With regards to winter here in Western Europe, there appears to be too much focus on the NAO without properly understanding the physics behind it and how it works.

I have said in recent weeks and since I began my winter forecast for this year that for us here in Europe, the NAO is probably of greater importance than the AO. Of course the AO is what holds or displaces arctic air from the pole, however, more often than not it’s the NAO that steers any cold air masses our way.

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Ok, so we have a mean -AO and that looks likely to persist through this winter but why is the NAO refusing to go negative? Another 2013-14 winter repeat? Let’s go back to basics. The reason for the AO going negative to start with is because of warming way up in the stratosphere, however, it’s WHERE that warming occurs that’s important.

Note in the latest heat burst (below) over Siberia that the warming stops at Hudson Bay while there’s cooling over the northeast Atlantic and Greenland.

We did see strat warming last year (like the above) and that’s what drove the cold North American winter but it stayed on the other side of the pole, thus we were on the cold side of the polar stratosphere and so a cold stratosphere on the European side means no Greenland high and no -NAO. Coincides with a strong west QBO which usually occurs during solar maxes.

Regarding the positioning of the strat warming to date, the same thing happening now, happened last winter, however the warming began earlier and stronger probably due to rapid snow growth in October over Siberia. We have an easterly QBO (opposite last year) and that means we have different dynamics in play with an apposing EASTERLY ZONAL wind at the equator. This means a weaker polar vortex further north and slower jet stream as a result. It also means the strat warming has greater influence than last year and it’s easier for SSWE’s to develop. With a weaker transatlantic jet stream/weaker PV, there’s a greater chance for the atmosphere to ‘build heights’ over the N Atlantic and Greenland and that’s where conducive SST’s come in. Once the atmosphere adjusts to the time of year with build-up of cold air, the oceans latent heat has influence on the atmosphere above. That’s why the tripole (warm-cold-warm) with warmest anomalies NORTH or WEST of the UK with cold pool underneath is so important and attracts positive heights

The grand process of ocean-atmosphere TAKES TIME. Continuous pressure put on the PV eventually punches through and a split or collapse occurs causing an SSWE. An SSW is when there’s a build up of warming on one side. Almost ALWAYS from Siberia towards the pole. Like a balloon, put enough pressure on it and eventually it pops forcing warming to spread over top. When you look back at history, most SSWE’s do not occur until late December or January and that means the NAO doesn’t go negative till then either.

Look back to most great cold spells or winters. When does the NAO have greatest influence? Mid to late winter, once the warming becomes too much for the pole to cope with and usually when the El Nino has feedback to the atmosphere.

Often the earlier a winter begins here in the UK, the quicker it ends. As impressive as November/December 2010 was, it started and ended early. Back in 1985 it didn’t turn cold till January and January/February and even March were brutal.

In 2009 it was the week running up to Christmas before things got interesting following a stormy, fairly mild start to December.

The upcoming 2 weeks looks interesting as we’re set to enter a stormy period. Check out some of the lows in the below surface charts. Severe gales, rain and yes snow will enter the UK/Ireland forecast. each low should have a cold slap on their backside since they’re coming down from the NW.

The latest 500mb height anomaly charts for the period between 240 and 360 is most interesting. Notice the negative heights over the N Atlantic/Greenland to start. Pay attention to the positive over Siberia which intensifies and pushes towards the pole but spreads out. By 360, heights are positive over the N Atlantic/Greenland.

Notice in the last two charts how there’s a strong positive building over the N Atlantic which appears to connect with the building heights between Scandinavia and Greenland. THAT would suggest more LOCKED IN cold for UK and West Europe.

Modelling, especially the GFS really struggles to see global pattern shifts. An intriguing period ahead.

See video for discussion… More tomorrow.

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