Model Output Discussion - 28th November

[Catacol]

Hi Catacol, I haven't got the foggiest what that chart means or where the data is from ? Does it show some form of slider pulling in an easterly

or not maybe ? could you explain please, Thanx

Red areas means high pressure. Blue bits low pressure. The red zone stretching along the northern side of the uk and down into the atlantic shows a signal for higher than average pressure across the 6 days in question with lower pressure over Europe. This means a leaning towards an north easterly flow in that time span, though the chart itself is a computer averaged result with human input on top (as someone pointed out to me a few days ago at the weekend there is no human input on top, so charts from the working week are probably best...) and being an average over such a large number of days means it can obviously vary a bit in that time span. The slider low for early next week, for example, was picked up in this model a few days ago and was visible by a noticeable shifting north of the average low pressure zone over Europe, reflecting the low pressure expected to zip through early next week. That signal has now receded again, suggesting to me that after the middle of next week any low pressure systems heading in from the atlantic will be heading for the channel/France or maybe even Spain at a push.

These NOAA charts are invaluable tools when looking at the overall shape of the pressure patterns. It is an ensemble mean product, meaning they take the average of a range of computer results which tends to provide a much better feel for the future weather patterns than a single computer run such as the GFS 12z freely available on sites like wetter and meteociel. There is also a very useful prognostic text analysis released each day that explains why they have drawn the map as it has been drawn, and which computer simulations they have preferred and why on each particular day. These are climate experts over at the climate prediction centre in the States.

So if you are looking for evidence of whether blocking is likely to remain then you have a computer average with human agreement from a range of models pointing to exactly that from day 8 (Thurs 6th) to day 14 (Weds 12th)

Avoid getting hung up on a single run from any model.

Direct link is http://www.cpc.ncep....10day/500mb.php

Edited November 28, 2012 by Catacol_Highlander

[bluearmy]

I have not looked back at the model outputs for a similar period in 2010 but, from recall, we were seeing fairly similar ebbing and flowing of chances for cold. In the end and apparently against the odds the cold emerged as if without warning. I cannot help wondering if the volatility in relatively short timeframe which we have experienced this week could end up with a similar rapid firming up into a very cold spell.

I wish i had the time to dig back to compare the detail.

egret - thats completely not true. i have lost count of the number of times this myth is brought back. the models counted down to the cold from pretty well two weeks away on gfs and 10 days on ecm. thats probably why we have seen so much despair with the current situation. you dont need to take the time to dig back. i already did it on several occasions and i remember it fairly well. (in the second cold blast mid dec 2010, several ecm runs took us to west based -NAO with the sinking p/v. that was the only problem)

[bluearmy]

the ecm 12z run is closer to the ens mean than the last two runs. it also fits in with quite a few gefs 12z members.

a few days ago we had gfs at one end of the spectrum taking energy due east into scandi and jma the other end with a slider to the west of ireland. well we still have the same two models at either end of the spectrum but the road has narrowed a bit. to be truthful, its narrowed away from the slider and further towards energy going to our east but not our northeast. i imagine ukmo/ecm are probably in the right area at the moment, unless we see some correction west. i doubt we'll see anything further northeast now.

[Catacol]

Rock solid support from the ensembles (next 7 days+) for GFS and ECMW also very similar to GFS at T168:

http://modeles.meteo...fs-0-168.png?12

http://www.meteociel...M1-168.GIF?28-0

Amazing how the models and ensembles are showing high confidence after such a scatter of late. Can't expect too much change in the next 7-10 days. GFS fails with the Atlantic ridge around T240 and suspect the same for ECMWF. Then things get interesting and I am expecting an Easterly for the last third/quarter December. As the jet takes another trip down south:

http://cdn.nwstatic..../384/hgt300.png

Hopefully Europe will have good cold pooling by then.

Worth being precise here, because 72 hours later (day 10) the ensemble pattern is thus:

I think you are therefore misreading the ensemble signal which is showing - as it has consistently shown for days now - a temporary rise in temp and westerly influence for about 48 hours, maybe 72, before defaulting back to this pattern above which is very similar to the coming weekend. Pressure building behind the low that comes through, with I suspect a tendency for any other systems from around about Friday 7th to come on a much more southerly track. This is in line with the waxing and waning of the atlantic ridge put out there by GP a good while ago.

To say second half to last third of December is therefore to undercook the evolving situation by a good 7 to 10 days at least. I agree, however, that by that stage cold pooling will be more prevalent, and so any flow we get from the east by the second half of December will be a good bit colder than it would be now.

[BlueHedgehog074]

Don't want to fill anyone's hope up (and apologies if others have already covered this), but the UKMO has potential to deliver a fairly wide-spread snow event for next Monday. One reason I say this is because while the Atlantic Low will be attempting to bust in from the West, it'll be bumping up against the colder Northerly/North-Easterly winds to the East at 96 hours:

This could help sustain the cold for longer, most especially to the East, before the front tries to spill in - their could be enough chilly air in place for snow to fall to low levels for a time. I would say, however, from looking at the UKMO chart that the further East you are, the better chances of seeing something wintry with colder air more likely to cling on for longer. Clearly, the higher up you are would increase the chances of snow better as well and help any sleet and snow to last longer. For South and Western areas, this would probably be more likely to be a rain event only, with any wintryness to the rain restricted to the hills.

I expect most of any low level sleet and snow to transform to rain drops as milder air from the Atlantic makes more of an influence at 120 hours - sleet/snow however could still be possible in the far North and East, and perhaps on the highest hills, too.

There's also this quote from the 6 - 15 day outlook from the Met Office: "A band of rain will move east across the UK on Monday, and this may be heavy in places. There is a chance of snow as this rain runs into cold air lying across the British Isles, with this most likely over high ground."

Although it does mention snow being most likely on high ground, it seems like from that summary that snow to lower levels wouldn't be entirely unfeasible.

The other models, such as the GFS and ECMWF, would probably deliver similar results, but I do think the little ridge ahead of that Atlantic Low to the West (which looks more pronounced on those models) would probably pump up a little too much milder air on it's Western flank curbing the chances of snow to low levels.

I know it does seem like I've been cherry picking a weather event, and the factors regarding exactly what weather we will get for late Sunday and into Monday could still easily change, but I would agree that the further South the advancing Low pressure systems from the West get, the better chances for wintry weather. Also, I do feel if the trough/lows to the East of the UK can hold on for longer with their cold Northerly/North-Easterly airflow before fronts from the Atlantic try to knock the cold away, the better chances we should see this rain falling as snow.

At least, despite some of the disappointments dished out on todays models, I agree when people mention that the colder air does seem like it will never hide from us completely. And although a little warm up could happen on Moday, the possibility of some potent North-Westerly winds dominating afterwards could unleash colder weather, some wintry, to the UK like a speeding blue hedgehog. Or, who knows, we could just see blocking re-strengthening to the North-West/North and North-East.

Edited November 28, 2012 by DiagonalRedLine

[chionomaniac]

That would be both the GFS and the ECM suggesting a northerly at around day 10 then. I suspect that this fits in with the north Atlantic ridge wax and wane that GP suggested recently.

The 12Z GFS ensemble H5 anomaly mean also backs this up.

One of the difficulties highlighted in the last couple of days and consistently mentioned has been how a split vortex reforms back to one. It is this that shows why the energy pulses across the Atlantic whilst it does so.

Looking at the 100 hPa chart for 5 days we see the split vortex, but with the Siberian segment about to become the dominant segment. As this occurs we will see a transfer of energy across Greenland from the Canadian segment as the vortex regrouping occurs.

However there is still uncertainty of how severely a forecast wave number 1 event will displace the vortex towards Eurasia. Fluctuations in forecasts will show possibilties such as the extended mid Atlantic ridge - here the 10 day 100 hPa chart shows the possibility

However this is unlikely to lead to long lasting Greenland height rises.

And just to totally contradict this earlier post and demonstrate how difficult these situations are to model, I have just checked the vortex situation forecast right out to GFS FI.

And guess what - there are signs of another vortex split occurring with the main bulk of energy being transferred to the Canadian daughter vortex.

Here are the 50hPa charts at distant FI from the GFS 6Z and 12Z runs!

That's two weeks away - and that pattern could give blocking to our north and/or northeast.

For those thinking that the strat vortex is going to rebuild and restrengthen the early signs suggest exactly the opposite.

Edited November 28, 2012 by chionomaniac

[Isolated Frost]

D7 H5 anomaly, for today 18z

Current H5 anomaly at 18z

A very similar pattern- though the current pattern is more amplified around the North Pacific and the Eastern Seaboard- the major difference is that the mean heights are primarily in the Arctic Basin, rather than the prediction that the mean heights would be positioned in Greenland- this has been the major downfall of any north-easterly/easterly, and the introduction of a northerly flow

[Paul]

As we're waiting for the 18z, thought now would be a good time to let those who may not have seen it know about the winter forecast which was issued yesterday:

[Bristawl Si]

Worth being precise here, because 72 hours later (day 10) the ensemble pattern is thus:

I think you are therefore misreading the ensemble signal which is showing - as it has consistently shown for days now - a temporary rise in temp and westerly influence for about 48 hours, maybe 72, before defaulting back to this pattern above which is very similar to the coming weekend. Pressure building behind the low that comes through, with I suspect a tendency for any other systems from around about Friday 7th to come on a much more southerly track. This is in line with the waxing and waning of the atlantic ridge put out there by GP a good while ago.

To say second half to last third of December is therefore to undercook the evolving situation by a good 7 to 10 days at least. I agree, however, that by that stage cold pooling will be more prevalent, and so any flow we get from the east by the second half of December will be a good bit colder than it would be now.

Thing is we saw similar charts a week ago, progged for this weekend.

So i'm not getting sucked in by charts a week or more out for the rest of the winter.

Not having a dig, but that's the way i'm playing it, if only to preserve my sanity!

[AutumnMists]

egret - thats completely not true. i have lost count of the number of times this myth is brought back. the models counted down to the cold from pretty well two weeks away on gfs and 10 days on ecm. thats probably why we have seen so much despair with the current situation. you dont need to take the time to dig back. i already did it on several occasions and i remember it fairly well. (in the second cold blast mid dec 2010, several ecm runs took us to west based -NAO with the sinking p/v. that was the only problem)

Perhaps he was getting confused between 2009 and 2010? Seems most likely especially given the timeframe.

[Isolated Frost]

Thing is we saw similar charts a week ago, progged for this weekend.

So i'm not getting sucked in by charts a week or more out for the rest of the winter.

Not having a dig, but that's the way i'm playing it, if only to preserve my sanity!

...the chart at 72 for this weekend is very similar to the one progged by Catacol_Highlander- so we have had these charts off last week verify...

[nick sussex]

the ecm 12z run is closer to the ens mean than the last two runs. it also fits in with quite a few gefs 12z members.

a few days ago we had gfs at one end of the spectrum taking energy due east into scandi and jma the other end with a slider to the west of ireland. well we still have the same two models at either end of the spectrum but the road has narrowed a bit. to be truthful, its narrowed away from the slider and further towards energy going to our east but not our northeast. i imagine ukmo/ecm are probably in the right area at the moment, unless we see some correction west. i doubt we'll see anything further northeast now.

BA is that compared to the 00hrs mean?

The 12hrs seem to have gone missing, no sign of them anywhere.

[Bristawl Si]

...the chart at 72 for this weekend is very similar to the one progged by Catacol_Highlander- so we have had these charts off last week verify...

The chart CH showed is for next Saturday 8th December

[swfc]

Or, perhaps some folks were simply too keen on ramping-up any runs that did show Snowmageddon, but ignored those (a sizeable number) that didn't?

Hi.To be fair pre the last 3-4 days the more respected posters on here have posting that the 500mb charts neg nao all pointed to a long ,blocked cold -very cold wintry outlook till mid december.Look on the old threads its fact!!!!.One of the main comments was watch the"trend"its screeming cold?Being told not to follow every run is fair enough but it didnt take many runs to bring the atlantic threw and blow the so called long termed blocked scenario out of the water.Altho the outlook is cool- cold i think moving the goal posts is very easy when things change and become watered down.You can put as many factors up leading to very cold weather as you wish but until"its in the reliable time frame"whats the point!The so called posters who app over react to each run showing cold and snow arnt actually helped when respected posters give out in some cases unrealistic long term forecasts!!!Anyway my take on the "current outlook"is late november early december unsettled,breezy and slightly colder than normal temps.

[Bobby]

.

Edited November 28, 2012 by Barb-

[mcweather]

That's two weeks away - and that pattern could give blocking to our north and/or northeast.

For those thinking that the strat vortex is going to rebuild and restrengthen the early signs suggest exactly the opposite.

Edited by chionomaniac, 26 minutes ago.

Pretty much in line with GP'S thoughs about the vortex becoming increasingly depleted by numerous attacks as we get into winter Chiono.

Could make for some interesting model watching in the run up to Christmas.

[Bobby]

Hmm, I think there could be a shot at some decent after around day 10 as some have mentioned with signs of a mid-atlantic ridge forming. If a low can cut through it to the south and we get some WAA then something interesting might be on the cards. The ECM hints at it this.

The GFS control shows what could happen if all goes well

Mid Atlantic ridge day 10:

Enery undercuts to the south/SE with troughing over Europe which is well supported, potential Beast from the East on the way.

A few other peturbations show something similar. More signs of blocking to the NE too... long way off yet but something to keep an eye on maybe. More interesting set of 12z ensembles anyway.

[Bristawl Si]

Hi.To be fair pre the last 3-4 days the more respected posters on here have posting that the 500mb charts neg nao all pointed to a long ,blocked cold -very cold wintry outlook till mid december.Look on the old threads its fact!!!!.One of the main comments was watch the"trend"its screeming cold?Being told not to follow every run is fair enough but it didnt take many runs to bring the atlantic threw and blow the so called long termed blocked scenario out of the water.Altho the outlook is cool- cold i think moving the goal posts is very easy when things change and become watered down.You can put as many factors up leading to very cold weather as you wish but until"its in the reliable time frame"whats the point!The so called posters who app over react to each run showing cold and snow arnt actually helped when respected posters give out in some cases unrealistic long term forecasts!!!Anyway my take on the "current outlook"is late november early december unsettled,breezy and slightly colder than normal temps.

Very good post - all 'types' got sucked in this time around incl. the 'Big Guns' on here

[bluearmy]

BA is that compared to the 00hrs mean?

The 12hrs seem to have gone missing, no sign of them anywhere.

indeed nick. conspiracy theories abound !!! missing data, blah blah blah. anyway, following on from ed's last post re the transfer back of energy from split vortex to split vortex - that concerns me in that with heights high around the pole, i feel this energy surge across to our north merely strengthens the northern arm. high risk strategy if we get the cold across us but would rather get some blocking to our north.

[Isolated Frost]

The chart CH showed is for next Saturday 8th December

Yes- but the point is that you said we had charts similar to that 10 days ago that haven't came off this weekend, but they have.

[Bristawl Si]

Yes- but the point is that you said we had charts similar to that 10 days ago that haven't came off this weekend, but they have.

In truth there have been numerous chart scenarios for this weekend over the last week - the one u posted could be the one correct one.

[Met4Cast]

Hmm, I think there could be a shot at some decent after around day 10 as some have mentioned with signs of a mid-atlantic ridge forming. If a low can cut through it to the south and we get some WAA then something interesting might be on the cards. The ECM hints at it this.

The GFS control shows what could happen if all goes well

Mid Atlantic ridge day 10:

Enery undercuts to the south/SE with troughing over Europe which is well supported, potential Beast from the East on the way.

A few other peturbations show something similar. More signs of blocking to the NE too... long way off yet but something to keep an eye on maybe. More interesting set of 12z ensembles anyway.

We were saying this 10 days ago too.

Back to chasing charts in FI then!

[Paul]

These charts may not be something too many are familiar with, but what they show is the sea level pressure mean from the ensembles as the usual contour lines and the colours show the spread, which is the difference between the highest and lowest ensemble run - ie a high spread means a large difference between highest and lowest so less certainty in terms of the outcome.

To put a baseline in, here's the chart for 24 hours and as you can see the spread is virtually all blue and therefore there's good confidence in the pressure pattern (as you'd expect)

By 96 hours, there's a bit more colour coming in there, with an increasing amount of spread surrounding developments of the low pressure system to the northwest of the UK with 5-8hpa 'uncertainty' within parts of that.

Roll it forward to 144 hours and the differences increase with the mid-atlantic, southern tip of Greenland and the east coast of the USA all areas of low confidence, and incidentally all important locations in terms of upstream development for the UK

168 hours and that increases

And by 240 hours, we're talking 10-20hpa of potential swing (on just this run alone) across much of the Atlantic, the UK and the eastern seaboard.

So, what to draw from this? 96 hours is the point to watch in terms of upstream & beyond that there's too much uncertainty to draw any real conclusions at this point. To coin an old phrase - 'more runs needed'

[Catacol]

Thing is we saw similar charts a week ago, progged for this weekend.

So i'm not getting sucked in by charts a week or more out for the rest of the winter.

Not having a dig, but that's the way i'm playing it, if only to preserve my sanity!

But these ensemble charts from a week ago for this coming week have verified? What was subsequently picked up was the zipper low which has upset everyone so much. Also weather is never a precise science, and the operational runs that showed deep cold coming towards us were typically deceptive. Watch them for enough years and you learn to take operational eye candy with a pinch of salt. Work with ensemble trends and you are never (rarely?) disappointed. Takes all the stress out of it. :-)

Edited November 28, 2012 by Catacol_Highlander

[Biggin]

These charts may not be something too many are familiar with, but what they show is the sea level pressure mean from the ensembles as the usual contour lines and the colours show the spread, which is the difference between the highest and lowest ensemble run - ie a high spread means a large difference between highest and lowest so less certainty in terms of the outcome.

To put a baseline in, here's the chart for 24 hours and as you can see the spread is virtually all blue and therefore there's good confidence in the pressure pattern (as you'd expect)

By 96 hours, there's a bit more colour coming in there, with an increasing amount of spread surrounding developments of the low pressure system to the northwest of the UK with 5-8hpa 'uncertainty' within parts of that.

Roll it forward to 144 hours and the differences increase with the mid-atlantic, southern tip of Greenland and the east coast of the USA all areas of low confidence, and incidentally all important locations in terms of upstream development for the UK

168 hours and that increases

And by 240 hours, we're talking 10-20hpa of potential swing (on just this run alone) across much of the Atlantic, the UK and the eastern seaboard.

So, what to draw from this? 96 hours is the point to watch in terms of upstream & beyond that there's too much uncertainty to draw any real conclusions at this point. To coin an old phrase - 'more runs needed'

Sorry mods, but Paul that is a quality post, so informative