An Autumn Of Two Halves

[Glacier Point]

Autumn has been characterised by an unusually long dry spell to start, followed by an abrupt change mid to late October. The forseable future looks unsettled and November will more than likely feature an upper low geopotential height anomaly.

The key question to ask whenever we get these anomaly patterns is why ? This is important as large and obvious anomalies help us understand the global weather pattern and how this might shape the medium to long range forecast.

If we look at the autumn divided into periods 1st September - 15th October and 16th October - 10th November in terms of the 500 hPa geopotential height anomalies:

We see the anomalous ridge over the UK during the first half and upper low centred over the UK during the second (bear in mind that this anomaly will increase in intensity c/o the unsettled weather this week).

So what's caused this turnaround ? The answer lies in the Indian and Pacific Oceans.

Reanalysis of outgoing long wave radiation anomalies identifies a negative (yellow and orange) anomaly centred in the western Pacific over the Equator during the first part of the autumn. This corresponds to the Madden Julien Oscillation (MJO) in phase 6. The Global Wind Oscillation (GWO) measures the state of extratropical as well as tropical winds and as such is the better measure of the global wind signal. Ougoing longwave anomalies give us an idea of where strong tropical forcing - thunderstorm and cloudiness -was located. Strong convective forcing results in powerful downstream anticyclones which generate Rossby Waves which drive the ebbing and flowing of the upper or longwave flow pattern from the Pacific eastwards.

Although its slightly difficult to make out, the GWO was attracted towards phases 3-4-5 during the first part of the autumn. The MJO plots show this slightly better with the tropical wave spending most time in the eastern part of the spider graph. The reasons why the wind signals were slightly behind the centre of tropical forcing was because of a residual La Nina type signal within the extratropical atmosphere.

The GWO composite for phase 4 clearly in September identifies an anomalous ridge pattern for NW Europe.

Since mid October, tropical forcing has shifted back west towards the Indian Ocean, tied into a convective signal which moves around the tropics.

The GWO and MJO wind signals have shifted left towards phases 7-8-1-2-3 due to a negative tendency returning to the atmosphere over the tropics. The composites for GWO phase 1 advertise the unsettled pattern well although phases 0, 2 and 3 are also unsettled.

The point about this is to demonstrate how important the location of tropical forcing and the global wind signal are in influencing the likely weather pattern from 7 days onwards.

Looking ahead, the extent of negative (easterly trade) winds across the Pacific is clearly apparent.

This is likely to hamper the eastward progression of the current wave, tending to weaken and slow the MJO. The large addition of easterly winds to the wind signal is likely to hold back the GWO around phase 4 although the MJO will want to drive it towards phases 5-6-7. The large increase in tropical convection of late may help to bring this about although we are now talking into December for the results of this in terms of the GWO and then the resultant weather pattern.

Reanalysis of GWO phases 4 and 5 for December suggest a continuation of low pressure in the Atlantic. This suggests that our unsettled spell has some way to go, probably extending well into December following a brief lull towards late November where the GWO composites hint at a more amplified pattern developing with height rises once more over Scandinavia. From my point of view, I have this unsettled pattern as the most likely start to December, much like the current CFS forecast for December. This then starts to bridge the gap between monthly forecast through to seasonal.

In terms of recent Decembers, this is going to contrast quite starkly, with December 2000 being the nearest likely period.

[mcweather]

Hi GP Thanks for this, funnily enough I was going to email you and ask what your thoughts were on the things to look out for in November from the prelim winter forecast and how they're progressing.

[Pennine Ten Foot Drifts]

Yep, thanks GP - fascinating and mind boggling in equal measure (as always !)

[LomondSnowstorm]

Very informative post. As long as we get a decent northerly toppler I'll be fairly happy with whatever else the next few weeks bring, as long as things change towards the end of December/early January.

[Polar Side]

Thanks GP for a very well explained post. I seem to remember it turning very cold just before Christmas 2000 and lasting until New Year with a notable Polar Low event !

Are your thoughts for December similar to your winter thoughts in your pre-winter forecast ?

[Typhoon John]

Thanks GP nice post, and still hinting at phases 7 and 8 for January. Certainly some interesting weather due over the next few weeks.

[Summer of 95]

Anyone remember Autumn 2002? That was just like this year only the two halves were even more distinct.

Bone-dry September, albeit it was cloudy where I was at the time (NE England). October started with a surprisingly warm spell before turning unsettled midmonth, there was a storm towards the end in the south which the north largely missed. So far an exciting autumn, until November which was one of the most depressing and miserable months I have ever endured (along with October 2004 and August 2008 in Shrewsbury). Consistently mild, no frost or snow, rain at some point nearly every day but above all it was incredibly dull. This month has been depressing but there has at least been more sunshine since Monday than I saw during the entire second half of November 2002.

The winter that followed was generally mild, quite snowy in the east, almost snowless in the west- I'm crying into my beer at the thought of that happening yet again.

[Glacier Point]

Thanks GP for a very well explained post. I seem to remember it turning very cold just before Christmas 2000 and lasting until New Year with a notable Polar Low event !

Are your thoughts for December similar to your winter thoughts in your pre-winter forecast ?

My thoughts at the moment for December would not be far from where the CFS is at - low pressure in the Atlantic, southward displaced jet, unsettled, not overly cold or mild, and yes slight positive height anomaly over Greenland. The historical anlogues for El Nino years within low angular momentum feature heavily.

[Thundery wintry showers]

Glacier Point's post, to me, hints at a very warm, wet early part of December, akin to early December 2000 and also early December 1979. These setups with frequent southerly winds often also feature excessive cloud cover- indeed, like happened in November 2002 (also November 1997 had only half the normal sunshine in Tyneside). It is also noteworthy that 1997/98 and 2002/03 were El Nino winters, so this kind of pattern at this time of year has been quite common in recent El Nino years.

Temperatures and sunshine levels have been near normal so far this month, and this is largely because the Atlantic influences have been intermittent rather than giving an unrelenting train of depressions. The Atlantic looks set to power up and I think GP's analysis is likely to be very close to the mark for late November and early December.

The late December 2000 cold snap resulted from the jet slowing and buckling south with high pressure over Greenland and low pressure over Scandinavia combining to unleash an Arctic outbreak. December 2000, however, was part of a decaying La Nina winter.

Edit: I see GP's post above actually suggesting close to normal temperatures- presumably this implies the Atlantic influence tending to stall against relatively high pressure to the N and E? This is also plausible, though I have my doubts about the resulting temperatures being anything other than mild.

Edited November 12, 2009 by Thundery wintry showers

[Glacier Point]

The extra factor here Ian is that I see some tropical wave projection through phases 5-6-7-8, in addition to the potential lagged impact of a warming stratosphere and the upwelling wave in the atmosphere being conducive to the jet being displaced further south and less mild airstream. If this verifies, the cold air now building over the Arctic will to some extent get displaced into the mid and higher latitudes.

[Snowyowl9]

At this point in time 2" it`s the wettest november since 2005 which was almost twice as wet at this stage,but we know what happened next.

As for november 2002 that was a totally damp squib with not even a single air frost.

November 2000 plenty of gales very wet, no frost,just one day when there was sleet.

I`m only looking a month ahead.

[Gavin P]

Very interesting GP. I think autumn 1959 panned out very similar to this autumn, although of course in that year the warm, dry weather actually started around April and lasted through to mid October/

So going forwards I take it from the implied pattern that your thinking a few pounds on a White Xmas may be worth a go this year?

Edited November 12, 2009 by Gavin P

[BLAST FROM THE PAST]

So going forwards I take it from the implied pattern that your thinking a few pounds on a White Xmas may be worth a go this year?

Hi Gav

Already on for 6/1 [fora few weeks].

BFTP

[chionomaniac]

The extra factor here Ian is that I see some tropical wave projection through phases 5-6-7-8, in addition to the potential lagged impact of a warming stratosphere and the upwelling wave in the atmosphere being conducive to the jet being displaced further south and less mild airstream. If this verifies, the cold air now building over the Arctic will to some extent get displaced into the mid and higher latitudes.

It appears GP then, as winter progresses all we will need is a trigger to displace that cold air southwards and in our direction. A SSW could be one such trigger but we cold still get cold without that if the MJO/GWO hit the right phases at the right time. Do you feel that we need the El Nino to engage the atmosphere more than it is doing presently?

[Fozfoster]

Hi Gav

Already on for 6/1 [fora few weeks].

BFTP

Just had a fiver @ 7/1 for snow in brum

[Glacier Point]

Latest infra-red satellite and low level wind analysis identifies the centre for tropical convection (deep cloud formations) to be edging slowly eastwards towards Indonesia.

This is consistent with MJO progression eastwards but weakening as it does so. Notice the continual stream of easterly winds from the east Pacific right across the tropics weakening the MJO wave tied into an anomalous trade wind surge.

The global wind signals (MJO and GWO) are in low angular momentum (Nina like) base state as a result of these east winds and strong mountain torques across the sub-tropics.

http://cawcr.gov.au/...Last40days.html

http://www.esrl.noaa...gcm/gwo_40d.gif

Notice how the GWO is lagged behind the MJO signal once more. MJO in phase 3, GWO in phase 2.

With continued eastward progression of the MJO, I can see the atmosphere stalling around a GWO phase 4 orbit once more (as it did in September and October) as the tendency for easterly winds orginating 30N - 40N in the eastern Pacific to continue (related to the PDO?). Both phases 4 and 5 of the GWO are likely to draw a very disturbed weather pattern for NW Europe into early December and confidence is growing about this type of pattern starting the winter of 2000/10. This makes the idea of a Euro High an outlier and low probability in my book, more like low pressure over Europe. The most severe weather impacts are however likely to be shifted southwards towards France as the jet starts to become southward displaced as heights rise over Alaska and Canada forcing the polar vortex towards the eastern side of the hemisphere - remember we are seeing very warm stratospheric conditions over this area right now.

As far as Christmas goes, the eastward progression of the MJO may help to force a very weak orbit in the GWO towards phases 6-7-8. The timing of this looks to be mid December. Perhaps the cold shot just falling short of Christmas. Best bet would be to buy now and sell the week before Christmas for snow on the expectation that the markets will be going bonkers for a white one at this stage.

Edited November 15, 2009 by Glacier Point

[chionomaniac]

It's a brave call trying to predict the phases and timings of the MJO, GP!

The winter of 1989/90 is practically the only one I can find (since 1974/75) with regular orbits, with very little stalling or time in the circle of death!

http://cawcr.gov.au/bmrc/clfor/cfstaff/matw/maproom/RMM/phasediag/pd.1989.12.1.gif

I do like the betting analogy though.

Edited November 15, 2009 by chionomaniac

[Gavin P]

Latest infra-red satellite and low level wind analysis identifies the centre for tropical convection (deep cloud formations) to be edging slowly eastwards towards Indonesia.

This is consistent with MJO progression eastwards but weakening as it does so. Notice the continual stream of easterly winds from the east Pacific right across the tropics weakening the MJO wave tied into an anomalous trade wind surge.

The global wind signals (MJO and GWO) are in low angular momentum (Nina like) base state as a result of these east winds and strong mountain torques across the sub-tropics.

http://cawcr.gov.au/...Last40days.html

http://www.esrl.noaa...gcm/gwo_40d.gif

Notice how the GWO is lagged behind the MJO signal once more. MJO in phase 3, GWO in phase 2.

With continued eastward progression of the MJO, I can see the atmosphere stalling around a GWO phase 4 orbit once more (as it did in September and October) as the tendency for easterly winds orginating 30N - 40N in the eastern Pacific to continue (related to the PDO?). Both phases 4 and 5 of the GWO are likely to draw a very disturbed weather pattern for NW Europe into early December and confidence is growing about this type of pattern starting the winter of 2000/10. This makes the idea of a Euro High an outlier and low probability in my book, more like low pressure over Europe. The most severe weather impacts are however likely to be shifted southwards towards France as the jet starts to become southward displaced as heights rise over Alaska and Canada forcing the polar vortex towards the eastern side of the hemisphere - remember we are seeing very warm stratospheric conditions over this area right now.

As far as Christmas goes, the eastward progression of the MJO may help to force a very weak orbit in the GWO towards phases 6-7-8. The timing of this looks to be mid December. Perhaps the cold shot just falling short of Christmas. Best bet would be to buy now and sell the week before Christmas for snow on the expectation that the markets will be going bonkers for a white one at this stage.

Very interstting, thanks GP. So we're looking at an increasingly at a stormy and increasingly cold pattern to start winter?

On his latest video blog I see Joe B'astardi is comparing this coming winter to 77/78. Is that a similarity your picking up?