Storm Forecast
Valid: Sun 06 Jul 2008 06:00 to Mon 07 Jul 2008 06:00 UTC
Issued: Sat 05 Jul 2008 21:58
Forecaster: SCHLENCZEK

SYNOPSIS

An intense low pressure system over the British Isles is forecast to merge with another low pressure system over Scandinavia. Ahead of the associated upper trough, hot and moist air is advected into the central Mediterranean region, Italy and Austria. At 300hPa, a 50 m/s upper jet streak over the British Channel will translate eastward. At lower levels, a frontal zone stretches from SE Portugal via central France, SW Germany, Austria and NE Balkans towards Greece and the Black Sea region, separating hot and moist air on the southern side from a rather cool airmass in the northwest and northeast. An upper low over Ukraine / Russia will move northeastward while weakening. High pressure will dominate the flow pattern in the eastern Mediterranean region and northern Africa.

DISCUSSION

...Extreme SE France, S Switzerland, Italy, Austria, Czech Rep, Slovenia...

During the afternoon, diurnal heating as well as low level WAA will lead to about 1,5 kJ/kg MLCAPE over northern Italy. At this time, an upper ridge should suppress convective initiation while vertical shear strengthens. In the afternoon / evening hours, an upper level vort max should overspread SE France and Italy from the southwest. In the vicinity of the cold front, moderate CAPE and about 20 m/s deep layer shear should be present. Veering winds will lead to 150 - 300 J/kg SRH3 and a few mesocyclones will likely evolve. Organized multicells and supercells will pose a threat of large hail and perhaps some damaging gusts. GFS as well as mesoscale models show only little precip over northern Italy where instability should be best. In the late evening / night hours, a MCS with attendant severe gust threat may form over SE France / Switzerland and should move northeastward.

At the moment, convective initiation is somewhat uncertain and the expected storm coverage would not justify a level-two threat. An upgrade may follow if widespread convection occurs.

...Eastern Iberia...

Ahead of the upper low, southwesterly winds advect hot and moist air into northeastern Spain / Balearic Islands. Deep layer shear should increase to about 25 m/s and some hundred J/kg of CAPE should be available. Although a small upper level vort max should provide enough forcing, models show only little precip. If storms form during the day, they will likely be accompanied by large hail and isolated severe gusts. Storm coverage is expected to be low but a level-one threat should be warranted.

...Northern France, Belgium, Netherlands, northern Germany...

In the vicinity of the cold front, weak CAPE should be produced during the day in an environment with about 25 m/s deep layer shear and some thunderstorms may evolve. An isolated severe gust is not completely ruled out but severe threat is too marginal for a level area.


...British Isles...

In the wake of the trough, showers and thunderstorms will evolve in the cool maritime airmass. Deep layer shear should stay between 10 and 15 m/s and severe weather is rather unlikely.
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