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Convective Outlook - Sun Oct 19th

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martinastro
Martin Mc Kenna
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« on: October 19, 2008, 03:18:37 am »



Storm Forecast
Valid: Sun 19 Oct 2008 06:00 to Mon 20 Oct 2008 06:00 UTC
Issued: Sat 18 Oct 2008 23:42
Forecaster: SCHLENCZEK
SYNOPSIS

In the left exit region of a strong 85 m/s jet streak over the North Atlantic, rapid cyclogenesis is forecast between Iceland and Ireland. This low pressure system will intensify during the late afternoon with SFC pressure forecast around 955 - 960 hPa while moving NNE-ward. Very strong winds at all tropospheric levels should be present between Iceland and the British Isles where severe non-convective gusts are most likely to occur. Global NWP models are quite uncertain whether a dry intrusion will develop in the wake of the cold front which may allow a narrow convective line or not. This region has to be monitored during the day.

Another low pressure system over Scandinavia will affect the Baltic Sea during the afternoon with cold upper air (-30C) overspreading a region with SSTs around 12C. Some weak upper-level disturbances are present over Iberia and southeastern Europe where thunderstorms are most likely to occur.

DISCUSSION

...W Ireland, Scotland...

The cold front / upper trough of the intense cyclone is forecast to cross the northwestern half of the British Isles in the late night / early morning. Even though GFS does not expect much convective activity, a few shallow convective lines could develop if instability is sufficient. In this case, the region discussed here will need an upgrade because of an enhanced tornado / damaging gust risk as 15 m/s low level shear, more than 30 m/s deep layer shear and 400 J/kg SRH3 will likely be in place. Later on, some weakly electified, probably non-severe, thunderstorms should develop in the vicinity of the upper trough.

...Baltic region...

Showers and thunderstorms will likely develop in the vicinity of the upper trough that overspreads the Baltic region in the morning / early afternoon. Low level lapse rates close to 10 K/km and locally enhanced low level shear are forecast and a brief tornado cannot be discounted. The tornado threat as well as the storm activity itself will gradually decrease during the afternoon as the trough moves eastward towards western Russia. Given the short time-frame and a relatively small area with adequate conditions, a threat level was not introduced.

...W Mediterranean...

Ahead of the upper low, diurnal heating will create some hundred J/kg of CAPE over the Balearic Islands and northeastern Spain. Deep layer shear is forecast to stay between 10 and 15 m/s which will allow a few multicells with isolated large hail / severe gusts. In some places, 0-3 km shear may be enhanced by orographic features / sea breeze and some rotating updraftes may evolve as well. Tornadoes are not completely ruled out but overall threat is too low for a categorical risk.

...Aegean / Ionian Sea, eastern Mediterranean...

Some almost weak thunderstorms may develop in a region with weak pressure gradients and low to moderate instability. Except for an isolated funnel / waterspout, severe weather should not occur with those storms.


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