[Model Output Discussion - cold spell to end Autumn]

Wow at the 12z GFS, blizzard/snow storm of the century for northern and western parts of UK next Friday into Saturday!

Caveat that GFS tends to overblow depressions at that range, but the fact that it's been showing for a few runs or more now, worth keeping an eye on.

The slow-movement of the deep low over the N Sea very unusual and appears to be tied in with the stationary long wave trough over Europe and its removal from the strong NLy jet further west meaning it doesn't have the impetus to move quickly on like normal depressions that move west to east or southwest to northeast.

Alarming charts!

 

[SE and East Anglia generalweather discussion 08/11/2017 onwards]

In Paddock Wood, Kent outside doing a tree survey and got this lot of snow heading for me, already a few flakes before the main lot

[SE and East Anglia generalweather discussion 08/11/2017 onwards]

First snow flakes of the season from a shower driving through west Kent earlier this morning, hit a patch of black ice on a wet bend of a country lane too, managed to recover fortunately.

[Model Output Discussion - cold spell to end Autumn]

Something for coldies to make sleep easier tonight, the 12z EC det. was a warm outlier toward the end for London

 

[Model Output Discussion - cold spell to end Autumn]

Really think some are rushing too quickly in presuming that a failure in the northerly next week is a dead cert based on 12z EC and 18z GFS. Yes there’s a good possibility it may transpire this way, but I personally would put low confidence in model guidance after T+144. Here’s why:

- I have noticed and posted recently about the models perhaps not handling with too much confidence the meandering southern and northern branches that extend east over the Atlantic from a split in the jet off eastern N America. Take early next Wednesday the jet profiles are different in orientation and strength over the N Atlantic sector 12z v 18z, this has a knock-on effect of trough and ridge / surface high and low placement and movement. 18z essentially flatter with the northern arm

12z18z

- 12z EC standard deviation charts for ENS and the deterministic run highlight a relatively higher degree of uncertainty to the NW and N of the UK in particular for MSLP and 500mb for mid-next week too

So a few reasons at least why to sit on the fence rather than resign to the idea that the 12z EC and 18z GFS ops are correct and say ‘at least its only early December’

The 12z EPS mean does flatten out the ridge in the Atlantic too in the medium to extended range, but this hides the uncertainty beyond t+144 I think tonight. 

 

[Prospects For Cold & Wintry Weather In Coming Days]

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[Model Output Discussion - cold spell to end Autumn]

The meandering southern and northern branches of the split jet stream over N Atlantic sector causing plenty of uncertainty and low confidence with model guidance in the medium range ... because they can’t seem to resolve where to place the ridge between the two branches and where the southern and northern arms meet this side of the Atlantic. 

GFS blows up lows on the northern arm on the 00z then drops them on the 06z in the same timeframe, and along the southern arm it can’t decide how far east to move the lows toward Europe, combine that with cold arctic air being pushed south from the pole by the -AO height pattern developing. It turns into rather a roller coaster of evolutions on each new run!

[Model Output Discussion - cold spell to end Autumn]

As I mentioned last night, the split flow over the N Atlantic isn’t doing any favours wrt to maintaining the N Atlantic block far enough north, increased energy going into the southern extension of the jet over the Atlantic comes with increased troughing between New Foundland and the Azores - which pushes the Mid-Atlantic ridge toward western Europe.

The reason for the split flow is the block currently over the Atlantic and a zonal flow upstream over N America which stretches back all the way to E Asia.

The flow over N America does amplify in the medium range, as the strong jet over E Asia relaxes to allow the flow to buckle downstream with NA west coast ridge and deepening east coast trough ... the hope for coldies is that this amplification translates downstream to build the N Atlantic block towards Greenland again, allowing another Nly reload. But there are no guarantees on this evolution, particularly if the trop PV over N Canada migrates low heights  further east over Greenland -which would mean the jet flattening out over the Atlantic rather than amplify. 

 

[Model Output Discussion - cold spell to end Autumn]

17 minutes ago, s4lancia said:

18z says "nah, I haven't got a clue actually".

I don't think the models have so much picked up a new signal, more just highlighting how slight nuances in the flow, earlyish on, can send us down one road or another in this type of setup (unfortunately that usually doesn't work the same when we have a strong zonal Atlantic though)!

So basically, nothing to panic about yet. Could and probably will all change again tomorrow (for better or worse )

I think the trend away from protracted amplification of the upper flow continuing into the medium range stems from the split flow that develops off eastern N America. The southern arm extending east out into the Atlantic tries to recurve NE mid Atlantic which forces the Atlantic block toward Western Europe, while also giving a west-based -NAO. Whilst the northern arm flattens out over the top preventing the cold arctic getting far enough south.

The saving grace later in the 18z GFS run and, indeed, the 12z EC at day 10, is that the flow buckles big time over N America as a long wave ridge develops over the west coast, which teleconnects downstream to amplification over the N Atlantic to allow a Nly reload, but not as soon as early next week as them models were advertising before. A lot could change before then, so more sitting on the fence until we see another Nly reload.

[Model Output Discussion 01/09/17]

A look at the 12z EC operational run against the ensembles for London, suggests no cold outlier at any stage, it warms up a little late next weekend and early Mon 4th, if anything a little too warm compared to the average, before a big drop in temperature with the Nly reload by Tuesday 5th. The average staying 5C or below right out to the 11th Dec for London. Certainly no worries there ...

[Model Output Discussion 01/09/17]

Russian Goliath meets Atlantic Goliath?

[Model Output Discussion 01/09/17]

12 minutes ago, Dkeane3 said:

This amazing northerly shown by the gfs, what does it show for snow prospects countrywide?

At 9 days out, I would no more trust a chart showing snow than an email in my spam box wanting to pay me money if I do a small favour transferring money into my bank account then back out again but, some organised snow coming down the country on Tues 5th?

I'm sure the models will try and ruin our dreams at some point subsequent runs by throwing in some 'pesky shortwaves' from the NW to slow the progress south of the deep cold ... surely?

[Model Output Discussion 01/09/17]

That's both ECM and GFS ops suggesting a Nly reload around Mon 4th Dec. Hopefully it will tap into some deeper cold, as indicated, than we see this week, thanks to less WAA into Greenland and the arctic area around it prior ...

[Model Output Discussion 01/09/17]

The jet riding over the top of the mid-N Atlantic block is like a wriggling snake on the 18z. It flattens out next weekend to allow less cold air originating from the NW Atlantic to move toward the UK before the jet buckles north again to allow a northerly to reload to end the weekend and start the following week! No matter how the upper flow tries to supress the block it just bounces back north even stronger! The block is there in that region for the long-run I think ...

[Model Output Discussion 01/09/17]

45 minutes ago, lorenzo said:

All in - tremendous model viewing, and something we have not had to enjoy for a few years. Lets make the commentary on said viewing as cool as it can be..

certainly not run-of-the-mill standard fare for late November. My interest is not so much in this coming week's weather, which looks cold but mostly dry away from coastal areas. I'm more fascinated in watching the bigger picture unfold for the end of November and ahead for the first few weeks of December.

Having seen the extended EPS for the last several runs, blocking looks to remain in place near Greenland for a protracted period, of 10-15 days at least lasting well into early December. That is a big plus with regards to keeping the flow across the UK from a cold direction.

Upstream, troughing stretching from Siberia to Alaska above ridging over SE Asia will continue to drive a strong east Pacific jet which extends its strong zonal flow energy over N America, keeping cold air bottled up over N Canada and driving a strong jet over the top of the Greenland block. This energy tending to perhaps flatten the N Atlantic/ S greenland ridge at times to allow some troughing to extend SE towards the UK, bringing surface lows with it. This pattern upstream looks to continue for the next 10 days at least.

However, in early December, a pattern change looks to take place upstream over the N Pacific, perhaps in the second week, where the strong east Asian/East Pacific looks to weaken, which will allow a long-wave ridge to form along or off the west coast of N America. We then end of with a three-prong ridge attack towards the pole in the trop = NE Pac/Greenland/W Russia ...

... which will surely put pressure through wave breaking on the polar vortex, certainly potential for cross polar ridging which maybe why the AO forecasts indices are nose-diving in early December.

All this while the MJO is forecast to remain weak, with little influence on the northern hemispheric patterns for the foreseeable.

[Model Output Discussion 01/09/17]

6 minutes ago, snowbunting said:

Its the heights over Greenland that are truelly astonishing. Promising lots of Nly outbreaks. One of these is going to create fun and games. 

That’s a sea level pressure chart, rather than heights, often the pressure seems extremely high over Greenland, but this is this the model trying to calculate the pressure over the high plateau covered in ice that dominates Greenland rather than representative of sea level pressure. 

Chart below shows similarly high pressure at the surface over Greenland, while above are low heights (blue) indicative of upper troughing.

[Model Output Discussion 01/09/17]

Wouldn’t be too despondent over the 12z and 18z GFS op desire to introduce less cold air from the NW and W. The 12z EPS Clusters don’t inspire all out confidence for the idea of the high res runs this evening to lose the ridge influence to the north and bring in troughing from the NW and back the flow to a NWly later next week and weekend. 5 clusters, which indicate we could go down this path or we could see Atlantic ridging build strongly enough NE to block troughing extending down from the NW. No clear signal yet, albeit we are entering a blocky pattern and the models are always keen, often too keen, to break blocks down.

 

[Model Output Discussion 01/09/17]

26 minutes ago, northwestsnow said:

 

PS look how cold  Southern Norway is!! Im sure i heard some folk say there is no cold air to tap into..

That part of Norway is high elevation / mountains just inland, hence low temperatures shown which aren’t reflective of sea-level.

T850s don’t always truly reflect the surface temperatures by rule of thumb of course, though the air on arrival in UK after crossing over the North Sea at the time of your chart will be modified somewhat compared to when it left Scandinavia. 

[Model Output Discussion 01/09/17]

Only just caught up on today's model output, as been busy, but looking at the 12z/18Z GFS they are certainly an improvement on the 00z and 06z GFS. To my eye this because they are evolving a more stout block over Iceland and into Greenland with help of the arctic high.  

The 00z/06z GFS highlighted my reservations on relying on a mid-Atlantic ridge creating a block while most of the jet energy is going round the top of the ridge, as this inevitably leads to the ridge sinking with little energy/troughing undercutting the block.

But both 12z/18z GFS along with 12z ECM high res show the merging of the arctic +ve heights/ridge and the Atlantic ridge building a block over Greenland that prevents the toppling from the NW that GFS was previously indicating. Though the 18z does eventually implode from the west in FI.

Some mouthwatering synoptics continually on offer over recent days, especially ECM, but I think we should temper expectations a little, as on face value, the charts are suggesting cold and below average next week but nothing exceptional and certainly no snowmaggedon for many. The cold generally doesn't look too deep on current projections so there'll be snow around for sure, but mostly settling over the hills during the day, maybe lower levels at night, for a time. Where skies clear it will be frosty at night and cold sunshine by day, add in a wind chill from the cold wind, it will certainly feel more seasonal than the 16C I seeing on my thermometer at 11pm last night!

We could be entrenched in this cold and blocky pattern for a while given the probable stubborness of the block to our W and NW and that the models, especially GFS, are often too keen to break it down. Which takes into early December. Longer term, some of the MJO forecasts hint at it orbiting, albeit low amplitude for now, towards colder phases 7/8 in December, which combined with possible still weak strat PV via weakening zonal wind forecasted, increased wave 1 activity and warming in the strat could help more blocking in December. 

So interesting times ahead!

 

[Model Output Discussion 01/09/17]

Strong signal from the 00z EPS 500mb mean/anomaly for ridging NE toward Svalbard and Norway by day 10, the extended EPS to day 15 maintaining ridging signal in this area, while deeply -ve anomaly is maintained over mainland Europe with hints of slight -ve anomaly over N Atlantic undercutting +ve anomaly over Greenland and Svalbard.

Last night's CPC update to its 6-10 day and 8-14 day 500mb heights/anomalies back up this idea too.

So it will be interesting to see if GFS has a change of heart and goes with stronger ridging in the Atlantic rather than the toppling from the NW.

[Heavy Rain, Gales and Snow Risk For Rest Of The Week]

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[Model Output Discussion 01/09/17]

12 minutes ago, tight isobar said:

So from around 192hrs its atlantic waa -v- canandian migrating lobe and its army of shortwaves.

Can we get an' arm of decent advetion into greenland and further into the pole in time to disrupt the canadian eastward progression..and hold the ridge??

50/50 as thing are.

Its really is on the edge....

After the weekends cold shot!

 

This the downside of not having a proper Greenland block in place next week, but relying on stout Atlantic ridging north instead. The GFS is trying to push the trop Polar Vortex over Canada further east toward Greenland while strengthening in the medium range - which puts pressure on the N Atlantic ridge to the west to flatten out. 

Certainly the Atlantic ridge slackens in the extended EPS, but still with a Euro trough anomaly, so could be we revert back to NWly flow eventually after another arctic reload next week. The caveat is that the models may not be handling the blocking over the Atlantic and PV over Canada too well, so we may end up seeing more protracted blocking. Indeed 06z GFS FI teases us with further amplification upstream, with ridge building back over the top of troughing sliding SE into Europe and the UK gets an Ely.

[Model Output Discussion 01/09/17]

49 minutes ago, nick sussex said:

Not liking this mornings outputs which seem to be shifting things west .

The shortwave needs to run se and end up to the se of the UK not the sw as that pushes a ridge north ahead of it into the UK.

The UKMO is a shocker this morning both it and the GFS develop an even more west based set up. The latter eventually pushes the ridge to the se away but now we’re dealing with events well  outside of the reliable timeframe which means even more time for more variables to pop up.

ECM  holds onto it’s Amber warning, the GFS and UKMO are now moved to Red!

Yes, that upper shortwave/secondary low ejecting east over the N Atlantic from the upper trough over northern Canada late weekend and early next week could throw a spanner in the works with regards to how quickly it clears SE - which could scupper a second arctic reload next week after the brief cold NWly blast this weekend.

The EPS clusters quite widely spread over how quickly this shortwave trough clears through, some hold the trough back far enough west and long enough to prevent any deep cold from getting far south, though generally the guidance suggests it will get SE eventually to allow a reload form the north. Thankfully no dramas with regards to this feature from 06z GFS, which doesn't hold it back west.

If we can be confident the shortwave trough clears SE quickly as per GFS and ECM, more runs needed, we then need the block out west to hold to allow deep cold over northern Norway and Svalbard to get drawn this way next week. As although the synoptic evolution is working with help of blocking to the west, we are not really seeing any deep cold yet in the reliable timeframes. Though that's not to say we can't see some snow maybe overnight at lower levels, certainly high ground in the north and west is looking favourable from Friday.

[Model Output Discussion 01/09/17]

1 hour ago, karlos1983 said:

My computer was the issue. Works now.

 

Clear there that the ensemble mean supports the high res run on the 00z run, be interesting if the trend continues, because although we expect the operational runs to chop and change, even the ensemble guidance has made some seismic shifts over the past week in the extended range. We've gone from a Euro high signal in the medium range to a Euro trough in the space of a few days' runs.

Seems that the models may be less reliable when the polar vortex is not driving the show, but high latitude blocking and trop waves are. Though there is the chance that the PV will organise itself eventually as we head into early met winter.

Interesting that yesterday's zonal wind forecasts on the Freie Universitat Berlin site are showing a downward slide in 10-30hPa zonal winds after they were showing an uptick - which suggests the strat PV may not be in such a hurry to re-organise and strengthen for now?

The MJO doesn't really seem to be a key player in the troposphere patterns evolving, as it has been in COD since the beginning of the month and looks to remain in COD for a bit or perhaps emerging in low amplitude in various phases according to which model you look at. Even the strat looks increasingly amplified on Berlin at day 10:

Certainly plenty for coldies to be optimistic about looking at the signals coming from various medium range pointers ... for now.

[Model Output Discussion 01/09/17]

As John mentioned earlier, the models firming up on a meridional pattern developing downstream over the Atlantic and northern Europe by the end of the week, as a deep trough digs down over eastern N America, a ridge pushes north to Greenland while a deep trough digs south over Europe.

Longer term, the 00z EPS extended to day 15 and the EPS clusters suggest -ve height anomaly / troughing over mainland Europe while +ve heights remain to the W and NW - which always a good sign. However, we've seen a lot of chopping and changing over recent days from the ensemble guidance, so I would personally want to see more runs to firm up on this signal for potential for pressure rises to the N and NE above the Euro trough leading to a flow E of N eventually.

Although the models are agreeing on the broadscale 500mb pattern, there are some difference from the morning's operational / high res runs with regards to the surface. Both 00z UKMO and GFS high res runs had a low in the west and a low to the NE Thursday night / early Friday, GFS has low to the W of Scotland while UKMO a low to SW of Ireland. Both delay the onset of cold artic flow expected to sweep south this weekend, by this secondary low that develops along the cold front, while the ECMWF high res hasn't got this low to the west and brings in colder air quicker to end the week.

 

However 06z GFS seems to remove this deep low near the west the 00z had early Friday!

00z and 06z GFS for 00z Friday -  spot the difference easily west of Scotland!

00z 06z

This differences so early on at the surface, with regards to secondary low development, seem to be why the BBC may be a little uncertain with regards to the potential for cold air sweeping south to end the week and through the weekend and its extent and coldness.

UKMO has this secondary low (note I don't like using the word 'shortwave') running across southern areas Saturday - delaying the cold - GFS and ECMWF don't have it though.

GFS and ECMWF bring another secondary low SE from Greenland / Iceland area early next week, differences in track this far out, which could reload a N or NWly flow from the arctic by Tuesday. Thereafter potential for pressure rises to the north which may allow a more NEly flow to develop - big uncertainties as early as this weekend with regards to arctic blast, so although interesting and nice to see the potential next week, it maybe worth not looking too far ahead for now!