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Thunderstorm Outlook - Sun Aug 31st

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martinastro
Martin Mc Kenna
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« on: August 31, 2008, 12:32:06 am »



Storm Forecast
Valid: Sun 31 Aug 2008 06:00 to Mon 01 Sep 2008 06:00 UTC
Issued: Sat 30 Aug 2008 23:18
Forecaster: SCHLENCZEK
SYNOPSIS

An Omega-like upper flow pattern with a ridge over central Europe, an upper cut off low near Iceland and a low pressure complex over Russia will weaken during the next days and a zonal upper flow pattern is expected to establish over the eastern part of the North Atlantic. There are only a few regions in Europe with a considerable amount of instability: Thunderstorms should develop ahead of an upper trough over the Iberian Peninsula that will shift eastward during the period. In the vicinity of the trough, a strong 40 m/s jet streak at 300hPa translates from Gibraltar to Sardinia. Another focus for deep moist convection is expected over western Germany, eastern France and the Benelux countries where advection of warm and moist air at lower levels will lead to moderate instability. Over the eastern Mediterranean, moderate instability will be present in the vicinity of an upper trough near Greece. A rather weak cold front over western France will strengthen during the period when it moves across Belgium / Netherlands and western Germany. Non-severe cold air convection with showers and thunderstorms should be most likely west of Ireland and over western Russia.

DISCUSSION

...Extreme eastern Spain / western Mediterranean...

In the vicinity of a weak surface cold front, about 1 - 2 kJ/kg MLCAPE will form over eastern Spain during the day. Deep layer shear in order of 15 m/s should be present but recent GFS runs indicate strong CIN and very weak QG forcing. A few isolated storms with large hail and damaging gusts may develop but confidence for convective initiation is too low to issue a higher threat level at the moment. If indeed convective initiation leads to some intense long-lived storms (small MCS / supercells) an upgrade will be issued.

...France, Benelux countries, southwestern Germany...

Diurnal heating will lead to about 1,5 kJ/kg MUCAPE over parts of France, Germany and the Benelux countries. As CIN is expected to be rather low, temperature anomalies, orographic effects and other mesoscale features may lead to widespread initiation. In most parts of the region to be discussed, deep layer shear is 10 m/s or less. Some weakly organized multicell storms are expected to form, capable of producing isolated large hail and isolated severe gusts. As storms will tend to cluster, the hail / gust threat will diminish later on but local flash floods cannot be discounted. A marginal level-one threat seems to be warranted for those areas where greatest instability is expected.

...Greece, Aegean Sea...

Weakly organized thunderstorms will form in an environment with more than 1 kJ/kg CAPE and weak deep layer shear. Instability and shear are forecast to decrease during the period and severe convective weather should be unlikely.


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