Extended Forecast
Valid: Mon 07 Apr 2008 06:00 to Tue 08 Apr 2008 06:00 UTC
Issued: Sat 05 Apr 2008 18:24
Forecaster: TUSCHY

SYNOPSIS

A persistent blocking pattern , known under the name of " Rex blocking " in the US, goes on and results in a strengthening SW-erly flow over SW and central Europe. Downstream of this constellation, a warm and humid airmass spreads northeastwards. Numerous disturbances rotate around the col and the downstream side of the strongly positive tilted trough, supporting a few areas of enhanced thunderstorm chances and in combination with increasing thermodynamic / kinematic parameters also an augmented severe thunderstorm threat.
Warm and stable conditions prevail over extreme western Russia while cold air still covers our extreme northern parts of the forecast area.

DISCUSSION

.... Portugal and Spain....

A well - known problem with such a pattern is the timing, when those disturbances come out of the base of the main upper-level trough. This uncertainty is still present in latest standard deviation maps and hence our confidence for this forecast area is not excessively high.
A weak surface depression builds east / northeastwards with the main pressure falls over far northern Spain / southern Bay of Biscay. Moisture in the PBL gets somewhat better during the day as LL wind field veers to SW-erly directions during the morning hours. This happens in response to an upper-level impulse approaching NW Portugal during the daytime hours. Lapse rates at mid-levels are weak during most of the daytime hours as weak ridging ahead of the impulse weakens / flattens but they become somewhat better during the evening hours over west / northwest Portugal and Spain. Impact of diabatic heating could become important regarding onset of convection but past events displayed thick Ci-shields in the range of strong subtropical jets with limited diabatic heating so we have to monitor this, too.
Scattered thunderstorms will evolve during the early afternoon hours, increasing in coverage and intensity during the afternoon / early evening hours. Some veering is present at lowest levels and if storms can root into the boundary layer an isolated tornado report will be possible but otherwise neither wind gust nor hail threat looks too imminent for a level-1. We hence decided to stick with a broad thunderstorm area although parts of it could be upgraded in later outlooks.
Thunderstorm activity will diminish during the night hours.

... E - Slovenia, NE - Croatia, Hungary, Slovakia and western Romania...

A depression over NE-Italy shifts east / northeastward, approaching Poland during the midday hours from the south. A combination of steep mid-level lapse rates and modest moisture advection at lower levels results in a broad area of a few 100 J/kg CAPE. Placed under the left exit region of a 50m/s streak at 300hPa, a combination of upper-level divergence and LL convergence along an eastward moving surface cold front should support enough lift for scattered thunderstorm development in a near uncapped environment. At the moment extreme western Romania and south Hungary look most favorable for large hail as steep mid-level lapse rates and deep helical flow overlap. Due to the long forecast range and still some uncertainties regarding the exact path and strength of the depression we wait with drawing a level area.

Numerous showers / thunderstorms are forecast to develop over parts of United Kingdom and N-France and Belgium. LL CAPE over central UK is slightly enhanced and we can't exclude an isolated funnel report as LCLs stay low but overall this weakly sheared environment precludes any severe thunderstorm threat.