General Model Output Discussion

[Thundery wintry showers]

Looking at the models it is an inevitability that the cold air will gradually be mixed out from the south, so whether we get a widespread white Christmas or not depends on how quickly this happens. The ensembles still show a good deal of spread with some starting this process as early as 22/23 December, a bit like the GFS operationals, while others delay it as far as 26/27 December with a large number of the Manchester ensembles staying below -5C 850hPa on Christmas Day. Ultimately the timing of the breakdown has not moved over the last few days (nor the spread of uncertainty on it) but its intensity has downgraded somewhat, and a more gradual change would reduce the chance of snow cover thawing abruptly on 23-24 December.

In the near term, some downgrades to the intensity of the cold but upgrades in terms of precipitation, so instead of it being very cold but mainly dry for many, it is likely to be cold and snowy in the north and marginal in the south. The key to all of these changes is the Arctic low slipping south over or to the west of us rather than to the east, cutting off the input of Arctic air and resulting in it being wetter but a little less cold.

[SteveB]

Gp has just posted in the technical thread if anyone is interested in the longer term.

[Nights King]

The met office maps show rain moving through the south west on monday. Will it not fall as snow in the west country as it bumps into cold air or is it likely going to go with out a battle ground event occuring? Maybe something like we saw feb?

Id be devastated to go through whole cold spell and not see a covering of snow.

Edited December 17, 2009 by mullender83

[Thunder Snow]

Looking at the 18z precip charts for saturday, Scotland/Ireland and Northern England are likely to see a spell of snow.

[Dingwantssnow]

T-63 Low looks to be more east

[mickpips]

Looking at the 18z precip charts for saturday, Scotland/Ireland and Northern England are likely to see a spell of snow.

Certainly confuses me why people even bother to look at ppn charts ... they are nothing more than a very outside indication of any precip , unless of course they follow any fronts etc

Looking into the xmas period I still think , as i have always done , taking a middle ground between diverging outputs usually gives you the best clue as to what is likely to happen. The divergence between the models and the eventual breakdown in terms of timescale would lead me to beleive Christmas day itself will likely still be cold enough for snow anywhere north of the midlands .... but time will tell and this weekend will most likely see the various model outputs come to an agreed evolution

[Thunder Snow]

LOL a big blob missing cumbria LOL

[Barb]

Well the 18z so far is keeping the cold over us better but that low pressure that appears on Tuesday would ideally go further south through the channel which would bring plenty of snow to many areas. Bit too far north now though.

[Dingwantssnow]

It's a very complex set-up next week, and one which is unprecedented since people have been discussing the NWP on the internet in the UK.

If the first Low is there on Tuesday, then whilst it could bring less cold air to the South, it can't really make inroads too far North - look at the upper circulation - there is no where for it to go other than back South or decline in situ. The more intense any PPN, the greater this being of snow as it hits the entrenched cold air.

Any breakdown will be to 'less cold' rather than mild until any new signals are picked up on next week. But this is a difficult time for the models and by any standard a rarefied set-up.

I take it your talking about the low thats comming from the SW at T-177/180?

Edited December 17, 2009 by Dingwantssnow

[davehsug]

Ian is quite right. It is an immensely complex situation. I think the only thing that is certain is that the low is not going to do exactly what any of the models has predicted. In my experience depressions just don't meander around in the same way as anticyclones.

If it can get far enough south as one or 2 models have hinted, it could well get mixed up with the lows further south & get caught up in the general west to east flow, leaving us back in an easterly flow.

Alternatively it could go further west and open the door to the southerlies.

I just can't see it wandering around, sometimes heading west sometimes east, then south, then north again.

If anyone has examples of depressions that have behaved like that in the past, I'll gladly defer.

[Barb]

If it can get far enough south as one or 2 models have hinted, it could well get mixed up with the lows further south & get caught up in the general west to east flow, leaving us back in an easterly flow.

Yes that's the holy grail, the low moves east to our south bringing heavy frontal snow and then easterlies to keep the freeze going. It could easily happen. Also very easily could stay too far West/North pump up rain and mild south westerlies. Who knows.

[Dingwantssnow]

No, the one before then at around T108. T180 might as well be a month away such is the uncertainty of this set-up.

Oh thats the one that has come all the way from the north has a poke and a sniff about to our west bit like its had too much and then shuttles off back up north later in the run. Never seen anything like it before.

Edited December 17, 2009 by Dingwantssnow

[BLAST FROM THE PAST]

Fred a quick question, do you see this cold spell lasting through to the end of January?

There is a propensity for cold pattern to show its hand this winter. No we will have milder spell before renewed cold in Jan which I anticipate a 2 week cold spell. If the period between Christmas and New Year doesn't turn milder then yes the cold could really become lasting....but that maybe asking too much.

Richard et al, seen your posts....keeping a real close eye on the ECM over next couple of days as it doesn't want to let the cold pattern go...and one has to take note of that.

I have just read GP's update and so i will add this....we seem to be heading the same direction re this winter.

BFTP

Edited December 17, 2009 by BLAST FROM THE PAST

[casparjack]

There is a propensity for cold pattern to show its hand this winter. No we will have milder spell before renewed cold in Jan which I anticipate a 2 week cold spell. If the period between Christmas and New Year doesn't turn milder then yes the cold could really become lasting....but that maybe asking too much.

Richard et al, seen your posts....keeping a real close eye on the ECM over next couple of days as it doesn't want to let the cold pattern go...and one has to take note of that.

BFTP

Don't you think it's ironic that only four or five days ago it was the ECM that 'downgraded' in one run and the whole thread was on the verge of jumping off a cliff in anticipation of the next run whilst the other two principal models held with the cold/colder theme, then ECM came back in line and here we are tonight. Now it's the ECM that's leading the idea that we might not even have a mild interlude, just a less cold spell whilst things reload.

I feel that there is definitely more to come from this winter...

[Barb]

My opinion is that there's still decent potential for a cold Christmas for southern half of Britain but it seems to be gradually slipping away, just about holding on. All could change though, should have a better idea by the weekend - could make or break it.

And lol this thread as dead as a doornail tonight. I guess most posters are in the SE.

[scottishandy]

Another interesting evolution in FI tonight. The low comes all the way south, hits the channel then about turn and back up the North Sea! A run of SEasterly's for the festive period, then we get an attempt at height rises into Scandanavia, then at T348 a reset of the start of our current patterm with some nice WAA heading up to Greenland!

If only I could belive that the fronts running S in the next couple of days would give Central Scotland more snow than GFS is trying to say it will.

[David-kig]

nt a bad run keeps the cold till 25th even in the south

[Iceberg]

All three models improve the chances of it lasting until Christmas day now (even really in the south), disconnecting the low pressure link up and preventing the southerlies.

Personally the one thing I would say is that there will be alot of snow about somewhere for the first few days of the week.

Changes even at 72Z makes the whole thing very unreliable though.

[Iceberg]

It does seem as if it will(but the meto are really playing it down atm, maybe they want to get today out of the way, to reduce distraction to the public. It's there on all the charts still meto, ECM and GFS.

[Turnedoutniceagain]

would you have an idea on how much that will bring and how long snow reg be here looks like 6 hours on the gfs forecast are they realiable

David, is there any chance of some punctuation in your posts ?? I've read though your last 4-5 posts & don't have a clue what you're on about !!

[Frosty.]

The 00z runs have upgraded the longevity of the cold spell until after christmas day, especially the further north you live. The E'ly which is bringing bands of heavy/thundery & drifting snow will fizzle away by midnight tonight or no later the 6am tomorrow, kent seems to be the worst affected area with thunder snow. The balance of power in this wintry spell has so far been in the south and east of england but there is going to be a dramatic shift with these areas having a largely fine and sunny weekend apart from the odd flurry, the main snow threat then changes to scotland, nw england, n.ireland with strengthening NW'ly winds and frequent snow showers sometimes merging into longer periods of snow with drifting. Next week is a very complex messy picture, the cold air could linger all week and snow will feature in the forecast but the worst affected areas are still to be determined although it's a safe bet that most of scotland will have the most severe conditions from this weekend onwards..The ECM 00z T+240 chart is very interesting :o

[asjmcguire]

Hum,

16 Day Detail etc on Netweather still showing 18z GFS?? Last updated 22:39:00

[West is Best]

You occasionally encounter on here the phrase 'the models are struggling', which sometimes contains mere wish-fulfillment. However, at the moment I think there's something in it. The synoptic patterns showing up on the model runs are somewhat unusual by the standards of recent years. After the 18z operational mild outlier we now have some fairly odd 0z runs. ALL 3 models seem to be struggling with the Greenland high, and the direction and positioning of the lows moving southwards. Neither the UKMO nor ECM are particularly good for cold, keeping most of us in southern Britain on the wrong side of the systems: better for Scotland though. The GFS, on the other hand, is very interesting: all the more so when you see the ensembles which show 1. great scatter (for both very cold and very mild) and that 2. in far FI the operational disappears into its own mild world.

It may be that the mild conveyor wins out this battle and normal synoptics will be resumed. However, looking at these runs I am not currently convinced. There is enough at the moment to suggest we might be in for a prolonged cold spell.

Edited December 18, 2009 by West is Best

[Frosty.]

saturday nights snow will bring snow to the south west is that still on???

Yes, according to the GFS 00z, you will get overnight snow into sunday morning. It's really the areas which are currently worst affected which will probably be the brightest this weekend although not immune from flurries.

[Terminal Moraine]

It may be that the mild conveyor wins out this battle and normal synoptics will be resumed. However, looking at these runs I am not currently convinced. There is enough at the moment to suggest we might be in for a prolonged cold spell.

I agree. GFS has been toying with the idea of milder weather moving in around christmas for some time but seems to alternate between that outcome and one of cold persisting, at least north of the Midlands. There's also a good deal of ensemble agreement on the cold weather holding on until at least the 25th, beyond that the operational is amongst the mildest of the members.

As I view each run from each of the major players I am left with the feeling that, beyond about next Tuesday, everything is rather vague and messy.