Storm Forecast
Valid: Sun 04 Jul 2010 06:00 to Mon 05 Jul 2010 06:00 UTC
Issued: Sun 04 Jul 2010 06:12
Forecaster: SCHLENCZEK
A level 1 was issued for N Morocco and N Algeria mainly for large hail and severe wind gusts.
A level 1 was issued for NE Spain and SW-central France mainly for large hail, excessive rainfall and severe wind gusts.
A level 1 was issued for N Ireland and Scotland mainly for severe wind gusts and tornadoes.
A level 1 was issued for N Italy and Slovenia mainly for excessive precipitation and isolated large hail and to a lesser extent for severe wind gusts.
A level 1 was issued for N Switzerland and SE-central Germany mainly for isolated large hail and excessive precipitation and to a lesser extent for severe wind gusts.
A level 1 was issued for N-central Sweden and Norway mainly for severe wind gusts and an isolated tornado.
A level 1 was issued for Belarus and NE Ukraine mainly for excessive precipitation and large hail and to a lesser extent for severe wind gusts.
SYNOPSIS
A plume of very warm and moist air is present over central / eastern Germany on Sunday morning ahead of a large Atlantic longwave trough. The frontal boundary which separates this subtropical airmass from cooler maritime air will shift eastward while weakening. Two other weather systems of interest within the warm airmass are an upper cut-off low over NW Morocco and another upper low over Romania. Some cold air convection can be expected in the vicinity of a strong cyclonic vortex over the British Isles. Apart from this region, most of the convection will develop in an environment with weak pressure gradients and marginal shear.
DISCUSSION
...N Morocco, Strait of Gibraltar...
A hot and unstable airmass is advected northwards into the extreme northern parts of Morocco. Ahead of the approaching upper low, strong southerly winds are present in the lower and mid troposphere, leading to 15 - 20 m/s deep layer shear. Recent soundings (Casablanca 00Z, Gibraltar 00Z) show a strong cap around 700 hPa. An upper vort-max associated with the upper low may provide enough lift for initiation but even then there will be only a few storms. As enhanced 200 - 250 mē/sē SRH is in place, some storms may become supercellular and will pose a threat of large hail and isolated severe wind gusts. Near the centre of the low, some back-building storms are observed on IR imagery. Locally, excessive rainfall produced by such storms may lead to flooding.
...SW-central France, S-central Germany...
Near the cold front over central Europe, moderate values of MLCAPE are observed by soundings from S France and S Germany. Diurnal heating will lead to an increase of instability to around 1000 - 2000 J/kg MUCAPE. Most of the instability will be elevated with LCL heights forecast around 1500m AGL, locally only a few hundred metres above the ground. NE Spain and SE Germany will have the greatest chance for some elevated multicells which tend to produce large hail and severe wind gusts. Over the Massif Central and near the Alpes, orographic lift and low LCL heights may lead to rather widespread development of very slow moving multicellular storms which will produce torrential rainfall, leading to flash floods.
...N Italy, Slovenia...
Some moderate instability is present over N Italy in an environment with 10 - 15 m/s deep layer shear. Diurnal heating, some orographic lift and a weak upper vort-max may lead to initiation and primarily slow moving multicells are expected to develop. These storms may produce isolated large hail and isolated severe wind gusts but the main threat will be local excessive rainfall with flash floods. Convective activity is expected to start in the early afternoon and will continue into the late evening.
...Scotland, N North Sea...
The core of a strong cyclone will cross N Ireland and Scotland in the early afternoon with some instability produced by strong upper level cooling at the tip of the upper trough. Most of the CAPE should have its origin from strong QG lifting of a potentially unstable layer near the cold front where WV satellite images show a dry intrusion. There will be only a few hundred J/kg CAPE but this will partially overlap with 25 - 30 m/s deep layer shear in the northern parts of the cold front. Low level winds will become very strong (approx. 25 - 30 m/s at 850 hPa) and a lot of severe non-convective gusts are expected to occur in the warm sector region. Near the cold front, gusts may be convectively enhanced and could reach even locally up to 34 m/s. A circular storm-relative hodograph is forecast for the transition region of the cold front / upper trough and low-topped supercells are most likely to occur there. Given strong LLS and low LCL heights, any rotating storm may produce a tornado. The severe wind gust / tornado threat will not last very long as the region with instability should have passed Scotland after 18 UTC.
...N-central Norway / Sweden, N Finland...
Warm and moist air has overspread most parts of Scandinavia and a few hundred J/kg CAPE are forecast in the vicinity of a 50 m/s southerly upper level jet. A small DCVA maximum is found near the right entrance region of the jet over the central parts of Norway / Sweden and QG lift should be sufficient for convective initiation. LCL heights will be rather low and locally enhanced LLS (10 - 12 m/s) may allow an isolated tornado as some storms will likely contain mesocyclones. An isolated large hail / severe wind gust event is not ruled out either.
...Ukraine and Belarus...
Weak vertical shear, LCL heights around 1 km AGL and moderate values of MLCAPE characterise the airmass over Ukraine and Belarus. Most storms tend to be slow moving multicells which may deposit large amounts of precipitation in a short time-frame. Some isolated large hail / severe wind gust events are also expected.
...Other regions...
Around 500 - 1000 J/kg MLCAPE are expected over SE Europe in an environment with 10 - 15 m/s deep layer shear. There is no real focus for severe thunderstorm development but some storms may produce large hail, severe wind gusts and excessive rainfall.
