Model Output Discussion - The final stretch to Christmas

[Vorticity0123]

With Christmas now on the doorstep, the weather charts are turning interesting from a cold perspective. This all seems to begin with a very small scale low pressure system which is expected to pass this Friday or Saturday. How will this small-scale feature influence our weather, and what are the long-term trends after this passage? To answer these questions, I will give a look at what various models are showing.

 

Current synoptic situation

 

For the first 96 hours, I will use the GFS charts as a guide for the synoptics.

 

The current synoptic situation is given below:

 

GFS 18Z surface pressure + 500 hPa heights (colours) T + 0h.

 

As of speaking, we are experiencing a zonal flow (red arrow), with relatively high pressure located over central Europe, and low pressure activity located near Scandinavia. This complex area of low pressure is associated with some weak troughing (indicated by the blue colours on the map edging slightly southward over the southern tip of Scandinavia).

 

The zonal flow over the past few days has led to anomalously mild conditions, with average temperatures about 1 to 3 degrees above average last week¹.

 

There is also a notable cut-off trough present to the southeast of Newfoundland (green colours, emphasized by the blue circle), associated with a remarkably strong low pressure area there.

 

However, the main focus for the short-term weather will lie in the area blue-encircled to the west of Greenland. If one looks very closely, a dip in the 500 hPa heights (indicated by the blue colours) can be observed. This dip is in part responsible for the formation of the low pressure area affecting the UK on Friday or Saturday.

 

Short-term developments

 

24 hours later, the 500 hPa trough has moved east and extended somewhat southward, as can be seen in the link below:

 

http://www.wetterzentrale.de/pics/Rtavn241.gif (GFS 18Z surface pressure + 500 hPa heights, T+24h.)

 

The trough can be seen just to the northwest of Iceland (blue colours). Also, a surface low is located at the position of the trough, but this is not the surface low that will move over the UK. The low of interest is located to the west of Ireland, as a sub-1025 hPa low pressure area (if one can call it as such).

 

Another 24 hours later, the low pressure has deepened somewhat into a sub-1010 hPa low pressure area now located near London.

 

http://www.wetterzentrale.de/pics/Rtavn481.gif (GFS 18Z surface pressure + 500 hPa heights, T+48h.)

 

Note that the minor 500 hPa trough previously located near Iceland is extending toward the low pressure area located over London.

 

On the 850 hPa temperatures chart of the same time, the low pressure area is also beautifully visible, with relatively warm subtropical air flowing into the storm from the south, and cold 850 hPa temperatures moving in behind the low from the region near Iceland.

 

(GFS 18Z 850 hPa temperatures, T+48h.)

 

What is also remarkable is that there is a large pool of rather cold 850 hPa temperatures located over and to the east of Scandinavia.

 

For the long-term prospects, it is important to focus on the low pressure area located over Nova Scotia. On the southeastern side of the system, a significant stretch of warm 850 hPa temperatures is being advected northward. If one combines this with the 500 hPa pressure chart, one can see that at this position, ridging starts to edge northward as well (just to the east of the warmest 850 hPa temperatures).

 

Looking at another day later, the low pressure system has moved well into Europe, which can be seen in the following link:

 

http://www.wetterzentrale.de/pics/Rtavn721.gif (GFS 18Z surface pressure + 500 hPa heights, T+72h.)

 

The system may deliver some harsh conditions in the Benelux, with snow and a northeasterly storm all within the range of possibilities (currently, there is too much uncertainty to say anything specific about this).

 

More notable is the warm air advection (WAA) that has continued to the east of the low (500 hPa and surface) over Nova Scotia. This can be seen by the orange colours edging northward toward Iceland. Furthermore, this WAA has shifted somewhat to the east. As a result, high pressure at the surface starts to build in the vicinity of Iceland. In the days following, this high pressure activity will become established near the UK.

 

Medium-term (un)certainty

 

As a result of the warm air advection advertised above, a settled spell seems to be becoming more prevalent over the UK. There appear to be developing some minor discrepancies between the models then. For comparison, the links to the GFS and the ECMWF 5 days out will be given:

 

http://www.wetterzentrale.de/pics/Recm1201.gif (ECMWF 12Z surface pressure + 500 hPa heights, T+120h.)

 

http://www.wetterzentrale.de/pics/Rtavn1141.gif (GFS 18Z surface pressure + 500 hPa heights, T+114h.)

 

The main feature that can be observed is that the GFS has the center of the surface high pressure area over the border between France and Germany, and the ECMWF has it around Normandy (much more to the west). If one looks at the 500 hPa heights (which are probably a better guide here), the GFS is somewhat more progressive in terms that it has the 500 hPa ridge (orange colours) moved further to the east than the ECMWF has. However, these differences at the 500 hPa level are not very significant (on the basis of large-scale patterns, not what will be experienced at the surface). To show this, take a look at the spaghetti diagram of 120 hours out of all GFS members:

 

(GFS 18Z spaghetti contour lines of 516, 552 and 557 hPa height, T+114h.)

 

A short explanation: the lines above originate from all GFS members from the 18Z run. These indicate lines of equal pressure (being 516, 552 and 557 hPa height). When the lines are close together, it means that all members agree on a significant kind of pattern. When the lines are all over the place, the uncertainty is large.

 

As can be seen, clear patterns can be seen in the diagram, with the 500 hPa ridge extending toward Scandinavia from the southwest (northward-edging lines). On the other hand, a very deep trough is located over central Europe (southward-edging lines). The trough is partly caused by the low which will over the UK in about 2 days' time.

 

This is a good indication that the spread for 5 days out is relatively small. Only the orientation of the (500 hPa) ridge over the UK seems to vary some.

 

Given that there is quite some certainty that a ridge will be present over the UK (at 500 hPa and at the surface) it seems that a more settled pattern will start to emerge after Christmas with also somewhat cooler temperatures than we have been used to.

 

Long-term trends

 

Looking at the long-term trends, uncertainty (logically) increases significantly. The NOAA 8-14 day 500 hPa anomalies show that some kind of a ridge is likely to be present over Scandinavia, but one needs to see more consistency to be certain for this. They can be seen in the link here:

 

http://www.cpc.ncep.noaa.gov/products/predictions/814day/500mb.php

 

The ensembles from the GFS and ECMWF at day 10 (not indicated here) generally agree on the idea of some kind of high pressure activity remaining over northern Europe, but it is uncertain to which extent this will be.

 

Also, the ECMWF ensembles for De Bilt, the Netherlands, show that after about 8 days, there is a range of about 15*C in maximum temperature between the warmest and coldest member, showing there is a large degree of uncertainty after that period. They can be found here:

 

http://www.weerplaza.nl/15daagseverwachting/?r=midden&type=eps_pluim

 

Conclusion

 

After the passage of the low pressure area in about 2 days' time, it seems that a dry period will emerge influenced by high pressure activity, with some frost at night also a good possibility. How long this high pressure activity will be able to sustain itself is still up to debate, but there are indications that this period may last more than just a few days. Finally, what rests is to wish a merry Christmas to you all :smiliz19: .

 

Sources:

¹ http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/regional_monitoring/wctan1.png

http://www.cpc.ncep.noaa.gov/products/predictions/814day/500mb.php

http://www.wetterzentrale.de/topkarten/fsenseur.html

http://www.weerplaza.nl/15daagseverwachting/?r=midden&type=eps_pluim

Edited December 24, 2014 by Vorticity0123

[Daniel*]

Well I like the GFSP bitterly cold air treading over, with SST (sea surface temperatures) much warmer than what we can expect at this time of year, this will aid in explosive convection... like 2010 proportions. Finally I'd like you to wish you all a very Merry Christmas, I wish you all well and hope for prosperity as we head into the year 2015 - for each and every one of you. Events this week play testament to how precious our time on this planet is, so I ask of you to follow your dreams & most of all be happy for what you have.

[SW Saltire]

Yeah she said a pattern change during the middle to end of December, so she was definitely right about that. But the weather can make fools out of even the most knowledgeable posters.

 

Nope, I can't remember who made that post

Haha.

I do agree none of us know where we are heading next and the weather is very unpredictable.

I'm just very unconvinced that the GFS (old) has any clue how to resolve the upcoming pattern or even any model for that matter.

Did the day 10,9 or 8 mean (since we are on the subject) show the low taking anywhere near the path it's currently projected to take? Did they show the amplification of the pattern behind that low? Did they show the cold surface high thereafter at 8,9, 10 days out?

Perhaps people can prove my suspicions wrong, but I suspect they did not suggest any of those things (any more than fleeting op runs)

[Hocus Pocus]

EDIT: clearly, if the southward bias of 12z GM & E4 proves correct (despite UKMO shift slightly N in modified output), then snow threat also shifts much further S).

Lol, how long have been posting here Ian. You should know now as soon as there's a whiff of snow all logic goes out of the window. Merry Xmas to you and your family by the way.

[Cheese Rice]

Only trouble is, we are no closer to expressing confidence re outcome versus where we were 24hrs ago...UKMO go for track somewhere halfway-house between their 00z and 12z GM. Arpege looks essentially similar. But this saga will run to wire, with very fine thresholds to discriminate PPN type (thus ignore e.g. GFS snow charts, but I'd hope most people already do anyway until T+12 hrs!!)

 

You've got a much larger data set than I do so its nice to have your insight. The EURO4 has just updated and highlights the uncertainty with a definite southerly track.

 

 

 

Bit of a mess, I'm not convinced a southerly track would bring copious amounts of snow to the south. Looks to marginal. The further north the low tracks the better (depending how you look at things).

[kumquat]

Euro4 looks like snow into the Brecon Beacons if nowhere else on that frame. Would be great if posters could get to grips with the words "to" and "too". Yep, I'm a spelling nazi and that one always grates.

 

Confident we'll see some snow in the air in the West Country. Even that would be enough for me.

[karlos1983]

Merry Christmas everyone. Love this forum, learnt so much over the past few winters. The weather is a passion of mine, but outside of this forum its deemed as geeky and nobody gets it. So cheers everyone I hope you all have a great day.

Karlos

[Cheese Rice]

You've got a much larger data set than I do so its nice to have your insight. The EURO4 has just updated and highlights the uncertainty with a definite southerly track.

 

 14122618_2418 (1).gif

 

Bit of a mess, I'm not convinced a southerly track would bring copious amounts of snow to the south. Looks to marginal. The further north the low tracks the better (depending how you look at things).

 

The Net Weather NetWx- SR/MR model has just updated singing the same tune as its previous output.

 

SR

 

MR

Edited December 25, 2014 by Cheese Rice

[nick sussex]

Merry Christmas to everyone. Lets hope Santa can deliver some festive weather !

 

The output just doesn't want to read the script, the GFS looks similar to its 12hrs run, the GFS P a bit more interesting especially with those shortwaves heading towards the UK, akin really to Santa and his sleigh trying to deliver some very cold air and snow but will he make it down the chimney! Sorry for all these Santa analogies but hey its Christmas!!!

 

If you would have asked me this morning which was more likely to verify it would have been the GFS P as at that point NCEP were bigging up the Alaskan Ridge and were all for the more amplified side of solutions upstream.

 

However tonights trends seem to have departed from that script so its really back to where were a few days back with more uncertainty with possible solutions which could range from dry, cold and frosty, to wow Santa arrived on his sleigh and delivering a shortwave gift of some snow or some battleground scenario with troughing running se in towards the UK, to basically low heights flattening the high and bringing in less cold conditions.

 

Lets hope Santa has read all our letters and will deliver some nice outputs tomorrow! :smiliz34:

[Eugene]

Well i am shocked by the doom tonight, was expecting all models to show a very flat pattern yet i see some great runs, GFS 18Z ensembles only have a few flat members with the rest with varying degrees of cold blocked weather, just a few days ago there wasn't much chance of any type of cold for next week now we have plenty of interest.

[blizzard81]

Merry Christmas everyone. Would just like to echo Nick's words above. Cmon now Santa, we are snow starved here in the uk!

[Bazz1053]

Merry Christmas everyone, I don't post but very enjoy reading all your posts. When it comes down to cold/snow you just don't know what you are going to get until it happens !

Have a good one all.

[Kentspur]

Merry Christmas and a Snowy New Year (hopefully

;-))

[Penrith Snow]

The updated MetO +120 fax chart is interesting as the 528 dam line is as far south as Rome and Sardinia in Europe but well north of the Arctic Circle in Greenland, Indeed it's off the map!

Does that count as WAA in the Greenland area?

Crazy charts again and FI begins at +96 max.

Andy

[fUnKy]

merry Christmas all. I don't post in here as they would just get moved with my level of knowledge but just want to say love reading the exchanges on the models so all have a great one and look forward to learning more from you all next year.

[Sideways Snow]

First post, and until I get to grips with these models, the last post for a while! Just want to say thank-you to you all ( especially frosty! ) for such addictive reading and viewing, here's to another fantastic year on the best weather forum on the net... Happy Christmas!!!!!

[95 Degrees]

Merry Christmas weather watchers :w00t: , hope everyone has a wonderful day, here's to a besterly for the next 3 months .

[snow fest]

Hi everyone, been reading these posts for years and been a member for a while, just want to say thanks for all the time other put in to create a great thing to read. And a message to Frosty, keep the faith dude! you crack me up.

Happy Christmas and heres to a few flakes here and there over the next few days!

[geordiekev]

Oh well, another white christmas bet down the drain. Second year in a row that the winnings were in touching distance for the Newcastle station being only hours from paying out at 6/1 from mid November, but I'll still be glued to the radar maps in between today's festivities hoping for a speck of the pink stuff over Albemarle weather station, 200m above sea level and near east coast so fingers and toes crossed. At midnight showed sleet showers only a few miles away.

Close but probably no cigar, but yet again fascinating watching the winter unfurl and seeing some of our winter regulars and exceedingly good posters.

Merry Christmas to the Mods who work overtime at this time of year and may everyone have a merry and safe Christmas

Edited December 25, 2014 by geordiekev

[snowking]

Merry Xmas to all and I hope everyone asked Santa for a big Greenland warming!

Have a great day and may Aberdeen see some snowfall so that the bet I've had on since late October comes in!

SK

[fergieweather]

Happy Christmas all. As a final sign-off before indulgence tomorrow, I cite the UKMO Dep. Chief an hour ago, reference whatever you may read and/or assess into the future: "...confidence tapers off quickly toward the end of next week, and further changes in emphasis are possible in the coming days."

Enjoy the festive period!

[Allseasons-Si]

Just managed to have a few minutes break from wrapping all the crimbo presents to wish you all a happy Christmas and a prosperous new year,buy heck this year has flown :smiliz23:

 

all though it won't be a white Christmas for the most of us bar a few wintry fleets in the NW wind ,it certainly will feel seasonal which will be a bonus from the crud we had last year,this time last year our wintry/snowy hopes where fading fast,but this winter it is a lot more seasonal,please santa deliver that huge sack of presies,i meen snow this winter :smiliz19:

as for the models,there is plenty to talk about,shortwave/low dropping sse then a NE er maybe followed by a easterly with high preasure hopefully parking over scandi and sustaining an easterly flow over the uk,what more can you wish for :w00t:

 

have fun everyone,it has been another great year of model watching whether it be spring convective showers/storms/summer heat/autumn storms to looking for cold from late autumn,never a dull moment in here

 

stay safe peeps.

 

 

[meathead]

Got to laugh at some people on this forum. Just last night a lot of you were saying we are guaranteed an easterly. Today the same people are saying we are not. Kinda confusing to newbies and people who can't read charts. I don't know why people are analyzing charts that are outwith 5 days. The next 4 days are going to be cold pretty much everywhere in the UK, after that we will just need to see what happens

Edited December 25, 2014 by meathead

[Jamesjane]

Got to laugh at some people on this forum. Just last night a lot of you were saying we are guaranteed an easterly. Today the same people are saying we are not. Kinda confusing to newbies and people who can't read charts. I don't know why people are analyzing charts that are outwith 5 days. The next 4 days are going to be cold pretty much everywhere in the UK, after that we will just need to see what happens

Im also a newbie..i just try to follow certain posters..maybe i could suggest Gibby for a totally

Unbiased view..others include nick sussex, TEITS, steve murr and Tamara (i understand her even less) but its a good education..

Don't panic anyway regarding the easterly being touted..according to the BBC weather forecast, anything after boxing day is up for grabs..they still haven't got the incoming low pressure narrowed down to where it will eventually

Depart from our little island..

Apologies to anybody i haven't included in my respected peer list as i know everyone on here is more learned than me..

[Mucka]

GFS, GFS (p), UKMO all show quick collapsing ridge and sinking block with the jet running over the top at the mid term.

 

 

So not much joy there I'm afraid but at least UKMO might offer a second bite of the cherry if the upstream was more favourable.

 

GFS singing; "We wish you a merry Christmas and a zonal new year!"

 

 

GFS (p) actually goes on to produce a New Years day storm.

 

 

GEM singing form the same hymn sheet I'm afraid

 

 

Personally I think the output is laughable and FI is about 72h but we will see.

 

Anyway no point in getting too hung up about it and we still have some seasonal cold to come with snow chances boxing day and some frosts.

Edited December 25, 2014 by Mucka