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Convective Outlook - Sat Aug 23rd

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martinastro
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« on: August 23, 2008, 12:33:29 pm »

From ESTOFEX



Storm Forecast
Valid: Sat 23 Aug 2008 06:00 to Sun 24 Aug 2008 06:00 UTC
Issued: Fri 22 Aug 2008 22:40
Forecaster: SCHLENCZEK
SYNOPSIS

Two upper troughs over the western parts of Europe will be the focus for convective storms on Saturday. The first one, initially located over the Netherlands, is forecast to travel eastward while digging to the south. On both flanks of the trough, a 40 m/s jet streak at 300 hPa is present, leading to strong speed shear in the vicinity of the cold front and the trough axis. The other trough with its axis from extreme SW France towards SW Portugal around 06 UTC on Saturday will also move eastward towards the Balearic Islands on Sunday morning. A well-defined frontal boundary stretches from Belarus via eastern Germany / Czech Republic, Austria, northwestern Italy and Gibraltar towards northwestern Morocco. It separates hot and moist air over the Mediterranean region and eastern Europe from cool air over the east Atlantic and western Europe. At lower levels, pressure gradients should be rather weak except for the surface low over Poland and the frontal wave west of Ireland. Hot and dry conditions will continue over the southeastern parts of Europe.


DISCUSSION

...Extreme SE Spain...

Ahead of the approaching upper trough, advection of an EML and LL moisture convergence led to a significant amount of instability. Soundings from Barcelona on Friday 12 UTC showed around 1,7 kJ/kg MLCAPE and similar values are also expected on Saturday. Capping inversion is rather weak over the southern parts of the Iberian Peninsula and some weak QG forcing should be sufficient for thunderstorm initiation. Deep layer shear in order of 15 m/s is forecast with a 0-6 km shear vector almost perpendicular to the frontal boundary which should support discrete organized storms. SRH3 around 150 - 200 J/kg will support rotating updrafts and supercells will become a distinct possibility. Main threat should be large / isolated very large hail if some long-lived supercells develop. As steep LL lapse rates, low LCL heights and strong release of LL CAPE are forecast by GFS, a funnel / brief tornado cannot be discounted.

...NE Italy, Slovenia...

In the vicinity of the cold front, GFS simulates increasing instability with MLCAPE values around 1 kJ/kg in a strongly-sheared environment. Best conditions for severe thunderstorms should be expected for the Po Valley and the western parts of Slovenia where strongest SRH / LL shear is forecast. SRH3 in order of 200 J/kg combined with 20 m/s deep layer shear and 10 m/s low level shear will provide a good environment for supercells that may produce widespread large / isolated very large hail and severe gusts. Although LL shear is not that strong, an isolated (possibly strong) tornado is not ruled out. Storms are expected to develop over northern Italy during the afternoon and move eastward. Later on, storms may merge into a severe MCS, capable of producing damaging gusts. Especially in the northern part of the risk area, strong forcing may lead to intense precipitation with an enhanced flash flood risk.

...Benelux countries, western Germany...

In the wake of the trough, diurnal heating and upper level cooling will create some hundred J/kg CAPE and thunderstorms will evolve. Rich BL moisture and steep LL lapse rates are expected which may allow some funnels / short-lived tornadoes. Rather low instability and deep layer shear will limit the threat of large hail and severe gusts. Storms will diminish after sunset.

...Eastern Poland, western Belarus, Lithuania...

Recent GFS runs show low-end instability in the vicinity of the cold front where deep layer shear in order of 20 m/s is present. If forcing is strong enough, some linearly organized multicells will evolve, capable of producing isolated severe gusts. At the moment, confidence is too low to issue a threat level.

...British Isles...

A frontal wave will cross the British Isles during the second half of the forecast period. GFS suggests that some low-end instability will develop in the small warm sector over Ireland in the evening. Wind speeds in order of 20 m/s at 850hPa are present near the cold front and a convective line with isolated severe gusts may evolve. Even though SRH around 200 J/kg is expected, tornadoes should be rather unlikely as most of the vertical shear is in the lowest 2 km which does not favour organized multicells or supercells. A threat level is not issued.
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