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Convective Outlook Ireland & UK - Sun (Updated Level 1 Risk)

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Author Topic: Convective Outlook Ireland & UK - Sun (Updated Level 1 Risk)  (Read 553 times)
martinastro
Martin Mc Kenna
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« on: November 22, 2008, 03:28:37 pm »

From Tony Gilbert of the UKWW



Slight Risk Snow Storms western & central regions UK & Ireland Through Sunday Period

Slight Risk of isolated Tstorm & weak tornado SE UK 06Z-12Z

Slight Risk Strong Tornado Development 18Z-03Z Sun-Mon SW Ireland

Low 988 mb Scotland maintains cold polar flow from NW. Ridging to the south introduces narrow warm, moist feed ahead of the initial cold front. Secondary feature (surface trough) introduces slight threat of snow across Wales, central UK, & SW UK regions. Wrapped occlusion brings slight threat of snow to western parts of Ireland through period. Strong upper jet stream enhance convective possibilities for SW Ireland. Whilst deep upper trough surges southward covering much of NW Europe.

Suggestion of snow is not guaranteed ATM and will require monitoring of models leading up to outlook period. Irish SST could well set off a few thunder snow storms which should move inland effecting N.Wales and central UK through Sunday afternoon.

An interesting combination of conditions may well trigger an isolated Tstorm SE UK during Sun morning with attendant risk of weak tornado development. UKMO synopsis paint a picture of sustained warm moist feed along the English Channel just ahead of the cold front. Cold dry mid level air is shown to surge south creating a moderately volatile atmosphere for a time on a local scale. It is quite possible that convection at the tail end of the cold front could sharpen quite dramatically and given the likely low level veer we should not rule out the odd tornado report for the SE.

Particular attention is specifically given to SW Ireland late in convective period where mid level winds are expected to exceed 110 kts. GFS develop CAPE offshore beneath this level. Given the projected steering winds, instability could well advect onshore within a highly sheared environment. Given the convective potential here, I see a defined risk of strong tornado development.

The above detail is based on current model output and is issued quite early for the said period. Hense the convective map and detail will require updating based on the most recent model output.


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martinastro
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« Reply #1 on: November 22, 2008, 08:39:04 pm »

UPDATE 7pm Sat

Whilst this forecast is issued as low risk, rather restrictive changes in the models reduce potential somewhat further.

Prime regions at risk of Thundersnow are now limited to NW Scotland, Wales and SW UK through afternoon and evening. Earlier potential for SE UK is now canceled due to now weaker fetch of mild air from the west. SW Ireland continues with 'Slight' potential for an isolated tornado event 00Z-06Z Mon (later in the period). Some good potential will exist for thunderstorm development overnight around the Channel Islands.

Risk of snow remains marginal, though a convective regime for west coastal regions in the UK looks pretty certain. The shift of upper supporting winds decrease the risk of organised cell development.

The outlook nevertheless remains open to changes based on new model updates. Still worth keeping an eye on IMO!

 

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martinastro
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« Reply #2 on: November 22, 2008, 11:50:09 pm »



Storm Forecast
Valid: Sun 23 Nov 2008 06:00 to Mon 24 Nov 2008 06:00 UTC
Issued: Sat 22 Nov 2008 23:22
Forecaster: SCHLENCZEK
SYNOPSIS

At the southern tip of a very intense 90 m/s jet streak that points towards Ireland, cyclogenesis is expected, leading to a 985 hPa surface low over the southern North Sea. The cold front / upper trough of this feature may allow convection over the British Isles and northwestern France. There is a high risk of widespread severe / isolated extreme non-convective gusts over northern UK in the second half of the forecast period.

The weather in eastern-central Europe is dominated by an intense 960 hPa cyclone. In the warm sector over parts of Russia, relatively warm and moist air is advected towards the Baltic States. This feature is surrounded by a 90 m/s jet streak at 300 hPa. The associated upper trough leads to unsettled conditions over the eastern Mediterranean.

DISCUSSION

...Ireland, Wales, SW-central England...

Some CAPE will likely be produced near the cold front over southern UK and the Channel region where rich BL moisture is present and LCL heights stay relatively low. An approaching upper level vort max should provide sufficient forcing for convective initiation. Good kinematic conditions with about 15 m/s low level shear, 600 J/kg SRH1 and 25 m/s background flow at 850 hPa will allow one or two convective lines with embedded mesocyclones, capable of producing severe gusts and possibly a few tornadoes. The cold front will cross the region in the morning / early afternoon.

Later in the period cold upper air overspreads the area, leading to a few hundred J/kg of CAPE. Showers and thunderstorms will form in a weakly-sheared environment with potential for organized severe weather being negligible. In the late afternoon / evening, an intense upper shortwave trough associated with the northwestern edge of the surface low will cross the region with sufficient QG forcing for ascent. A very strong gradient flow with wind speeds near 35 m/s at 850hPa will establish and a narrow convective line will likely produce severe gusts. SRH1 in order of 400 J/kg and 15 - 20 m/s low level shear may allow embedded mesovortices and a few tornadoes are not ruled out. Peak activity is expected between Sunday 15 UTC and Monday 03 UTC.

...Adriatic Sea...

In the northern part of the Adriatic Sea, very cold upper air and relatively warm SSTs will lead to low-end instability. Deep layer shear in order of 40 m/s should aid storms to organize into multicells, some of them may develop shallow mesocyclones. An isolated waterspout is not ruled out but overall threat will stay below the level-one threshold.

...Eastern Mediterranean, SW Turkey...

Ahead of the large upper trough over eastern Europe, about 500 J/kg MLCAPE are present in a region with 10 - 20 m/s deep layer shear with most of the shear in the 0-3 km layer. Some weak QG forcing is in place and about 200 J/kg SRH3 should allow some mesocyclones. Storms will likely produce small hail and isolated severe gusts.

...W Russia...

Latest GFS outputs show some signals for instability near the cold front of the large cyclone. Impressive kinematic conditions will persist in that region, i.e. more than 50 m/s deep layer shear, 15 - 20 m/s low level shear, strong QG forcing and about 400 J/kg SRH3. Multicells will likely organize into one or two lines that may produce quite a large number of severe gusts and perhaps an isolated tornado. The time-frame should be limited to Sunday 06 UTC - 12 UTC as instability and LL wind will weaken during the afternoon.
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« Reply #3 on: November 23, 2008, 07:04:49 am »

Have a nice weather!

P.S. That cyclone which passed across Ukraine yesterday (topic) was worth for a "level 1" by ESTOFEX classification.
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martinastro
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« Reply #4 on: November 23, 2008, 10:41:01 am »

Nasty here!!!! incredible anvils and mammatus. Already thunderstorms in N. Ireland. Powerful winds to. This is a day for the camera. I'm away out!!  Smiley
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« Reply #5 on: November 23, 2008, 11:20:37 am »

Good luck! Looking forward for your photos, Martin.  Smiley
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« Reply #6 on: November 23, 2008, 01:17:25 pm »

Quote
Powerful winds to.

You're not kidding, there's so much salt spray in the air it's actually hard to tell if it's raining or not.

Quote
This is a day for the camera.
To get its electronics wrecked?
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martinastro
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« Reply #7 on: November 23, 2008, 03:23:31 pm »

I'm back. That was a rough day! Gale force gusts, and intense hail showers from quite decent convection for this time of year. Plenty of big cbs around but mid level cloud hid the beauty of most of them. Was out for about 6 hours, now I'm freezing, wet, and knackered. Plenty of images but nothing really photogenic. Around 05.00 last night it was the worst I have seen it in a long time, it was complete horizontal torrential rain with flooded the streets. Waves of water where blowing across the roads at phenomenal speed. The street lights shook violently. Certainly fits in with a level 1 forecast. I don't think the tornado risk was high at all today because of a lack of significant wind shear (early days yet) but there was enough lightning around in N. Ireland and the Republic. Recent strike in Northern Lough Neagh. More crazy hail thumbing my window as I write this.

The camera certainly got a soaking (what else is new) but it's a tool at the end of the day. There would be no point in having it if I was afraid it might get wet in the elements. Convective images need to be taken where the action is for them to be effective in my opinion. But that's just me.  Smiley
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« Reply #8 on: November 23, 2008, 03:33:21 pm »

Sounds great.  Smiley
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« Reply #9 on: November 23, 2008, 10:21:44 pm »

Well, I finally got the images uploaded to the computer and they turned out a complete disaster!. I made a big mistake. I closed the aperture down to F/7.1 to get the depth, but it was extremely dark so I had to use a slower shutter. I noticed that the image has that 'breakdown' look giving a grainy or mottled texture which has destroyed them. At least I hope that's the reason. I recall I had the same problem with a mammatus display during the Summer taken with a high F/ number in low light levels. Bridge cameras must be very poor at this range. So, I have deleted every image I took today, but I left a few for here which don't show very much. Quite a disappointing result. Lesson learned. Made a short video too, nothing exciting, just some convection and hail.



Left moving cells



Intense hail curtains at the foot of Slieve Gallion



Bow and hail



Cb



Left moving convection and hail showers over Sperrins



CB

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« Reply #10 on: November 29, 2008, 09:06:45 pm »

Nice  Smiley
In the last shot the buildings are so small and the hail curtain is so big  Grin quite impressive
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