Storm Forecast
Valid: Tue 11 Jul 2006 06:00 to Wed 12 Jul 2006 06:00 UTC
Issued: Tue 11 Jul 2006 02:20
Forecaster: VAN DER VELDE

SYNOPSIS

A long-wave 500 hPa trough moves over Scandinavia and pushes an occluded cold front into western Europe and southern Scandinavia. A warm, unstable airmass is in place over much of Europe, but subsidence and divergence under the large high pressure area minimalize organized convection in most places. The cold front does provide a focus for organized convection, and the system that affected Finland yesterday is shifting into northwestern Russia, but with decreasing pressure gradients.


DISCUSSION

...southeastern Netherlands, eastern Belgium, northeastern France and western Germany...

GFS predicts MLCAPE above 500-1000 J/kg before the front, with 0-1 km average mixing ratio values of 12-14 g/kg, with midlevel vorticity advection barely touching the area. Just available 00Z soundings show that the warm 500-850 hPa layer has cooled and moistened significantly, and the mixing ratio is already 11 g/kg, so the model can be considered on track.
Deep layer shear will be weakening after the 00Z soundings, on the order of 10-20 m/s, which allows for long-lived multicells, that may develop large hail. It is also possible that isolated severe gusts may occur, taking into account the dry midlevel air in Trappes 00Z, especially in southeastern parts of the level 1, or if storms align and form a squall line, but current indications for this are weak, having no clear convergence zone or a strong vort max, and the front moves rather slow.

...Denmark, southern Norway and Sweden...

Convective precip is present in GFS, but CAPE fields show only very marginal values/sporadic patches, with coldest cloud tops barely -20 C. There will be a lot of lift, deep layer/low level shear in this region, enhanced SREH, and low LCLs.... but thinking is that instability may lag the cold front, and be conciding with downward motions. If organized convection does develop at the front, this area may require an update for severe gusts and a chance of isolated tornadoes.

...northwestern Russia...

A continuation of the Finland system of yesterday. Kargopol 00Z shows potential for more than 1500 J/kg MLCAPE (50 hPa) and Petrozavodsk 30 kts at 700 hPa combine into a good setup for severe weather. GFS indicates some decrease of kinematic values, however the situation still seems favorable for supercells and a linear MCS, with large hail and severe winds, respectively.

...Spain, Portugal...

With an upper cut-off low (300 hPa), GFS and MM5 hint at isolated convective precipitation. With very deep, dry boundary layers (00Z Gibraltar inverted-V), expect these storms to be very high based and with a chance of dry microburst (and possibly 'dry lightning' in already badly forest fire-affected regions)

...southern Italy...

00Z Brindisi shows potential for >1500 J/kg MLCAPE(50 hPa layer). Although shear conditions appear rather weak, do expect some isolated large hail/severe gust.