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Thunderstorm Outlook - Wed August 20th

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martinastro
Martin Mc Kenna
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« on: August 19, 2008, 09:54:27 pm »



Storm Forecast
Valid: Wed 20 Aug 2008 06:00 to Thu 21 Aug 2008 06:00 UTC
Issued: Tue 19 Aug 2008 19:26
Forecaster: GATZEN
SYNOPSIS

Long-wave trough remains over north-western Europe, yielding a westerly mid-level jet over western Europe and a south-westerly jet from central Mediterranean to Baltic States. The latter also extends into the lower troposphere over the Baltic States, where winds will likely reach more than 20 m/s at the 700 hPa level. Most of Europe is dominated by a cool or relatively dry boundary-layer air mass.

DISCUSSION

Baltic region

Along a cold front that moves eastward at the eastern flank of the upper trough, a frontal wave is forecast to develop over Poland in the range of a strong jet streak and associated DCVA. Warm air mass will spread southward in the range of this wave. Latest soundings indicate that lapse rates and boundary-layer moisture are quite poor over eastern Germany and Poland, but mid-level height falls in the range of approaching short-wave trough may lead to weak instability east of the frontal boundary.

At the surface, a weak low is expected to develop, and low-level convergence will be quite weak. However, forcing along the frontal boundary will likely result in initiation. Showers and thunderstorms will rapidly move north-eastwards given strong south-westerly winds, while low-level winds will be weak from the south or south-east. This will result in moderate low-level storm-relative helicity and updrafts may rotate. Isolated supercells will pose a threat of some large hail as well as severe wind gusts. A tornado is not ruled out especially hear the frontal wave where low-level buoyancy and vertical wind shear will be most favourable. Storms will move quickly to northern Baltic States and will weaken during the night hours.

Southern British Isles

At the flank of the long-wave trough, a weak frontal boundary moves eastwards over southern British Isles. To the south of this frontal boundary, southerly winds are forecast that will advect moist low-level air mass into Ireland. Given low-level convergence along the boundary and rather high low-level buoyancy due to increasing moisture underneath the cyclonic flank of the upper jet, thunderstorms are forecast to develop during the day. Weak vertical wind shear is expected and weak tornadoes are forecast. Later in the period, low-level convergence spreads eastward, but low-level moisture seems to be significantly lower. However, showers and thunderstorms are not ruled out, and an isolated weak tornado is not ruled out. Convection will weaken in the evening/night hours.

Northern Adriatic

Moderate vertical win shear and some instability due to moist low-level air mass will likely assist for some thunderstorms during the afternoon and evening, capable of producing isolated large hail.
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martinastro
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« Reply #1 on: August 20, 2008, 01:08:58 pm »

UPDATE 8.30 Weds from Tony Gilbert



Moderate Risk Scattered Gen Tstorms UK & Ireland 12Z-21Z

Slight Risk Weak Tornadoes blue and red boxes

Stronger convection likely late in period yellow box

Outlook rather more broad than earlier predictions with significantly moist low level flow. Convective map is broken down into sub boxes to help locate regions with higher risk potential.

Slow moving occlusion brings rain and showers to much of the UK and Ireland today. Most of the coverage looks to be under a weakly sheared environment. Though, special attention is given to S.Wales later in the day where moderate vertical shear and dry intrusion maybe introduced (red box)and hense the slight risk of tornadoes. Blue box over Ireland signifies a region of convergence within a highly unstable environment. It is within this region that weak convergence zone tornadoes may in fact develop . Though ATM I grade this risk as only slight.

Yellow box highlights significant potential instability as dry air overruns moist primarily after 15Z. We can expect to see a rash of isolated small tstorms as mid level air is rapidly cooled.

The occlusion will typically introduce lots of cloud and rain cooled air throughout the period though the coverage is so broad that small notches of insolation are very likely along the periphery where stronger convection can be generated and should become apparent on radar.



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brianb
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« Reply #2 on: August 20, 2008, 02:19:06 pm »

FYI there is evidently strong convection around up here - we had a short but heavy shower about an hour ago - but no signs of any electrical activity. There hasn't been much solar heating of the ground either, though the air does feel humid.
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martinastro
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« Reply #3 on: August 20, 2008, 03:13:47 pm »

There's some cumulus development here but as you mentioned, lack of ground heating will play a big role. Can't see any storms here today (so far) but I will be focusing on any funnel clouds which could form from any convection in the area. Instability is at its greatest for N. Ireland between 16.00 and 19.00 BST with over 400 kj CAPE forecast.
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