Storm Forecast
Valid: Wed 02 Jul 2008 06:00 to Thu 03 Jul 2008 06:00 UTC
Issued: Tue 01 Jul 2008 20:13
Forecaster: GATZEN

SYNOPSIS

East of amplifying trough centred over western Europe, a subtropical high ridges into Scandinavia. Warm air mass characterized by steep mid-level lapse rates as indicated by latest soundings over west Mediterranean spreads into north Mediterranean, Alps, south-western Germany, France, and Benelux to the east of a propagating cold front that is expected from the eastern Channel to eastern Pyrenees on WED, 18 Z. Most of the Balkans, eastern central Europe, and southern Scandinavia are dominated by dry and stable air mass in the range of the ridge axis. To the east, quite cool but unstable air mass is present in the range of low geopotential.

DISCUSSION

Eastern France, Benelux, western Germany

To the east of the cold front, an unstable air mass will spread northward. Some QG forcing is expected in the warm air advection regime and low-level convergence will likely assist for some convection in the night hours that will decay in the morning. During the day, diurnal heating will likely result in high CAPE over the eastern France region. New convection is expected to develop along of outflow-boundaries as well as further west along the cold front that moves eastward rather slowly. Most of the day, it seems that strong deep layer shear in the range of the jet at the eastern flank of the upper trough will not overlap with instability ahead of the cold front. Additionally, low-level forcing is expected to be quite weak. This may change in the evening hours as GFS indicates some cyclogenesis and a weak frontal wave along the cold front over eastern France. Thunderstorm cluster may organize along the leading gust front and severe wind gusts as well as large hail becomes more likely. North of the deepening frontal wave, increasing SRH may also support updraft rotation of more isolated convective cells, but current thinking is that storm coverage will be to high to allow for isolated cells. Embedded mesocyclones will be capable of producing isolated large hail and severe wind gusts, though. Tornadoes may also occur given favourable low-level veering profiles, but this will strongly depend upon the development of the surface frontal wave and convection. At this time, I would think that thunderstorm cluster will not be well-organized and chance for very large hail and strong tornadoes is not high enough to issue a level 2. An upgrade may be required in the early afternoon hours. Convection will likely spread into Benelux and western Germany during the night hours, where local severe wind gusts will be the main threat.

Italy to Alpine region

West of the ridge axis, warm air advection will likely produce some QG forcing. Strong instability is present and low-level forcing from sea-breeze fronts and mountains will likely be strong enough for daytime convection. Additionally, some outflow-boundaries of early convection north of the Alps will likely be focus of new convection. Given weak vertical wind shear, pulse storms are forecast capable of producing isolated large hail and local flash flooding.

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