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Model output discussion - the beast arrives

Paul

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27 minutes ago, Man With Beard said:

Going to be a strange week in here. Most of the country will be probably spend most time in the regional threads to discuss snow prospects within the following three hours. The only area not really included is the South West, so their attention will turn to Thursday/Friday which is the only hope down there of a snowfest.

It most certainly is not game over for the South West by any stretch. 100 miles north or south is nothing for a system to move even 36 hours before an event happens, and that's when no block is in place at all. There is a reason why the Met Office have a weather warning in the most severe category over the South West (at low likelihood at the moment simply because of how far out it is).

My personal opinion, from looking at the historical records we have, I feel that if anything, this low will not be further south but instead will be further west, before possibly splitting in the way shown on the UKMO.

North of the M3/A303, I don't think this will make a major difference - you will get an all snow event on Friday.

I'm not entirely sure what that would mean for areas south of that. I think the main factor will be the shape of the low. If it is extremely rounded on Friday morning, like the GFS shows, then it will slowly suck up air from the English Channel and out to the Atlantic. However, if it starts to lose its shape as it hits the block like this GEFS member:

gens-3-1-102.png

then the flow turns more easterly for the whole of the south and the 0C uppers line might stay right on the coast, probably resulting in snow for all except just maybe the very fringes of the coast.

gens-3-0-102.png

This is an entirely reasonable evolution from the current trends - much more likely than a sudden shift of track to the east or south.

Hi MWB - far SW (Cornwall/S devon) still have things to be looking at over the next two days, especially Wed if the wind veers after the short wave low passes through N England. 

Wrt Thu/Fri, yes very small margins, the slider back in Dec ended up going through the midlands/west country rather than NW england/east anglia and i think that was picked up at about T36.  I feel the Fax charts are now the best guidance for that system as we are in the range (<T84) of the modified charts now so should have an idea of Exeter's thinking.  My opinion from an IMBY view, is rain south west of a line Bristol to Brighton, but living in Exeter has given me a glass half empty take on weather when it cones to snow/Tstorms.  As long as it's not freezing rain!

Edited by swebby

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image.thumb.png.c55619133e431dac084597df54a606b0.png

06z for midday saturday, is cold air still holding on? A few more do, thanks to the secondary channel low that forms on Friday, many of the GFS runs however show the incoming main low curving back westwards before reaching Ireland meaning its hard for cold uppers to return.

Not a bad 06z but could be better, what I'd like to see is major trough disruption as it heads towards the channel, then forcing the low eventually eastwards instead. May still happen.

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4 minutes ago, Quicksilver1989 said:

image.thumb.png.c55619133e431dac084597df54a606b0.png

06z for midday saturday, is cold air still holding on? A few more do, thanks to the secondary channel low that forms on Friday, many of the GFS runs however show the incoming main low curving back westwards before reaching Ireland meaning its hard for cold uppers to return.

Not a bad 06z but could be better, what I'd like to see is major trough disruption as it heads towards the channel, then forcing the low eventually eastwards instead. May still happen.

The gap is closing! Bit like watching the poll trackers prior to a general election:)  Quick Qu Quicksilver - in terms of deciding what member is what, what time frame are you looking at - i.e Fri/Sat or further afield?

Edit sorry reread your post so ignore the Qu

Edited by swebby

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5 minutes ago, icedust said:

Polar Low?

Re the possible polar low overnight/early tomorrow. Investigating the upper air it seems the trough is under a minimum of 500mb temperature (about minus 36) which is moving west under the otherwise undisturbed baroclinic flow. Baroclinic to those not in the jargon means thickness lines being advected, in this case cold advection slightly wobbling, maybe wobbling enough to generate the surface trough, not evident higher up. In short, a small area of increased instability moving west to southwest will pep up  and merge the showers in this trough (may become a very small centre). This instability is being extra invigorated by the surface trough dragging in warmer surface air ahead of it from the north. So a polar low? well may be a case of semantics, but it is definitely away from frontal zones and will be unstable; so yes Polar Low will do.Print

Thck 500-1000hPa GFS Tu 27.02.2018 06 GMTHeight/Temp. 500 hPa GFS Tu 27.02.2018 06 GMT

I only called it a polar low because I heard a professional weather forecast presenter earlier this morning call it a polar low..anyway, whatever it is its going to dump up to 8 inches of snow across northern england tomorrow!...model output is incredible for this week, especially further north in terms of potential longevity..exciting stuff is an understatement!..bitter cold incoming.:):cold:

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Fairly solid agreement now on the London ens for less cold air to arrive in the south later this week until about Monday after this it looks like it could turn chillier again just how cold remains open to question

gefsens850London0.thumb.png.7b78bbe0a476a5c4ac375b55c5783db7.png

Edited by Summer Sun
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1 hour ago, icedust said:

Polar Low?

Re the possible polar low overnight/early tomorrow. Investigating the upper air it seems the trough is under a minimum of 500mb temperature (about minus 36) which is moving west under the otherwise undisturbed baroclinic flow. Baroclinic to those not in the jargon means thickness lines being advected, in this case cold advection slightly wobbling, maybe wobbling enough to generate the surface trough, not evident higher up. In short, a small area of increased instability moving west to southwest will pep up  and merge the showers in this trough (may become a very small centre). This instability is being extra invigorated by the surface trough dragging in warmer surface air ahead of it from the north. So a polar low? well may be a case of semantics, but it is definitely away from frontal zones and will be unstable; so yes Polar Low will do.Print

Thck 500-1000hPa GFS Tu 27.02.2018 06 GMTHeight/Temp. 500 hPa GFS Tu 27.02.2018 06 GMT

Has this been officially classified as a polar low?

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Where do we stand then with the end of the week, has the GFS proved to be correct with less cold or even mild air moving into the south by Saturday?

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2 hours ago, Frosty. said:

We almost get 499 dam thickness on the Gfs 6z on wednesday and some fabulous sub zero maxima of minus 3 or 4 c with heavy drifting snow and severe wind chill making it feel like minus 10 to minus 15c in the gale force easterly winds!..can't believe it's so quiet in here!:cold::D

Maybe in Yorkshire Frosty, but down here in the SE we have barely seen a covering this winter, let alone drifts.

we ve been let down too many times ...still hoping though 

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11 minutes ago, lassie23 said:

Where do we stand then with the end of the week, has the GFS proved to be correct with less cold or even mild air moving into the south by Saturday?

Less cold in the south does look to be the form horse at the moment so Kudos to GFS for spotting this first

UKMO for Thursday and Saturday - London goes from -1c on Thursday to 6c on Saturday and 7c on Sunday based on the current thought's of the met office

UW72-7.thumb.GIF.8a9f3584ce36b55fa914723eb1fd7152.GIFUW120-7.thumb.GIF.bf89c36fdadda97c707115dc89a927e7.GIF

Yorkshire north you are likely to remain in the colder air albeit the 850's won't be 'as low' but still a chance of snow

Edited by Summer Sun
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The GFS keeps up it's trend for Friday "correcting" the low and the snow northwards, while the ICON keeps it further south.

The first is good for me (and much of the country) and the second is not, don't know much about the ICON and so far this season all the GFS snow events have been exaggerated for my location (nothing stuck).

So which model is the best for accuracy, does anyone know, do we need to get Harry Hill to decide?

 

Edited by johnwirral

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Friday could see a spell of freezing rain in the south as the transition to less cold air begins this could cause major issues and needs to be monitored closely

The freezing rain shows as green

tempresult_xad4.gif

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6 minutes ago, johnwirral said:

The GFS keeps up it's trend for Friday "correcting" the low and the snow northwards, while the ICON keeps it further south.

The first is good for me (and much of the country) and the second is not, don't know much about the ICON and so far the season all the GFS snow events have been exaggerated for my location (nothing stuck).

So which model is the best for accuracy, does anyone know, do we need to get Harry Hill to decide?

 

Here the forecast is 7c and easterly on Friday, Rain. Saturday Southerly 9c rain. So much warmer than it has bee or will be for a few days. . Northern CI’s Snow forecast 10cm today into Tuesday,  Southern CI, maybe the odd shower, so a few miles makes a difference. All CI’s  have Thursday as the big snowy event. Guess our forecasters are with GFS! We shall see! 

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So GFS shows the show the low going up into Ireland and the icon shows its moving east along the channel...

Clearly some major changes going to happen over the coming days.

I personally dont like to trust the GFS when we get easterlies.

It just doesn't cope well at all with it and is always more progressive than the other models back into warmer temperatures.

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I find it ironic that looking past this week and the trend for a breakdown of sorts to less cold conditions, that it can be blamed on the lack! of a Canadian vortex, so the block to our north makes a rather speedy move west in its absence . How many times does this chuffin seemingly endless vortex scupper our winters? Now we get the mother of all easterlies and the darn thing ain't there to block or hold off the speedy westward progression. that's my rather simple take on it anyway

 

Still an amazingly cold week coming up!

Edited by Nicholas B
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52 minutes ago, johnwirral said:

The GFS keeps up it's trend for Friday "correcting" the low and the snow northwards, while the ICON keeps it further south.

The first is good for me (and much of the country) and the second is not, don't know much about the ICON and so far this season all the GFS snow events have been exaggerated for my location (nothing stuck).

So which model is the best for accuracy, does anyone know, do we need to get Harry Hill to decide?

 

Surely the exact opposite of what you say is true?

The low staying south keeps the UK in colder air for longer.

Edited by Johnp
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Strange winter......-NAO for the first and last days of the season only. 

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Feels like minus 35 / 40c (actual minus 22c) where our air is coming from in russia which is why we may see sub 500 dam thicknesses on wednesday as the 6z showed, certainly potential for it..such an exciting week with disruptive snow / drifting as well as sub zero maxima especially wed / thurs with frequent snow showers further east and blizzards spreading north across the uk later this week!:cold::)

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19 minutes ago, Johnp said:

Surely the exact opposite of what you say is true?

The low staying south keeps the UK in colder air for longer.

Not really if you live in the east of the country then there will be frequent snow showers and opportunities for them to merge together, these don't deliver for my part of the world and the further west you go. While the longevity will be shorter much larger area of the country is forecast be the GFS to get the frontal snow and possibly extremely rare blizzards, which was originally only impacting the south, and areas further north will stay in the cold over the weekend or longer. I'd rather igo out with a bang than a wimper...

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I notice the UKMO run was very consistent with yesterday's 12Z but the fax charts look similar to the ECM run. Will be interesting to see what the UKMO 12z does - it's not exactly alone as the GEM isn't dissimilar and ICON too.

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Afternoon all :)

If a week is a long time in politics, it's going to seem like and eternity in here as we watch the tribulations, machinations, projections and confusions of the Atlantic onslaught scheduled for a few days hence. Done and dusted it certainly isn't, "game over for the south", "all rain south of the M4" - bold calls for an event very difficult to predict.

The ENS are clearly calling for a slackening of the frigid air - no surprise, 72-96 hours of -10 to -15 uppers in late February is unprecedented and unsustainable so the coldest of the air would ease eventually. Whether that translates to "less cold" rather than "mild" depends on your perspective and perception I suppose but within an overall context of heights to the north and east, cold air isn't likely to be far away and it's quite possible some northern areas will never lose the cold.

Anyway, a quick look at some medium-range model output looking today forward to Thursday March 8th:

ECM 00Z OP at T+240:

ECM1-240.GIF?26-12

A complex evolution and very interesting. Pressure is high to the NE with a new HP developing over Scandinavia and to the far NW with a complex trough in mid-Atlantic and a shallow LP over SE England. Uppers over the country range from just below zero across the south to -6 or -7 over Scotland. It's entirely possible a T+264 chart would see rising pressure to the NE and the LP heading SE (along the line of the trough) toward Iberia.

GEM 00Z OP at T+240:

gem-0-240.png?00

Another interesting evolution. LP is centred over southern England and is moving slowly ENE. A chain of shallow LP extends SW from the Azores while pressure if high to the NW with a new HP cell coming off the eastern seaboard. The weather over the British Isles would be unsettled with rain for southern and eastern areas (based on positive uppers) and more wintry conditions further west and especially north.

GFS 06Z OP at T+234:

gfs-0-234.png?6

A complex trough controls the weather over the British Isles with one centre to the NW of Ireland a new LP moving from the SW. Pressure is higher to the NW and is building via a mid-Atlantic ridge. The British Isles is in 0 to -4 uppers so rain or showers for many with snow restricted to higher ground. Further into FI and the trough pulls away east and south re-introducing colder air even as new Atlantic systems approach from the SW but look set to move SE into Europe.

GFS 06Z Control at T+240:

gens-0-1-240.png

A complex trough sits just to the east with two LP centres over Scandinavia (really ?) and a smaller deepening LP to the SW but a transient ridge from Greenland covers the British Isles settling things down and re-introducing colder air with -8 uppers so frosty for most. Further into FI the battle between colder and milder air-masses continue but with warmer air coming across the British Isles from the SW by mid month.

The 06Z GEFS at T+240:

http://www.meteociel.fr/cartes_obs/gens_panel.php?modele=0&mode=1&ech=240

Surprisingly little spread so general agreement on LP dominance over the British Isles with a cold trough setting up as LP is held to the south or east. One or two members try to introduce milder conditions by forcing the trough north in mid-Atlantic but there's not much support for that at this time.

In summary, part of the process of winter becoming spring is the battle between the colder and warmer air-masses and the fascinating synoptics and weather these can produce. We are looking at it in microcosm this weekend but it's the dominant feature of the weather stretching through the first third of March. This week's exceptionally frigid air will pass but there's no rapid transition to spring on the cards - rather, any milder incursion (IF it happens) looks to be brief with a strong signal early next week for colder (if not as cold as currently) conditions to return with the mildest temperatures likely next Monday (10-11c in the south so only just on average) and values set to fall from there.

Clearly, the further north you are the more likely to remain in the colder air-mass so little chance of an appreciable warm-up in Scotland going forward. Further south it will depend on how the models end up dealing with the initial onslaught on Friday - it may be even if the bulk of the milder air stays to the south there will be a clear slackening of the frigidity in the south but the complexities of judging snow/rain/freezing rain events I'll leave to others at this time. ECM goes strongest on a return of heights over Scandinavia and some of the trough modelling from the other outputs just doesn't look feasible to me. As a final point, it might also be worth considering the accumulations of rain and snow-melt and the possible flooding implications if we get a protracted "battle ground" scenario between the competing air-masses.

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Looking at the GEFS 6z mean, northern uk stays very cold throughout the weekend and next week too whereas further south temps recover closer to average following the freeze although the mean shows it turning colder from the north again next week..longer term the temps recover generally but there are still some very cold blocked members out to mid march but with more milder options too..Enjoy this week coldies!:cold::)..if you're in the southeast, enjoy the Thames tickler!!:D

Edited by Frosty.
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3 hours ago, icedust said:

Polar Low?

Re the possible polar low overnight/early tomorrow. Investigating the upper air it seems the trough is under a minimum of 500mb temperature (about minus 36) which is moving west under the otherwise undisturbed baroclinic flow. Baroclinic to those not in the jargon means thickness lines being advected, in this case cold advection slightly wobbling, maybe wobbling enough to generate the surface trough, not evident higher up. In short, a small area of increased instability moving west to southwest will pep up  and merge the showers in this trough (may become a very small centre). This instability is being extra invigorated by the surface trough dragging in warmer surface air ahead of it from the north. So a polar low? well may be a case of semantics, but it is definitely away from frontal zones and will be unstable; so yes Polar Low will do.Print

Thck 500-1000hPa GFS Tu 27.02.2018 06 GMTHeight/Temp. 500 hPa GFS Tu 27.02.2018 06 GMT

Not totally convinced, compare with this case study - Snow and gales in eastern England from a North Sea polar low: 6/7 January 2010 - http://onlinelibrary.wiley.com/doi/10.1002/wea.740/full

Typically a warm core system, it wouldn't be under a 500mb temperature minimum, though this system is very shallow.

The 850mb Theta-e chart below shows a frontal structure already this morning along the west Norwegian coast (might well have been initiated by land/sea temperature/wind contrasts)

nmm-5-1-0.thumb.png.9d59eb84116a828b8a465dbedbee43d9.png

This is following a line of vorticity advection -

euro1_24hr.thumb.png.902b325a8817bc340e261167810c87ff.png

Edited by Interitus
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IMO, the only reason this is being described as a polar low is because it originated in the arctic !

its a feature that has been predicted from the middle of last week - I was always of the mindset that polar lows form without such predcitivity ?????

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Icon at just T-33 interesting look at much further SE the energy disrupting is going and its actually not far from phasing with that small low over Greece now that would be backtrack from models if we get clean undercut. I doubt they will go that far but shows just how nothing decided for weekend yet as massive changes even within 24 hour timeframe

12z

iconnh-0-33.png?26-12

12z

icon-0-33.png?26-12

6z

iconnh-0-39.png?26-06

6z

icon-0-39.png?26-06

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