridge and the trough further upstream, a southwesterly flow advects a mixed layer northward across France and into Germany. The trailing edge of the mixed layer will be the focus for convective development Monday afternoon and evening, as an approaching trough from the west provides forcing for upward vertical motion.


DISCUSSION

E France, Switzerland, S Germany, W Austria...

In response to the approaching trough, which should cause mid-level temperatures to drop, and diurnal warming and moistening of the lower troposphere, about 1000 J/kg of MLCAPE is expected to develop across E France. Deep-layer shear 20-25 m/s in the 0-6 km layer, should be more than sufficient for updraught rotation. Storm initiation will likely first take place over central and eastern France during the mid-afternoon, and later across the Jura and elsewhere.

After initiation, a mix of multicell and supercell storms will probably develop. The supercells will be capable of producing very large hail. Additionally, they may locally produce damaging winds.

It is expected that the storms cluster relatively soon as strong forcing overspreads the area in the early evening. Training storms may produce some very heavy rainfall amounts across eastern France, N and W Switzerland and SW Germany.

However, the models also consistently simulate an eastward progession of the storms and the cool low-level air along the plains north of the Alps, suggesting that bowing segments are possible. Those storms will bring a significant severe wind threat, gradually mitigated by diurnal stabilization as the storms move eastward.

NE Spain...

In response to the approaching trough, isolated to scattered storm development in forecast across the northeastern mountains of Spain in the afternoon. Strong deep-layer shear suggests that the storms may become supercellular. Their main threats will be downbursts, given the deep well-mixed boundary layer, and large hail. The storms will probably weaken and die out gradually after sunset.