Wed 10th Dec: Stornoway Report

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Wed 10th Dec: Stornoway Report

Postby Eddy Graham » 2014 Dec 10, 22:22

Hi Everyone

I simply can't not post something on the COL board today..!

We've been having some truly incredible weather here in Stornoway over the past few days, especially past 24hrs. And although the wind hasn't been as strong (nor the air pressure nearly as low) as on some of the days during last Dec 2013, there has certainly been some startling weather:
- From 3-4am this morning, through to 5pm tonight (Wed), we've had near-continuous thunderstorms all day! Many of the squalls have contained lightning, thunder (generally more subdued in tone than in continental storms), large hail (>0.5cm diameter), driving rain, sleet and some snow. The sheer number of thunderstorms has been amazing - I've lost count! (at least 5 separate storms had gone through by 2:30pm)
- Some of the squalls have been tremendous, almost astonishing, with massive hail downpours (giving coverings of ~1cm in a few seconds- 1 minute or so). I'd say 80% of the precipitation is graupel (pea hail), so it's a misnomer to say that it's snowing on the mountains - really, it's mostly denser, heavier graupel that falls from these squalls.
- The wind has been severe but not exceptionally damaging or violent, it was a severe gale 9 yesterday (Tues) morn with gusts to 72mph at the airport nearby. Today say W F8-9, gusts 11. But it hasn't been a steady storm like many previous ones, rather a procession of lulls (F4-6) followed by squalls to F9-11.
- I took fear when my large Stevenson Screen visibly shook in a squall this morning, and the contents blew out/down (graduated cylinders, datalogger) - I was lucky not to have a smashed thermometer!
- After a relatively dry November (75% precip) and also a dry-ish start to Dec, the past 4 days have brought 45mm and the rea garden is waterlogged again! (but not quite as bad as on Christmas Eve last year).
- The temp is oscillating between +1 and +4C. As I type, another heavy hail squall is going thro (temp +3.9degC), but the worst is over now..

More updates on my blog (by tomorrow at http://bit.ly/1qGLyJm), with best wishes to all COL members
Eddie Graham
(Stornoway town 04004)
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Eddie Graham
Stornoway, Na h-Eileanan an Iar (Outer Hebrides), Scotland.

Stornoway town COL station, 20 AMSL, local records back to 1873.
Hebridean Weather Blog: http://bit.ly/1IFAPJa
Twitter Weather Feed: https://twitter.com/eddy_weather
Eddy Graham
 
Posts: 82
Joined: 2012 Jul 18, 17:09
Location: Stornoway, Scotland

Re: Wed 10th Dec: Stornoway Report

Postby Admin » 2014 Dec 11, 22:17

Hi Eddie! I'm particularly surprised at the thunder. I believe that Scotland as a whole gets rather few thunderstorms, and I would think that the western isles would get even fewer. Presumably the few you do get are the winter type, because you are not part of a large land area for summer storms. So what was this? I didn't know that temperate latitude depressions were accompanied at all by thunderstorms, except for a few winter-type ones, and some brief squalls with thunder and the odd tornado when the isobars set themselves up right. This was clearly an exceptional depression -- have there been similar events in the past, or is this mass of thunderstorms unique in history? From a chart I saw, these storms affected much of N and C Scotland. Given the low frequency of "normal" thunderstorms, such events must stand out in the statistics.
Peter Wright (signed in as Admin).
Admin
Site Admin
 
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Re: Wed 10th Dec: Stornoway Report

Postby Eddy Graham » 2014 Dec 12, 09:05

Hi Peter - good to hear from you again!

Yes, the climate of the Hebrides is very similar to the coasts of N & W of Ireland with a winter peak in thunderstorms, and with very few in summer (if any) - those that rarely occur are usually imported from mainland areas lying to the E & S). In the six winters that I have been here now, we have usually had at least 1 winter thunderstorm each winter, and they are almost always of the "thundersnow" variety (i.e. low freezing levels promote lots of ice in the cloud, especially graupel, presumably therefore encouraging electrification). I don't have the exact stats to hand, but here are the approximate stats:
Winter 2009-10: We had at least 3 or 4 days of of thunder, typically 1 or 2 flashes in a single storm for an individual day
Winter 2010-11: I think there may have been 1 instance of thundersnow in Dec 2010
Winter 2011-12: Again, if I recall correctly, 1 instance
Winter 2012-13: None that I recall
Winter 2013-14: A violent thunderstorm w (of equivalent severity to continental summer storms) overnight on 5th Dec 2013 (also with a wind touching violent storm force 11). December 2013 had a total of 5 days of thunder (but were mostly single thunderstorms on individual days - except 24th Dec 2013 when we had 2 separate thunderstorms on the that day alone).
Dec 2014, Winter 2014-15 so far: We've had 4 thunder days so far in Dec 2014, but there were at least ~12 separate thunderstorms over 2 days on 10th/11th (also 3 consecutive days of thunder- 9th, 10th, 11th; and 4/5 days on 7th, 9th, 10th, 11th). Yesterday (11th) I made specific note of the time of each thunderstorm going through:
5am: Active thunderstorm during the night, snow and hail lying
8:47-8:51am: Another storm, power knocked out for whole island, severe hail
3.45pm: Another thunderstorm, snow and hail
5pm: Yet another separate thunderstorm, rain/hail followed by snow

My own impression is that the 5th Dec 2013 event and this event of the past 5 days are possibly unique (few people on the island remember such a repeated sequence of storms, though I accept that people's memories can often be heavily rose-tinted). I notice that sea-surface temperatures are considerably above normal around Scotland at the moment (see the chart on http://www.esrl.noaa.gov/psd/map/clim/sst.shtml), but considerably below normal in mid-Atlantic. I think also the depth of cold air must be important - this morning (Fri 12th) I see another set of thunderstorms moving into NW Ireland, and the IR-Sat image (see below) shows that their anvils are turning anticyclonically (indicative of surface cyclonic rotation, the same phenomenon that happens with hurricane outflow at the tropopause).
ScreenShot001.jpg
ScreenShot001.jpg (22.08 KiB) Viewed 1046 times

E.
Eddie Graham
Stornoway, Na h-Eileanan an Iar (Outer Hebrides), Scotland.

Stornoway town COL station, 20 AMSL, local records back to 1873.
Hebridean Weather Blog: http://bit.ly/1IFAPJa
Twitter Weather Feed: https://twitter.com/eddy_weather
Eddy Graham
 
Posts: 82
Joined: 2012 Jul 18, 17:09
Location: Stornoway, Scotland

Re: Wed 10th Dec: Stornoway Report

Postby Graham Easterling » 2014 Dec 13, 09:22

My own impression is that the 5th Dec 2013 event and this event of the past 5 days are possibly unique (few people on the island remember such a repeated sequence of storms, though I accept that people's memories can often be heavily rose-tinted). I notice that sea-surface temperatures are considerably above normal around Scotland at the moment (see the chart on http://www.esrl.noaa.gov/psd/map/clim/sst.shtml), but considerably below normal in mid-Atlantic. I think also the depth of cold air must be important -


I think you've hit the nail on the head. Even down in Cornwall we get an Autumn peak in thunderstorms, when the sea is warmest relative to the air in a polar maritime flow. Often a single crash bang affair. (Thunderstorms here are very rare in March April for the reverse reason.) That cold pool in mid Atlantic would have restricted warming until the warmer SST was reached near the Hebrides. Our last good thunderstorm was back in July mind you (lightning over Penzance http://metrouk2.files.wordpress.com/201 ... streak.jpg )

We do get a lot of hail in winter. 20 days with hail in just January & February this year.

Good to get reports from the north by the way.
Graham

Penzance Weather Station
Grid Ref:
SW464231 - Post Code: TR18 4TP
19m AMSL - Aspect SSE
In a SE - NW orientated valley

23 Years of Penzance Weather Records : http://penzanceweather.atspace.com/weather.html
Graham Easterling
 
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Location: Penzance, Cornwall

Re: Wed 10th Dec: Stornoway Report

Postby Len Wood » 2014 Dec 13, 12:55

Did anyone see the BBC 1 programme on Monday
'Wild Weather with John Hammond'. ?
You can catch it on iPlayer if you are interested.
Hammond along with some expert on thunderstorms in USA said you don't get hail in winter.
It is just graupel.
Ridiculous I thought.

They were referring to the multi layer monster hail you get in the USA.
Clearly the 'expert' had no knowledge of our set up for winter storms.
The Cbs in winter that give hail can be deep enough o give multi-circulation to an ice pellet and therefore layered hail in my opinion, not just graupel.
Len

Wembury, SW Devon coast
N50.33 W4.13
Altitude 83 m asl
Len Wood
 
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Joined: 2012 Jan 28, 16:12
Location: Wembury, coastal SW Devon, 83 m asl

Re: Wed 10th Dec: Stornoway Report

Postby Eddy Graham » 2014 Dec 14, 11:53

Thanks Graham, thanks Len - good to hear your weather news too!

Just to clarify, I just checked the definitions of "graupel" and "hail" on the American Met Soc Glossary:
Graupel: Heavily rimed snow particles, often called snow pellets; often indistinguishable from very small soft hail except for the size convention that hail must have a diameter greater than 5 mm.
Hail: Precipitation in the form of balls or irregular lumps of ice, always produced by convective clouds, nearly always cumulonimbus. An individual unit of hail is called a hailstone. By convention, hail has a diameter of 5 mm or more, while smaller particles of similar origin, formerly called small hail, may be classed as either ice pellets or snow pellets. (Hmm, bbut no mention of graupel here!)

So, in my above posts, I was calling the precipitation 'graupel' because it was mostly opaque (and therefore presumably caused by riming), rather than clear (which it sometimes is when melting) - however, given these AMS definitions and because their diameters were mostly above 0.5cm, I therefore should therefore replace my instances of 'graupel' above with 'hail' - Hope you know what I mean!

All the best, sincerely
Eddie
p.s. 3-6cm of 'proper' snow fell across Stornoway town yesterday morning, the children were delighted. But it is nearly all gone this morning after an overnight high of nearly 9degC.
Eddie Graham
Stornoway, Na h-Eileanan an Iar (Outer Hebrides), Scotland.

Stornoway town COL station, 20 AMSL, local records back to 1873.
Hebridean Weather Blog: http://bit.ly/1IFAPJa
Twitter Weather Feed: https://twitter.com/eddy_weather
Eddy Graham
 
Posts: 82
Joined: 2012 Jul 18, 17:09
Location: Stornoway, Scotland

Re: Wed 10th Dec: Stornoway Report

Postby Eddy Graham » 2014 Dec 14, 22:43

p.p.s. Yet another thunderstorm this afternoon in Stornoway (Sunday 14th Dec) during a violent hail squall and wind gusts over 60mph - we therefore now have equalled last Dec (2013)'s final tally of 5 days of thunder for this month so far.
Eddie Graham
Stornoway, Na h-Eileanan an Iar (Outer Hebrides), Scotland.

Stornoway town COL station, 20 AMSL, local records back to 1873.
Hebridean Weather Blog: http://bit.ly/1IFAPJa
Twitter Weather Feed: https://twitter.com/eddy_weather
Eddy Graham
 
Posts: 82
Joined: 2012 Jul 18, 17:09
Location: Stornoway, Scotland


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