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Nick F

Thursday's Arctic Surge

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Thought I’d start a thread to follow the developments of the potent arctic front modelled to surge south on Thursday, which bares some similarities to the potent arctic front that moved south & brought thundersnow on 28th January 2004.

So what’s the trigger for this arctic air to surge towards the UK later this week? Currently the very cold arctic air is bottled up way to the north in the arctic circle, with the coldest air beneath the Polar Vortex centred over Baffin island and the arctic ocean north of Canada, the cold air is being kept up there by the strong jet stream which is moving N and NE out of the Canadian Martimes towards Sern Greenland.

post-1052-0-78930400-1292278771_thumb.pnpost-1052-0-31831200-1292278855_thumb.pn

The Polar Vortex elongates eastwards across Greenland over the next few days with the jet streaK at its base heading towards northern Norway, the cold vortex extending east across Greenland along with the left exit of the jet streak deepens a low rapidly over Svalbard early Wednesday.

post-1052-0-41525100-1292278961_thumb.pnpost-1052-0-55929400-1292278936_thumb.pn

The jet stream in the meantime is undergoing major amplification all the time with a ridge building north over Greenland/Labrador while downstream the jet over the Norwegian Sea begins to pull south as a fragment of the polar vortex drops south towards the UK from the arctic circle together with the deep cold arctic airmass.

post-1052-0-14266800-1292279055_thumb.pn

As the jet streak ploughs further south down the Norwegian Sea, the left exit of the jet ‘abandons’ the low near Svalbard and instead develops a low over western Norway moving SE, this is the trigger at the surface to create a surge of arctic air south across the UK on Thursday.

post-1052-0-73730600-1292279097_thumb.pn

There is a notable thermal gradient immediately behind the cold front moving south on Thursday, as intimated by the closeness of the 528 and 510dam thickness lines on this evening’s fax charts for Thursday.

post-1052-0-12561300-1292279149_thumb.pnpost-1052-0-16620900-1292279175_thumb.pn

As 12z GFS suggest at the moment, the back edge of the precip along the cold front should be see an overlap of air with low enough dew points (at or below 0C) and 850-1000mb thickness (below 129dam) for snow to fall. Timing of front passage and the parameters mentioned of course subject to change. Look out for narrow squall line convective elements on the radar too on the back edge of the front as cold air undercuts, not something the models may pick up on yet.

I will update this thread over the next few days when I can with charts, satellite imagery, observations, etc to follow how this unfolds. But please feel free to add information and comments on what could be an interesting day.

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I cannot remember a more potent looking Northerly than the one being shown for thursday, this is going to bring severe blizzards to the n.isles and northern scotland and make travel very dangerous due to drifting, blowing snow..add to that sub zero temps and snow showers also to w and sw scotland, n.ireland, wales and sw england. Much of inland uk looks sunny once the cold front sweeps away south but a few snow flurries will follow. Into the weekend it looks potentially snowy for a much wider area but maybe a sleety mix near the south coast.

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Very interesting Nick and thanks.

Certainly feel that this cold front has the potential to be one of the most active, in terms of a rain to snow event and plunging temps in a short period of time, seen for many a year.

If you'll allow me to post the archive chart for 28th Jan 2004, for comparison. Remember these charts are some 16/18 hours before the event.

http://www.wetterzentrale.de/pics/archive/ra/2004/Rrea00120040128.gif

http://www.wetterzentrale.de/pics/archive/ra/2004/Rrea00220040128.gif

Archive charts of another active cold front in Dec.1967

http://www.wetterzentrale.de/pics/archive/ra/1967/Rrea00119671208.gif

http://www.wetterzentrale.de/pics/archive/ra/1967/Rrea00219671208.gif

Regards,

Tom.

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Thank you for more information about this "polar blast". In the 3 years of observing the posts in these model forums I don't think I've seen as much excitement and anticipation. I'm sure we will be in for surprises after the front moves through. With that unstable airmass some serious snowfall is possible almost anywhere! smile.gif

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If I could just add this chart as well from Feb.1969, which someone mentioned the other day.

http://www.wetterzentrale.de/pics/archive/ra/1969/Rrea00119690208.gif

http://www.wetterzentrale.de/pics/archive/ra/1969/Rrea00219690208.gif

Note the kinks in the isobars, tell-tale signs of troughs/polar lows.

This event produced 6 hours of snow in the E.Midlands and south-east, a foot in places with much deeper drifts.

In places temps fell from 4c at midday to -3c in the snow and -14c overrnight.

Many thanks to Trevor Harleys marvellous site.

Regards,

Tom.

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Thanks for taking the trouble to post this interesting summary, Nick. The Model Output Discussion often has its gaze a week or more ahead, which leaves a bit of a void on the forums between what the weather will do in the next day or two and what might happen six or seven days ahead. Therefore, I think it's very useful to have threads like this one which fill the gap.

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GFS seems to think that the cold front will be an all snow event for Leeds, so it looks like we should see sufficient undercutting.

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Thanks Nick.F.

this event to me is most interesting and exciting, its the timing of the arctic air catching up the cold front hitting the warmer air that produces thunder? this warmer air would be lifted up into huge cumulonimbus?

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The fax chart looks very tasty now two cold fronts right next to each other. Id normally post pic but im on mobile.

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If I could just add this chart as well from Feb.1969, which someone mentioned the other day.

http://www.wetterzen...00119690208.gif

http://www.wetterzen...00219690208.gif

Note the kinks in the isobars, tell-tale signs of troughs/polar lows.

This event produced 6 hours of snow in the E.Midlands and south-east, a foot in places with much deeper drifts.

In places temps fell from 4c at midday to -3c in the snow and -14c overrnight.

Many thanks to Trevor Harleys marvellous site.

Regards,

Tom.

Thanks for that Tom.

I remember this one in particular,having got stranded travelling home from work nr.Coventry.

I think this Thursdays event could be as severe,very much going to be a last minute type of forecast as it comes south.

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For me the exciting thing is watching the weather change radically as it sinks south, just watching the temperature drop, it amazes me how some people still are not prepared for this, its not just the temperature but the actually feel like is going to be very severe. I wait in anticipation, and have found all day I keep re-checking the charts in case I am missing something.

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A look at the most recent t+00 for 12z today shows a low deepening between Svalbard and NW Greenland coast.

post-1052-0-77992700-1292342092_thumb.pn

The cold front on the chart roughly marks the boundary of the deep cold arctic air and less cold martime airmass, as seen on the 850hPa temperature chart:

post-1052-0-02902100-1292342521_thumb.gi

... this cold air bottled up over the arctic circle by the belt of strong upper winds, as shown on the 300mb chart below - which is holding back the deep cold arctic air from escaping south for now:

post-1052-0-28924500-1292342558_thumb.gi

However, over the next 24-36 hours we see the jet amplify and buckle north to the west of Greenland and south towards the UK, so the deep cold air bottled up in the arctic will spread south towards us by early Thursday.

post-1052-0-08333200-1292342952_thumb.pn

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posted in the regional thread, my concern at modification around the coast when NW england has quite a strong WNW'ly wind coming off a 9/10c SST in the irish sea, would this make it alot more marginal for snowfall resulting in rain at the coast and send temps up to +2/3c? as i cant see them staying near freezing with an onshore wind.

any views on this would be welcome, thank you

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posted in the regional thread, my concern at modification around the coast when NW england has quite a strong WNW'ly wind coming off a 9/10c SST in the irish sea, would this make it alot more marginal for snowfall resulting in rain at the coast and send temps up to +2/3c? as i cant see them staying near freezing with an onshore wind.

any views on this would be welcome, thank you

Can't see a problem for the coast - we will have some seriously deep cold uppers - SST's won't come into play.

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I cannot wait for this cold spell but why do the weather forecast always mention showers instead of proper snow? This is the one which could be talked about for many years does anybody know what could be causing this unusuall winter weather and why it's not mild like others?

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posted in the regional thread, my concern at modification around the coast when NW england has quite a strong WNW'ly wind coming off a 9/10c SST in the irish sea, would this make it alot more marginal for snowfall resulting in rain at the coast and send temps up to +2/3c? as i cant see them staying near freezing with an onshore wind.

any views on this would be welcome, thank you

Im across the Irish Sea from you, in a coastal town, and all too-often experience marginal conditions during easterlies or northeasterlies. I wouldn't be too concerned, unless you live close to sea-level or right on the coast itself. As long as the winds are blowing in off the sea, temps near the coast will stay above freezing. It will still snow, but theres a greater chance of it thawing at times.

Its amazing what a short distance away from the coast, and little bit of altitude will do. There could be lying snow a mile or two inland at 50m asl, while closer to the coast you will get nothing but wet snow that quickly thaws.

You may find that many of the showers will fall as soft hail rather than actual snow.

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Im across the Irish Sea from you, in a coastal town, and all too-often experience marginal conditions during easterlies or northeasterlies. As long as the winds are blowing in off the sea, temps will probably stay above freezing, Im afraid.

Its amazing what a short distance away from the coast, and little bit of altitude will do. There could be lying snow a mile or two inland at 50m asl, while closer to the coast you will get nothing but wet snow that quickly thaws. I wouldn't be too concerned, unless you live close to sea-level or right on the coast itself.

You may find that most of the showers will fall as soft hail rather than actual snow.

In most cases you would be right - this is different however. 850mb temperatures over the Irish Sea will be below -10C, ensuring lying snow right down to the coast.

Sea surface temperatures as a factor will be overridden by bitterly cold Arctic air.

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Quite a good satellite image earlier this morning tying in with the 6am fax chart showing the main cloud band near Faroes associated with an occlusion ahead of the main cold front moving through Iceland and falling temperatures behind:

post-1052-0-21309100-1292406884_thumb.jppost-1052-0-76396200-1292406928_thumb.pn

0900hrs surface analysis shows the kink in isobars and wind shift with the passage of the cold front:

post-1052-0-83908900-1292407099_thumb.gi

The airmass is increasingly mild from the west ahead of this cold front though with winds from the west, with surface temps and 850hPa temps (below) rising from the west across Scotland and points north and west, with colder air towards the SE and mainland Europe:

post-1052-0-29005700-1292407394_thumb.gi

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Meteogram for Manchester over the coming few days shows the sharp drop in the 0C isotherm and the 1000-500 hPa thicknesses tomorrow, along with the wind shift to the north, the Relative Humidity also decreases after passage of the cold front – indicating the dry nature inland of the cold arctic airmass:

post-1052-0-49538100-1292408606_thumb.pn

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Nick, apologies if this may be deemed off topic for your thread.

You mention about the milder air coming into Scotland and Western areas ahead of the Cold Front.

It, in my opinion, is very possible that the highest temperatures of December may be recorded in the next 24 hours with

those figures gradually happening from N-S or NW-SE and then the rapid plummet during and post Cold Front.

As an example, the highest I've recorded this month is 6.7c and it's quite likely that that figure will be exceeded

later tonight and tomorrow morning before the freezing conditions set in. Then it will take quite some change for such

temperatures to be reached again in the rest of the month.

What do you think Nick?

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What do you think Nick?

Could well be Andy, already 7-8C in the Western Isles right now, it will be close to freezing here this time tomorrow!

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Temperature still 7C across the Faeroe Islands just ahead of approaching cold front seen on latest fax,

post-1052-0-42423100-1292424819_thumb.pn

while over Iceland, temperatures have fallen close to freezing. Akureyri on the northern side of Iceland has seen quite a steep drop in temperature between 6 and 7am this morning (5 deg C):


Observations for AKUREYRI, Iceland (BIAR)
Location: 65.68N 18.08W 27 meters
1400Z 14 Dec 2010 to 1400Z 15 Dec 2010
STN TIME ALTM TMP DEW RH DIR SPD GUS VIS CLOUDS Weather
DD/HHMM hPa C C % deg m/s m/s km
==== ======= ====== === === === === === === ==== ======= ======= ============
BIAR 15/1400 1028.1 0 -9 51 340 5 10.0 SCT052
BIAR 15/1300 1028.1 0 -12 40 320 3 10.0
BIAR 15/1200 1028.1 0 -9 51 330 5 10.0
BIAR 15/1100 1028.1 1 -6 60 350 12 19 10.0 SCT030 BKN050
BIAR 15/1000 1026.1 1 -2 80 340 9 10.0 BKN026
BIAR 15/0900 1024.1 2 -1 81 360 13 10.0 BKN023 R-S-
BIAR 15/0800 1024.1 3 1 87 350 10 15 10.0 BKN050
BIAR 15/0700 1023.0 5 1 75 350 10 10.0 BKN050
BIAR 15/0600 1022.0 10 -1 46 290 6 10.0 BKN050
BIAR 15/0500 1022.0 9 1 57 150 2 10.0 BKN050
BIAR 15/0400 1022.0 9 1 57 260 8 18 10.0 BKN050


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Nick, what are you thoughts on the snow potential for Yorkshire from the cold front? Yesterday seemed to show conflicting forecasts, with the GFS indicating a sleet to snow event, but the BBC showing a 90% rain event.

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