Storm Forecast
Valid: Thu 07 Jun 2007 06:00 to Fri 08 Jun 2007 06:00 UTC
Issued: Wed 06 Jun 2007 21:55
Forecaster: GATZEN

SYNOPSIS

Weather situation did not change significantly. High over Scandinavia remains, while several weak upper cut-offs are situated over northwestern Europe. Low geopotential centered over northern Mediterranean has weakend compared to WED. Strong westerly jet at the southern flank of this upper low remains.

DISCUSSION

Benelux

A surface low is expected to develop over France/southwestern Germany during the day, providing warm air advection over Benelux. Given steep mid-level lapse rates and rather rich moisture in low-levels, instability will likely realize. However, do not see that dewpoints will reach around 20°C as indicated by latest GFS model run. Additionally, low-level air mass may be relatively cool. Expect that thunderstorms will form during the afternoon/evening hours that may be elevated over some places. Given increasing north-easterly surface winds and therefore increasing vertical wind shear, chance for organized convection will be increased, too. A few multicells are forecast to develop, while only low chance for large hail is expected. Thunderstorms may go on well into the night in the warm air advection regime.

Northern/central Italy, Alpine region and southern/western Germany into France, northern Spain, Czech Republic and Poland

Boundary-layer moisture has improved over France as indicated by latest soundings, and quite steep lapse rates especially below 600 hPa are also present. This will result in CAPE during the day due to diurnal heating. Weak CIN is present, and convective initiation is expected in the range of low-level convergence zones/outflow boundaries of old convection or upslope flow regimes in hilly terrain. Quite strong buoyancy at relatively low levels will likely result in strong updrafts initially, while weak vertical wind shear is expected to inhibit organized convection over most places. Isolated large hail can not be ruled out with convective cells in the early mature stage. North and west of this area with rich low-level moisture, latest soundings indicate inverted-v-profiles and at least weak instability. Thunderstorms are expected to develop during the day, while a few strong downbursts are forecast to be the main threat. Intense precipitation and local flash flooding is forecast to be a significant threat over a broad area.