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Severe Weather Potential - Ireland & UK - Mon Jan 19th

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Author Topic: Severe Weather Potential - Ireland & UK - Mon Jan 19th  (Read 532 times)
martinastro
Martin Mc Kenna
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« on: January 18, 2009, 04:03:14 pm »

Tony Gilbert posted this on the UKWW. It looks interesting and would apply to several people who live within the box on these boards...



A thread is started regarding the potential for severe weather on Monday.

Particular attention is given to the second cold front feature moving into Ireland by mid afternoon and then through the UK by evening. Strong low level jet running inland central Ireland and then through to north central UK acquires strong low level vertical shear potential as a direct result of surface friction. Upper troughing during this period peaks and should deliver exceptional lapse rate potential. CAPE is driven inland from relatively mild SST. Whilst mid level flow looks rather weak by comparison there is nevertheless the significant increase in vertical shear to 650mb which is ample for separating updraft/ downdraft.

Models and synopsis forecast continue to shift and leap. So an early forecast is not practical ATM IMO.

On a personal note based on what I can see ATM we could well have the best chance for a significant tornado event, by comparison to recent weeks. In addition to this based on the level of weak CIN across Ireland in particular there is potential for a notable isolated severe cell development during this period.



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martinastro
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« Reply #1 on: January 18, 2009, 07:24:04 pm »

From ESTOFEX



Storm Forecast
Valid: Mon 19 Jan 2009 06:00 to Tue 20 Jan 2009 06:00 UTC
Issued: Sun 18 Jan 2009 18:34
Forecaster: TUSCHY
SYNOPSIS

A channel of low geopotential heights runs from S-Greenland to the North Sea and dictates the flow over the NE-Atlantic and NW-Europe. Numerous disturbances, embedded in this strong northwesterly flow bring unsettled and cold conditions for NW-Europe. Weak pressure gradients over the rest of Europe, warm mid-levels (Mediterranean) and cold/dry air (E/NE-Europe) result in stable conditions. Surface pressure falls over the western Mediterranean during the night hours, passing in an evolving depression south of the Alps.

DISCUSSION

... Bay of Biscay, parts of France, Belgium, the Netherlands, UK, Ireland, NW-Portugal and N-Spain ...

Frigid air from the frozen Baffin Bay and Davis Street is on its way to W-Europe along the south side of this extensive cyclonic vortex. Airmass modifies over increasingly warm SSTs (10-12C SW/S of Ireland) while mid-level airmass remains very cold with readings at 500hPa running well below -30C. So the environment is favorable for widespread CAPE release over the highlighted area and an increase in shower/thunderstorm activity from west to east during the day. EL temperatures drop to -30 to -50C, parcel layer depth increases in average to well above 1000m and the wind field is shaped cyclonically with various embedded disturbances. The same, messy picture at lower levels as numerous convergence zones cross the highlighted area from the west. Hence, thunderstorms can pop up everywhere over the E-Atlantic and NW-Europe but we want to highlight the areas, where conditions for more concentrated thunderstorms are more likley, including strongest CAPE fields, an uncapped airmass, climatology and synoptic lift. Winds at 850hPa are strong with 20-25m/s, so severe wind gusts in this well mixed airmass are possible with each shower/thunderstorm and a level-1 is needed. In addition, and isolated large hail/tornado event is possible, the latter one especially onshore, where ageostrophic deflection helps to increase LL directional shear.

... Ireland and parts of UK between 12Z and 00Z ...

The focus during that period will be a rapidly eastsoutheastward moving short-wave, crossing Ireland in the afternoon and S-UK in the evening hours. GFS had this feature in its forecast for the past few runs although the strength of this disturbance still fluctuates somewhat. The pressure gradient increases during the passage of this wave and GFS augments winds at 850hPa to 30-35m/s over central/south Ireland, decreasing slightly over SW/S-UK, so severe wind gusts are well possible. The main question remains how robust moisture advection turns out to be just ahead of this wave, which looks marginal at best for now. GFS has a diurnal driven instability signal over UK, decreasing during the evening hours. In addition, temperatures below 3km warm up slightly, also reflected in increasing capping over SW-UK and slightly higher LCLs. The main risk will be severe wind gusts, but the tornado risk has to be monitored in upcoming model runs,too.
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brianb
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« Reply #2 on: January 18, 2009, 09:22:38 pm »

Don't know what half those words mean Grin but there is something strange in the upper atmosphere - managed to get a bit of observing between the last shower & the high cloud spreading in ahead of the next front, which seems to be here now. The seeing was abysmal, could hardly use x77 because star images are smeared into big fuzzy blobs, in fact it was even difficult to focus my small scope (8 cm refractor) at x24. This sort of smearing often seems to occur when there is a jet stream around, but I can't remember it ever being as bad as this.
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martinastro
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« Reply #3 on: January 18, 2009, 10:06:01 pm »

Yes...I can see the other front approaching on the radar images. It's heading our way. Haven't done any observing this evening myself but I reckon there MIGHT be a good clearance after this front before dawn...comet Lulin could be worth getting up for?. I'm setting the alarm anyway. At least the Moon is waning and the sky is getting darker. Good luck Brian if you getting up for the comet.

I'm wating to see about any convective updates for Mon, probably won't be any until morning. I don't think I will see much here as it always seems to be the W areas which get all the action in Winter....but you never know. I will be keeping an eye out anyway.
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martinastro
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« Reply #4 on: January 18, 2009, 10:51:55 pm »

Convective outlook is updated. Scroll down page to latest chart....

http://www.ukweatherworld.co.uk/forum/forums/thread-view.asp?tid=28151&posts=5

Looks like only W and S areas affected.
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brianb
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« Reply #5 on: January 19, 2009, 09:01:03 am »

Quote
I reckon there MIGHT be a good clearance after this front before dawn
Clearance apparently just arriving now (0900)
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martinastro
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« Reply #6 on: January 19, 2009, 03:44:47 pm »

That was really annoying...I set my alarm again and woke up to clouds. My sleep pattern is really messed up these days.
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brianb
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« Reply #7 on: January 19, 2009, 03:57:34 pm »

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.I set my alarm again and woke up to clouds. My sleep pattern is really messed up these days.
Shouldn't mess you up too much - cancel alarm, take quick look outside, if cloudy then just go back to bed - I rarely sleep through the night even without an alarm.
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martinastro
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« Reply #8 on: January 19, 2009, 04:49:27 pm »

I meant over the last 48 hours I have been sleeping very little, not going to bed until before dawn then up again early morning, a bit over the place. At least when it does clear I will be ready for it.

Lightning at NW and W coast at the moment.

Still snowing here.
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