Storm Forecast
Valid: Wed 13 Sep 2006 07:00 to Thu 14 Sep 2006 06:00 UTC
Issued: Wed 13 Sep 2006 07:15
Forecaster: GATZEN

SYNOPSIS

Broad high is placed over eastern Europe. To the west ... amplified trough is present with an axis from Atlantik W of British Isles to Iberian Peninsula. While this trough propagates very slowly to the east ... and short-waves spread eastward across Iberian Peninsula ... European high ridges into southern Scandinavia. At lower levels ... dry/ stable airmass is present in the range of the high over most of Europe ... while tongue of warm/ moist air mass originating from western Mediterranean is advected into western Europe and North Sea region. Models indicate low-level convergence zone in the range of the warm air axis east of a frontal boundary/ cold front that travels eastward over British Isles, western France, and Iberian Peninsula that will the the focus of today's discussion.

DISCUSSION

Northern France, Benelux, southern North Sea

Latest models agree that low-level convergence will strenghten over western Europe today as weak upper short-wave trough travels northward over France and British Isles. While weak surface low pressure is expected that spreads northward into British Isles ... quite strong WAA could be expected at the north-eastern flank affecting western and northern France, Benelux, and eastern British Isles today. Strongest WAA should be over The Channel and eastern British Isles. Latest soundings show that steep low to mid-level lapse rates are present from northern France to north-western Germany ... and rather moist low-level air mass is also present over a broad area ... and daytime insolation is expected during the day yielding CAPE up to 1000 J/kg over Benelux region today. Further west ... moist and neutral to slightly unstable air mass is present in the range of the convergence. Expect that convective activity will have its maximum in the afternoon/evening hours ... when low-level convergence should be maximized and WAA should be strongest in the range of a LLJ forming at the eastern flank of the surface low ... greatest chance for severe thunderstorms should be near the convergence zone ... where steep low-level lapse rates and high CAPE present over Benelux/north-western Germany will overlap with strong vertical wind shear in the range of the frontal boundary over British Isles ... and given veering profiles ... a few mesocyclones may form at the eastern border of the convergence where isolated cells will be most likely. Large hail and severe wind gusts should be possible with every supercell that develops. Further east ... weak vertical wind shear will be present over Germany ... and chance for organized convection should be weaker. However ... an isolated event is not ruled out. From The Channel to eastern British Isles ... CAPE should be rather weak due to neutral lapse rates ... but thunderstorms may form in the WAA regime during the day. Given strong LLS at the northern flank of the surface low ... chance for tornadoes should be enhanced if significant low-level instability will develop. However ... this seems to be the limiting factor, though ... as low levels should be quite stable. Best potential seems to exist near the British coast ... where models indicate some instability east of the convergence line.

Western France

Latest soundings do not indicate sufficient instability over south-eastern and central France due to dry low-level air mass that spreads westward. During the day ... models show increasing southerly flow over southern and western France that advects moist low-level air mass from Mediterranean Sea north-eastward. In the range of the convergence ... at last weak instability should develop over western France ... and thunderstorms should form. Given quite strong vertical wind shear ... embedded mesocyclones are not ruled out ... posing a threat for isolated large hail and severe wind gusts as well as tornadoes given strong LLS.

Western Mediterranean, southern France

Latest Palma de Mallorca sounding is missing ... but there are no indications that rich low-level moisture has decreased significantly compared to yesterday ... and thermodynamic profiles should still be favorable for deep convection. Weak upper vort-maxima are expected to travel northward ... and given some vertical wind shear around 15 m/s DLS over numerous places due to complex upper flow ... a few thunderstorms may organize ... capable of producing isolated large hail and severe wind gusts. Thunderstorms should merge into one or two MCS in the evening hours as QG forcing strengthens in the range of another short-wave trough curving around the main trough over Iberian Peninsula ... and threat for severe wind gusts should increase. Apart from severe thunderstorms ... intense rain/ flash flooding is quite likely.