November thoughts

[Glacier Point]

Crucial month in terms of establishing patterns for the Winter but a good one to have a punt on a pattern that just doesn't want to give up.

Key players:

1) Global Average Angular Momentum (GLAAM) continues to bump along at very negative values, aided by La Nina. This is likely to make patterns slow moving with less zonal wind flow predisposing a more convulted pattern and increased vertical planetory wave dispersal resulting in blocking over the Polar Field.

http://www.cdc.noaa.gov/map/images/aam/glaam.gif

2) The Arctic Oscillation has jumped around from negative to positive of late as the polar field has cooled rapidly resulting in lower pressure at the surface. On the flip side, the impact of the easterly Quasi Bienniel Oscillation (QBO) and low GLAAM have resulted in a generally warm stratosphere and upper troposphere with height rises now manifested all the way up the atmosphere.

http://www.cpc.ncep.noaa.gov/products/stra...OND_NH_2007.gif

http://www.cpc.noaa.gov/products/precip/CW...hgt.ao.cdas.gif

Some cooler anomalies have just started to show up. These are not reflective of any widespread cooling just yet and will likely fade due to forcing of planetory wave dispersal and the easterly QBO. Any cooling event now will impact in about 3 weeks so this is not of immediate focus either.

I think we will continue to see volatility in the AO, perhaps with more predisposition to -ve values with possibly two major troughs in the AO delivering a neutral-negative value overall, say -0.3 to -0.5. Current AO forecast suggests the first possible negative plunge:

The main key to the AO is going to be the mean jet pattern forced by Pacific SSTA.

3) La Nina conditions continue to persist in the equatorial Pacific.

http://www.bom.gov.au/climate/enso/indices.shtml

Currently at moderate strength and likely to perist at such strength given sub-surface cold anomalies and resurgence of easterly surface winds likely to cause more upwelling. Most model guidance is for a continuation of such conditions:

http://iri.columbia.edu/climate/ENSO/curre.../SST_table.html

However, for those thinking of a strong Nina through the Winter, consider that we have not gone through a strong Nina in the christmas pudding and it could just be vulnerable to sudden bursts of westerly winds at the surface. Already we have seen an unexpected development of warmer surface and subsurface anomalies of late.

La Nina is likely to force a high pressure anomaly in the eastern / central Atlantic:

4) Pacific Sea Surface Temperature Anomalies lagged for 3 months are likely to drive the following pressure anomaly pattern:

... more high pressure in the Atlantic..

Note also the pattern over North America, trough in the NW, blocking over NE Canada which represents something of a difference to what is being programmed for the next 7-10 days. These suggest a big positive Pacific North America (PNA) pattern. These +PNA events are a feature of La Nina winters but tend not to be long lived and I suspect the underlying neutral / -PNA pattern will prevail, particularly so as the Pacific Jet will stiffen as the thermal gradient tightens, blasting away the ridge in the west and freeing up the Arctic to a more -AO pattern. Currently the +PNA is not going to do us any favours.

5) Atlantic SSTAs continue to show a slightly -ve North Atlantic Oscillation (NAO) signature, as it has done all Autumn:

October z500 anomaly showing high pressure all the way

Forecast anomaly based on this SSTA pattern

If anything, more of a signal from the monthly lagged SSTA to bring the now familiar high pressure closer to the UK:

Putting this all together is a signal for strong high pressure solution in the eastern/central Atlantic. Phases of the +PNA early on in the month will tend to bring the high towards the UK as jet flow is enhanced over the eastern US. During the more likey -PNA pattern which I think will prevail overall, the high will shift westwards allowing more of a north-westerly flow and perhaps more of a prolonged -NAO -AO pattern to sustain itself.

So, a dry month. An average temperature month as well with the high pressure anomaly centre just to the north and west of the UK with chances for cold incursions first 10 days and days 20-30.

[johnholmes]

brave man, you will have lots of supporters if it is correct. I cannot fault your arguments.

[Gavin P]

Impressive piece of work, GP. Would I be right in saying this is a classic November set-up in a La Nina scenario?

[Nick F]

Yes, a good forecast there GP, and certainly can see where you are coming from. Quite unusual to have a -ve NAO signal developing next week, despite the low heights programmed over Greenland/arctic circle for the mean time - all courtesy of the mid-latitude high undercut by azores low - maybe a response to strengthening of the Sub-tropical jet due to ex- TS Noel's energy arriving across the Eastern seaboard.

As for the +ve PNA pattern developing next week, it may be slower to change back to -ve again than the models want, but the La Nina signal is for the -ve PNA to dominate, so can't see the Eastern trough/Western ridge over N America persisting all of November, but no certain signs when it will relax yet.

[Glacier Point]

Thanks guys, it certainly does look like a Nina forced pattern at the moment, but it the remainder of the Pacific and Atlantic SSTA also want to go there so something of a rock solid bet for high pressure solution.

One thing about these projected cut off lows and -NAO is that the Pacific SSTA forcing is shown to induce an anomalously strong sub tropical jet flow into North Africa which would explain these features, and any ex TDs which get 'sucked' into the circulation.

[kold weather]

Nicely presented forecasted GP and for what its worth I do agree with you about the mean pressure positioning, there is a very evident reason to call for pressure to be higher over the Atlantic.

IMO its a loaded gun set-up that depending on what the AO does could either trigger or remain dormant.

On a practical level I wouldn't be surprised if we have a month a little like november 2005, in that the high pressure always remains close and tries to give northerlies but its not until the AO dips that the northerly shots hit us with any force. Before then most will head off into Europe and we may end up just on the western side. The exact temp range IMO will depend on exactly where the upper high pressure sits for most of the month, even a small shift (maybe 100-300 miles...which on hemispheric scales is quite small!) could lead to as nuch as 1C difference in the CET IMO.

Edited October 31, 2007 by kold weather

[BrickFielder]

Yes good forecast. Last months 250mb wind anomalies pattern across the Pacific matches up well to a strong la nina pattern.

Last Month.

Strong La nina

Subsurface analysis shows la nina is likely to peak in and around el nino region 3.

As la nina begins to weaken and seas warm around el nino region 1 and 2 we might expect the pattern to change as follows.

If a strong MJO event occurs then region 4 and region 3 areas will begin to warm and we could be looking at.

Not applicable for November but relevant to later in the winter.

What I was struck by was the stratosphere chart with a noticeable warming wave going up into the stratosphere and should make its way downwards for the later part of november. This I think will set up a blocked pattern.

Using the AO index could be a little risky as the pacific and atlantic do not always show the same signal. The NAO looks to stay slightly positive , although in some respects this could reflect a more negative NAO type pattern.

[The PIT]

So far so good first cool plunge almost bang on time.

[Glacier Point]

Half way point reached - so far so good:

Very much characterised by the mid Atlantic anomaly which has a striking resemblance to the November analogues for the Pacific Basin SSTA (not just La Nina - includes the -PDO pattern as well).

The AO has declined to very negative values:

http://www.cpc.ncep.noaa.gov/products/prec...x/ao_index.html

with a possible second trough towards the end of the month as forecasted.

Temperatures heading towards an average or possibly slightly below and, despite recent rainfall, below average precipitation thus far.

Just for fun.... using the same analogues for Pacific Basin SSTA rolled forward:

December

January

February

..a strong suggestion for a similar pattern to December, but January and February look to have potential to be governed by other factors with much weaker forcing in the Atlantic.