Forecast Update
Valid: Fri 16 Jul 2010 12:00 to Sat 17 Jul 2010 06:00 UTC
Issued: Fri 16 Jul 2010 11:45
Forecaster: TUSCHY

A level 2 was issued for parts of Switzerland, parts of Germany, W-Czech Republic and NW Austria mainly for large hail (significant hail possible), strong to severe wind gusts and excessive rainfall.

A level 1 surrounds the level 2 for the similar risk, including an isolated significant hail event.

A level 1 was issued for parts of France, Benelux and NW-Germany mainly for severe wind gusts, large hail and to a lesser extent for a tornado.

A level 1 was issued for parts of Finland and the White Sea mainly for a few severe wind gusts and large hail reports, but also for an isolated tornado event and excessive rainfall.

SYNOPSIS

Please refer to the main outlook, issued at Thu 15 Jul 2010 20:24 Z.

DISCUSSION

... N-Italy, Switzerland, Austria, SE/E Germany and the Czech Republic ...

Soundings upstream all indicate an increase in geopotential heights / WAA, especially at lower levels, as 20°C isotherm worked its way to the north/northeast. Beneath this plume, near isothermal conditions prevail, as seen in latest Vienna sounding. So despite BL air mass featuring high moisture content with dewpoints now already around 20°C, warm layer atop may limit initiation throughout the daytime hours, mainly over E-Austria and parts of the Czech Republic. Initiation may be bound to the topography and hence we did not want to downgrade all areas, as each thunderstorm, developing and moving slowly in this air mass may produce intense rainfall amounts and large hail.

However, the concern arises along the western fringe of this plume of hot air, e.g. NW-Italy, Switzerland and SE/E/NE Germany. No change in respect of QPF signals, which remain marginal at best for such an environment. Weak subsidence, gradually warming BL and warm LL may keep thunderstorm activity limited during the daytime hours, although each storm already will be in a favorable environment for organized convection, given latest sounding wind profiles. Large hail and strong wind gusts will be the main hazard with that activity. Given numerous drier layers at mid-/upper levels and still increasing BL air mass moisture (enhancing CAPE thickness in hail growth layer), an isolated significant hail event will be possible.

Main initiation however awaits thermal trough/short wave during the evening hours, when mid-level lapse rates steepen. An overlap of 15 m/s DLS, 20 to 25 m/s 1-8km shear and moderate to high CAPE from SE-Bavaria to the north/northeast enhance the chances for organized multicells / isolated supercells with large hail, strong wind gusts and excessive rainfall amounts. Steepening lapse rates at mid levels may counteract gradually cooling BL, which still features high 0-1km mean mixing ratios, so all kind of CAPE values feature high magnitues until well into the night. The main reason for upgrading is the hail risk, which seems to have increased during the past runs, as significant hail will be well possible in such an environment. Otherwise, a cluster of thunderstorms probably moves off of the western Alps to the north/northeast during the night with locally high rainfall amounts, large hail and strong wind gusts. This level 2 highly depends on more isolated activity ahead of this cluster, where maximized hail risk can be expected.

No significant changes have been done for the rest of the outlook, neither for the high-end level 1 over France/Benelux/NW-Germany (severe wind gusts being the main hazard, next to large hail and an isolated tornado event) nor the level 1 over Finland and the White Sea ( severe wind gusts/large hail and tornadoes, more isolated in nature).