October thoughts

[Glacier Point]

A new month and a likley transitional one towards the winter trend.

The Stratosphere looks reassuringly benign right now with no evidence of any organised cold pooling, indeed the latest polar cross sections continue to show marginally warmer anomalies developing at the edge of the atmosphere along the Meridian migrating steadily east:

Zonal comparisons also look OK, with options open for any warming event down the line. Also worth noting that the zonal series has a very similar look to the same period in 2005:

http://www.cpc.ncep.noaa.gov/products/stra...JAS_NH_2007.gif

http://www.cpc.noaa.gov/products/stratosph...ALL_NH_2005.gif

As a response to these benign conditions and low GLAAM, we continue to see an absence of strong upper zonal anomalies with no organised stirring of the polar vortex:

Reanalysis of a strongly -ve QBO and low GLAAM October-November period suggest something of a strong ridge to develop over western Siberia into the Arctic with lower pressure over the near continent.

All of this augurs well for tropospheric factors to take the lead into October.

La Nina continues to develop with cool subsurface anomalies being successfully upwelled by easterly surface winds across the equatorial Pacific. The latest ENSO data suggests region 3.4 dropping to -0.91 which is threatening moderate Nina status.

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

This is now starting to drive a strong high pressure cell in the Pacific which is likely to drive a strong jet into the western side of North America, shown well here by 00z 26/09/07 mean height comparisons:

In terms of the Atlantic, this will start to force lower pressure by shifting ridges from the west to the east coast of the USA effecting a subtle monthly change. The Atlantic however continues to be forced by a negative NAO signal - warm anomalies to the north, cooler ones to the south and further north - and is generally not conducive to the establishment of a strong jet - quite the reverse.

Reconcilling these two is possible, and a -ve NAO for October is very likely with an upper trough situated to our west. A Scandinavian ridge is an inevitable response to this troughing with mild conditions likely at first as the circulation remains static and from the south although there is some support (by the QBO / GLAAM combo) for the ridge over Scandinavia to be undercut by the upper trough setting up more of an easterly flow.

You'll notice I haven't mentioned the Arctic Oscillation yet.

The operational GFS has tried to build a highly negative AO of late although surface pressures over the polar field have remained slightly -ve (+AO) most probably due to rapid cooling of most of the surface. A prospect of a +AO and -NAO teleconnects for some very warm plumes to work their way northward but I do not see the AO staying +ve for long, in fact, I have a strong suspicion that this will dip sharply -ve towards the second week of the month as those huge surface anomalies over the polar field and the forcing from the QBO/GLAAM take effect. Worth noting that of the four similar QBO years where we switched from a westerly to an easterly phase in the Spring with a rapidly developing and strongly -ve value, two of them ended up with a strongly -ve AO (the others were asociated with a strong Nino and a neutral value).

This leaves us with a possibility of a -NAO -AO combination which teleconnects to something below average with the likley transition of the Scandinavian ridge west and NW towards Iceland and Greenland as the month progresses. Slight dips and peaks in the AO could lead to the core of the high wobbling around more towards the UK.

So, a mild start to the month with showery rain to the west changing to much more cooler conditions with frost towards the second half of the month. Overall I think an average month but masking some swings from one extreme to the other. If the AO does not dip -ve, there is however the potential for a very warm month as we become locked into a month-long south / south-easterly draw.

[BrickFielder]

The Stratosphere looks reassuringly benign right now with no evidence of any organised cold pooling

I would largely agreee about the northern hemisphere but there has been some significant warming of the upper layers over the southern pole. Could we perhaps be seeing the first sign of a rare southern hemisphere stratospheric warming like we saw in 2002. This will almost certainly drive the AAO (Antarctic oscillation index) negative and lead to a blocked pattern across the southern hemisphere which will lead to a transfer of energies across the hemispheres and induce a more blocked pattern in the nrothern hemisphere.

While I mostly agree about the GLAAM and the GOW (Global Wind Oscillation) being low we should not discount the fact that global frictional torque (Mountain) is increasing to bring these up. This I think adds up to poleward shifted ridges in the short term with a strong pattern shift to come.

[Chris Knight]

A new month and a likley transitional one towards the winter trend.

I presume that's why it has a " ber " ( brrr!) in the name

[Glacier Point]

Some thoughts on the pattern so far....

The Pacific Jet remains strong, fired up by a very robust high pressure cell underpinned by La Nina and strengthening thermal gradient in the N-PAC. La Nina is now in a moderate condition with region 3.4 showing up -1.2 anomalies with substantial subsurface -ve anomalies in evidence.

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

The easterly trades, supported by the strong easterly (QBO) upper flow, will ensure that we have La Nina in place for the next four to five weeks at least. Interestingly, the QBO may just have peaked which would signal a rapid decay into the winter which would also be terminal for Nina as the warm westerly's in the western Pacific could snaffle subsurface upwelling veiling the equatorial Pacific with warmer waters. But for now, moderate Nina in play.

The predicted impact of the strong Pacific jet has been for a shifting of the trough and ridge eastwards, a deep trough now evident over the mid Atlantic and ridge moved further east over the UK and near Continent. This is being underpinned by the SSTA signal which continues to force a high pressure solution for our locale.

We do have a -ve NAO though.

http://www.cdc.noaa.gov/map/wx/forecasts/pn.png

Problem is, we also have a +AO, which, as I stated above, is a recipe for warm, warm, warm.

http://www.cpc.noaa.gov/products/precip/CWlink/daily_ao_index/ao_index.html

It looks to me as if the AO is entering an increasingly volatile state, predominating for the time being in highly +ve values but dipping back to –ve values every now and again. This will be worth monitoring as we enter the second half of the month towards November when it really must show signs of becoming more –ve if we are to see reasonable prospects for blocking.

Also worth noting, there are some pools of anomalously cold air developing at the outer stratosphere and penetrating down to the 100 – 200 hPa layers. In a westerly QBO regime, this would make it very difficult for any –AO to survive, but thankfully, an easterly QBO might just allow the cold parcel to propagate downwards to form a distinctive bi-polar vortex focused over Siberia. This is definitely worth monitoring over the next few weeks.

La Nina and the –NAO signal are unlikely to change in the next few weeks. Neither is the strong Pacific Jet. This leaves us with the prospect of either just not enough blocking to prevent residual jet flow in the northern Atlantic – maintaining the blocking high over the UK

Or,

Blocking to develop over polar field dramatically altering the pattern and shifting us to a –AO –NAO regime in one foul swoop. The GFS and ECM control runs have shown of late big tendencies to be either mild or very cold outliers, sometimes ridiculously so but they, and the huge scatter in the ensembles are telling us that potentially we are on the cusp of extreme varieties of weather.

The 00z 05/10/07 mean height comparisons pick out that predicament for the modelling with an increased trend for blocking over Canada to move steadily northwards. If that continues, the entire pattern will change and dramatically so but for now I sense more of the standoff is the more likely resulting in this mid latitude high sat over the UK until the AO shows its mettle.

Edited October 5, 2007 by Glacier Point

[Glacier Point]

z500 Pressure anomaly for October 1-21:

So far so good into October with the overall pattern correctly nailed and the Autumn forecast doing rather nicely:

http://www.netweather.tv/forum/index.php?s...p;#entry1050386

Forecast pressure anomalies Sep - Nov:

Things to note so far in the context of November and what lies beyond:

The polar field continues to look pretty innocent, perhaps even showing signs of being warmer than the same time in 2005:

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

http://www.cpc.noaa.gov/products/stratosph...ALL_NH_2005.gif

This really does leave the door ajar for some fireworks (stratospheric warming events mid to late winter and more blocking before then). Upper level zonal winds are also negative around the pole showing no signs of an organised fast polar vortex:

Currently the polar height anomalies look pretty favourable with lots of +ve anomalies showing up.

In light of the temperature profile shown above, I would suggest that this should enable this type of profile to continue and grow. It's at the end of the month we should see where the polar vortex is trying to set up. My hunch at this stage is Scandinavia and Siberia with the main one extending through the Gulf of Alaska. This cross section is bar far the most positive aspect of October for me thus far.

[The PIT]

At the end of the day the call for October wasn't bad at all. Infact went pretty well mild start cooler end.

[Glacier Point]

A new month and a likley transitional one towards the winter trend.

The Stratosphere looks reassuringly benign right now with no evidence of any organised cold pooling, indeed the latest polar cross sections continue to show marginally warmer anomalies developing at the edge of the atmosphere along the Meridian migrating steadily east:

Zonal comparisons also look OK, with options open for any warming event down the line. Also worth noting that the zonal series has a very similar look to the same period in 2005:

http://www.cpc.ncep.noaa.gov/products/stra...JAS_NH_2007.gif

http://www.cpc.noaa.gov/products/stratosph...ALL_NH_2005.gif

As a response to these benign conditions and low GLAAM, we continue to see an absence of strong upper zonal anomalies with no organised stirring of the polar vortex:

Reanalysis of a strongly -ve QBO and low GLAAM October-November period suggest something of a strong ridge to develop over western Siberia into the Arctic with lower pressure over the near continent.

All of this augurs well for tropospheric factors to take the lead into October.

La Nina continues to develop with cool subsurface anomalies being successfully upwelled by easterly surface winds across the equatorial Pacific. The latest ENSO data suggests region 3.4 dropping to -0.91 which is threatening moderate Nina status.

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

This is now starting to drive a strong high pressure cell in the Pacific which is likely to drive a strong jet into the western side of North America, shown well here by 00z 26/09/07 mean height comparisons:

In terms of the Atlantic, this will start to force lower pressure by shifting ridges from the west to the east coast of the USA effecting a subtle monthly change. The Atlantic however continues to be forced by a negative NAO signal - warm anomalies to the north, cooler ones to the south and further north - and is generally not conducive to the establishment of a strong jet - quite the reverse.

Reconcilling these two is possible, and a -ve NAO for October is very likely with an upper trough situated to our west. A Scandinavian ridge is an inevitable response to this troughing with mild conditions likely at first as the circulation remains static and from the south although there is some support (by the QBO / GLAAM combo) for the ridge over Scandinavia to be undercut by the upper trough setting up more of an easterly flow.

You'll notice I haven't mentioned the Arctic Oscillation yet.

The operational GFS has tried to build a highly negative AO of late although surface pressures over the polar field have remained slightly -ve (+AO) most probably due to rapid cooling of most of the surface. A prospect of a +AO and -NAO teleconnects for some very warm plumes to work their way northward but I do not see the AO staying +ve for long, in fact, I have a strong suspicion that this will dip sharply -ve towards the second week of the month as those huge surface anomalies over the polar field and the forcing from the QBO/GLAAM take effect. Worth noting that of the four similar QBO years where we switched from a westerly to an easterly phase in the Spring with a rapidly developing and strongly -ve value, two of them ended up with a strongly -ve AO (the others were asociated with a strong Nino and a neutral value).

This leaves us with a possibility of a -NAO -AO combination which teleconnects to something below average with the likley transition of the Scandinavian ridge west and NW towards Iceland and Greenland as the month progresses. Slight dips and peaks in the AO could lead to the core of the high wobbling around more towards the UK.

So, a mild start to the month with showery rain to the west changing to much more cooler conditions with frost towards the second half of the month. Overall I think an average month but masking some swings from one extreme to the other. If the AO does not dip -ve, there is however the potential for a very warm month as we become locked into a month-long south / south-easterly draw.

Just to tie this one up.

z500 anomaly pattern:

AO index for October:

Very much a month for the trough opening up in the Atlantic. Interestingly the same pattern is trying to persist but with the jet coming over the top leaving cut off lows.

The forecast AO volatility was good, two + spikes and one -ve one. I'm quite surprised that the overall CET was as low as it was with this.

The Autumn forecast update looks like this:

Forecast based on SSTA:

Observed so far:

Just shows how much the SSTA will influence the pattern given a low GLAAM