Model Output Discussion 01/09/17

[Bruegelian]

So having a La Nina suppresses amplification and hence HLB, but surely there must be plenty of winters where we had a Nina, but had some HLB episodes giving cold weather. I suppose its a matter of degrees like everything else.

[northwestsnow]

Looking at that day 10 ecm mean i would suggest there are a large number showing of members showing height rises from the Atlantic towards Greenland.

Now, just to get that to day 3

Edited November 9, 2017 by northwestsnow

[Catacol]

1 hour ago, Tamara said:

Best that I interject at this point.

That is a GEFS forecast which has a low angular momentum bias, notwithstanding that the background pattern is inclined to La Nina

In my opinion its rather the opposite way around with regard your post. What has been happening is effectively "destructive interference" with the La Nina state. There is currently a relatively higher GLAMM state than seen since mid summer - but still only taking total global atmospheric angular momentum a little above -1SD.

The atmospheric decoupling from the ocean base state is the result of a significant eastward propagating MJO wave during October

Its feedback loops from the eastward propagating tropical convection in October that added westerly wind inertia into the atmosphere, boosting atmosphere angular momentum tendency. There is a text book lag time of about 3/4 weeks for the amplification mechanisms created by this amplitude convection to manifest in the atmospheric circulation. Look at the massive leap in AAM tendency starting from the beginning of Octobers high amplitude wave which culminated  mid month. From -4SD to +3SD - quite some ying-yang elastic band effect set in motion   NWP is now showing the fruits of the considerable consequential amplification injected into atmospheric circulation taking into account that lag. 

The westerlies added to the system negate the overriding net total of easterly trades in global windflows which are a product of La Nina and consequent low atmospheric angular momentum.

The rossby wave trains from tropical convection propagate polewards and help to perturb a fledgling vortex. On the basis of the size of the October bounce in AAM tendency no surprise to see an early season vortex struggle.

The question is what happens when the MJO feedbacks wane, and the downstream tropospheric amplifying mechanism is lost. This allows the typical La Nina pattern(with low atmospheric angular momentum) to dominate with the jet flowing over the anomalous Nina ridges in the Pacific and Atlantic - possibly at the same time as vortex intensification occurs. That is not a ticket to a -NAO/-AO combination in the future. Quite the opposite - we can't be thanking crashing GLAMM for setting in motion the processes for "building blocks" to a sustaining cold pattern

Time will tell of course, but the pattern indicated by the models heading through this month to the approach of start of winter could simply be a vortex pre-intensification phase characterised by cooling upper layers in the stratosphere descending and flushing out the warmer troposphere/lower stratosphere negative zonal anomalies.

There is unlikely to be any further assist from the MJO until later in December, based on periodicity of the next likely amplitude MJO wave (45 to 60 days). And with La Nina consolidating and the wavelengths changed with the new season, a next amplitude wave could well have a lot more on its plate in terms of making headway across the Pacific to help amplify the pattern downstream

The North Pacific ridge, characteristic of La Nina, will likely dictate the downstream pattern to a further Atlantic ridge - and La Nina favours increased polar jet flow heading over the anomalous circumglobal ridges. The ticket to a -NAO is a southern stream c/o rossby waves triggered by convection passing the dateline. That comes with a weak Nino signature which is not at all likely to be present this side of the New Year, at the least

 

Great post Tamara. Very clear and cohesive. I would add only one element - and that concerns the nature of the current Nina and also the PDO. The PDO has been positive a long time helping pump up that aleutian ridge. However the temp profile in the North Pacific is currently trending close to neutral. In addition we have what looks to be an east based Nina. I agree that the approaching cold is likely to be a phase that will be replaced by a stormy westerly driven December... but when the next phase of the MJO arrives there may indeed be decent convection potential in the western Pacific and this MAY coincide with less forcing from the long serving +PDO. End result? Perhaps a decent probability of a -NAO signature in January.

Intended only as a bit of balance to your post - not contradicting it... and perhaps preventing anyone from becoming too gloomy reading it!

[Man With Beard]

Just seen EC control run and it has low pressure in the Channel between T240 and T264 with an easterly for northern areas of the UK, and heights above that.

Ensembles by D10 are a real mess on the whole, though.

[Catacol]

7 minutes ago, Bruegelian said:

So having a La Nina suppresses amplification and hence HLB, but surely there must be plenty of winters where we had a Nina, but had some HLB episodes giving cold weather. I suppose its a matter of degrees like everything else.

I think the key is to understand that “Nina” on its own is a simplification of a complex process. And anyway - the Nina forecast itself is just a forecast. CPC currently rate Nina through winter as 65-75% likely. That’s a rather large chance of neutral conditions emerging and chances for increased AAM on th back of the next phase of the MJO. I suspect December will end up dominated by NW winds and overall less exciting than the upcoming early taste of winter, but there is no need to see a washout winter yet.

[chionomaniac]

1 hour ago, Tamara said:

Best that I interject at this point.

That is a GEFS forecast which has a low angular momentum bias, notwithstanding that the background pattern is inclined to La Nina

In my opinion its rather the opposite way around with regard your post. What has been happening is effectively "destructive interference" with the La Nina state. There is currently a relatively higher GLAMM state than seen since mid summer - but still only taking total global atmospheric angular momentum a little above -1SD.

The atmospheric decoupling from the ocean base state is the result of a significant eastward propagating MJO wave during October

Its feedback loops from the eastward propagating tropical convection in October that added westerly wind inertia into the atmosphere, boosting atmosphere angular momentum tendency. There is a text book lag time of about 3/4 weeks for the amplification mechanisms created by this amplitude convection to manifest in the atmospheric circulation. Look at the massive leap in AAM tendency starting from the beginning of Octobers high amplitude wave which culminated  mid month. From -4SD to +3SD - quite some ying-yang elastic band effect set in motion   NWP is now showing the fruits of the considerable consequential amplification injected into atmospheric circulation taking into account that lag. 

The westerlies added to the system negate the overriding net total of easterly trades in global windflows which are a product of La Nina and consequent low atmospheric angular momentum.

The rossby wave trains from tropical convection propagate polewards and help to perturb a fledgling vortex. On the basis of the size of the October bounce in AAM tendency no surprise to see an early season vortex struggle.

The question is what happens when the MJO feedbacks wane, and the downstream tropospheric amplifying mechanism is lost. This allows the typical La Nina pattern(with low atmospheric angular momentum) to dominate with the jet flowing over the anomalous Nina ridges in the Pacific and Atlantic - possibly at the same time as vortex intensification occurs. That is not a ticket to a -NAO/-AO combination in the future. Quite the opposite - we can't be thanking crashing GLAMM for setting in motion the processes for "building blocks" to a sustaining cold pattern

Time will tell of course, but the pattern indicated by the models heading through this month to the approach of start of winter could simply be a vortex pre-intensification phase characterised by cooling upper layers in the stratosphere descending and flushing out the warmer troposphere/lower stratosphere negative zonal anomalies.

There is unlikely to be any further assist from the MJO until later in December, based on periodicity of the next likely amplitude MJO wave (45 to 60 days). And with La Nina consolidating and the wavelengths changed with the new season, a next amplitude wave could well have a lot more on its plate in terms of making headway across the Pacific to help amplify the pattern downstream

The North Pacific ridge, characteristic of La Nina, will likely dictate the downstream pattern to a further Atlantic ridge - and La Nina favours increased polar jet flow heading over the anomalous circumglobal ridges. The ticket to a -NAO is a southern stream c/o rossby waves triggered by convection passing the dateline. That comes with a weak Nino signature which is not at all likely to be present this side of the New Year, at the least

 

I know that I am not involved with a winter forecast this year, but if I was it would be definitely be on the lines of front loaded cold, followed by stormy mild wet heart around Christmas and New Year, with a possible mixed end -depending on any wave induced SSW. Any front loaded cold could last well into December. Do you think that the atmosphere is more Nina based presently and that this supports the early vortex disturbances projected?

Edited November 9, 2017 by chionomaniac
spelling

[Frosty.]

I'm really encouraged by the latest Ecm 12z op / ens mean compared to the 00z..Hope it continues tomorrow. I think there is more potential for cold / snow during mid / late November than we have seen for a good few years..Hoping the met office continue to indicate colder more blocked patterns with snow in their future updates too during the days ahead..coldies deserve some luck this season!

[knocker]

The anomalies are in pretty good agreement this evening.

Upstream we have twin vortex lobes Siberia and NE Canada with a trough down the north east N. America and the North Pacific ridge stretching into the Arctic to a hot spot at Franz Joseph Land. Downstream some ridging into southern Greenland but the key feature remains the low/trough associated with the vortex in the Iceland area and north west of the UK. Thus we have a strong westerly jet running out of the southern United States and across the Atlantic under and over the high pressure  This would portend unsettled weather as systems track east, perhaps north east very close to the UK, temps around average perhaps a tad below,

Towards the end of this period the pattern transition gets underway and in the 10-15 period we are looking at a trough orientated down the eastern seaboard and amplification in the Atlantic which involves ridging in the western Atlantic into Greenland and the trough to the north west dropping south either over the UK or just to the east. Thus a meridional flow N/NNW over the UK. So we could still be looking at surface lows dragging some Arctic maritime air down if they park the bus just the east of the UK but as has been said before this does all depend on the intensity and the orientation of the key players in the amplification and different outcome could quite easily take place with minor adjustments to the pattern. Something of course the det runs will need to sort

Can't post the eps but it has the trough slightly to the east but with a slight negative tilt which could be against CAA.

.

 

Edited November 9, 2017 by knocker

[bluearmy]

Looking at the eps mean chart for midnight sat 19th from the last five runs to see how consistent the output has been - oldest first

 

[feb1991blizzard]

2 minutes ago, bluearmy said:

Looking at the eps mean chart for midnight sat 19th from the last five runs to see how consistent the output has been - oldest first

 

The odd one out is the midnight run last night but arguably the best with the low anoms further South rather than just East giving transient N'ly.

[feb1991blizzard]

Just as a matter of interest (shame no chionomaniac winter forecast this year), is it Nick F this year? looks like it given he did the preliminary thoughts.

[Allseasons-Si]

The jma has changed it's tune too with regards to height rises ahead of the trough and now follows the gfs and ecm

[summer blizzard]

Looking at the Euro tonight the key difference is that the trough at day 8 is weaker and hence further south. This means that the flow is able to amplify and we get the northerly. 

GFS12z highlights the danger of a west based -NAO quite well however it also shows the fightback from the east as the trough becomes quite negatively titled. 

[Allseasons-Si]

and the means from both the gfs and ecm at 240 hrs,you can see why the AO and the NAO tank into neg territory from the blocking in the Atlantic with winds north of the BI trending easterly into NE America and an almost reverse of the winds in the polar regions(AO),we just need this trough to head further SE into europe to benifit the rewards of those easterly/northeasterly trade winds,still these are the means and are a broadbrush of many put into one so we treat it as just

ens mean outlooks still look good if not better than the previous ones with the AO tanking to sub -5.4(-5 yesterday) and the NAO tanking to nearly -3 (was -2 yesterday)

500 hpa height anomoly's still look good too with low preasure at mid latitudes bolstering the heights in the polar regions

so another day,another dolar what would your money on

[Cheshire Freeze]

Pacific ridging cut off even quicker this run than the 12z.

Could be more of a slog going forward on the 18z

[johnholmes]

59 minutes ago, knocker said:

The anomalies are in pretty good agreement this evening.

Upstream we have twin vortex lobes Siberia and NE Canada with a trough down the north east N. America and the North Pacific ridge stretching into the Arctic to a hot spot at Franz Joseph Land. Downstream some ridging into southern Greenland but the key feature remains the low/trough associated with the vortex in the Iceland area and north west of the UK. Thus we have a strong westerly jet running out of the southern United States and across the Atlantic under and over the high pressure  This would portend unsettled weather as systems track east, perhaps north east very close to the UK, temps around average perhaps a tad below,

Towards the end of this period the pattern transition gets underway and in the 10-15 period we are looking at a trough orientated down the eastern seaboard and amplification in the Atlantic which involves ridging in the western Atlantic into Greenland and the trough to the north west dropping south either over the UK or just to the east. Thus a meridional flow N/NNW over the UK. So we could still be looking at surface lows dragging some Arctic maritime air down if they park the bus just the east of the UK but as has been said before this does all depend on the intensity and the orientation of the key players in the amplification and different outcome could quite easily take place with minor adjustments to the pattern. Something of course the det runs will need to sort

Can't post the eps but it has the trough slightly to the east but with a slight negative tilt which could be against CAA.

.

 

Thus a meridional flow N/NNW over the UK.

Sorry k I do not agree, it is not a true meridional flow in my view but I stand to be corrected. There is not sufficient amplification showing across the chart, not even the small area you refer to.

Just my view on the chart you are using.

Not in the above post but reading other comments about the NAO.

Remember they are not predictors in the true sense, but reactors if you like to what the surface charts are suggesting for surface pressure differences between two areas, usually the Iceland area and the Azores area. years of watching this prediction shows that up to 5 days ahead, (much as the surface prediction) it is generally quite accurate. Beyond that and accuracy often tails off quite dramatically.

[Man With Beard]

EC monthly still pushing ridge north-west/trough south-east for 20-26 Nov

[northwestsnow]

Just now, Man With Beard said:

EC monthly still pushing ridge north-west/trough south-east for 20-26 Nov

Lovely chart for coldies

[knocker]

4 minutes ago, johnholmes said:

Thus a meridional flow N/NNW over the UK.

Sorry k I do not agree, it is not a true meridional flow in my view but I stand to be corrected. There is not sufficient amplification showing across the chart, not even the small area you refer to.

Just my view on the chart you are using.

 

Not a problem John I quite agree. It's funny I had a bit of a niggle when I typed it . It certainly isn't a true meridional flow, although the flow over the UK is correct.  Indeed the flow across the Atlantic south of the Greenland ridge is almost flat. Must try harder...............

[feb1991blizzard]

25 minutes ago, CreweCold said:

Pacific ridging cut off even quicker this run than the 12z.

Could be more of a slog going forward on the 18z

This is going to be worse than the 6z - a right dogs dinner, it will be a quasi- stationary trough centred over the uk with possibly runners dumbelling around the southern half of it basically with the synoptical setup not being progressed in any direction - just eating up ill-afforded time.

Edited November 9, 2017 by feb1991blizzard

[bluearmy]

I have been intrigued by the modelling of the greeny ridge over recent days. On occasions it's been marked as per tonight's eps and others it's looked much flatter. Indeed the NOAA cpc chart looks flatter but it covers  a week period. there is the chance that the greeny ridge could be cut off with the jet running underneath - that would promote a flatter look to the output. 

[Methuselah]

3 minutes ago, feb1991blizzard said:

This is going to be worse than the 6z - a right dogs dinner, it will be a quasi- stationary trough centred over the uk with possibly runners dumbelling around the southern half of it basically with the synoptical setup not being progressed in any direction - just eating up ill-afforded time.

Oh no! Time to put the word out: toycatchers wanted!

[Cheshire Freeze]

5 minutes ago, feb1991blizzard said:

This is going to be worse than the 6z - a right dogs dinner, it will be a quasi- stationary trough centred of the uk with possibly runners dumbelling around the southern half of it basically with the synoptical setup not being progressed in any direction - just eating up ill-afforded time.

The thing that is nagging me is that as T 0 approaches, the vortex is looking slightly more resilient than the modelling had previously suggested- yeah it keeps looking ragged in FI but... I've been watching the GFS today and even around 72 hrs I have noted that the vertical advection over on the Pacific side has been muted somewhat on successive runs (and deflected away from being polewards). These are relatively close range changes and is very reminiscent of last year before it all imploded.

Edited November 9, 2017 by CreweCold

[swfc]

Possibly the longest easterly ive ever seen.moscow area to esb.uk to the south of it lol.18z at its finest

[damianslaw]

GFS has a habit of forecasting pattern changes well beyond the 10 day timeframe at say 14 days out, then often wobbles as the pattern change timeframe enters the 10 day timeframe, only to revert back to what it showed a week previously once on the 7-8 day timeframe - so expect further wobbles next couple of days, but then it will latch onto a theme and hold it.. so I'm not surprised to see such volatility and change today.

ECM though has a habit of doing about u-turns 6/7 days ahead of expected pattern changes, with this in mind wait until Sunday before we see some harmony between both models..