We live in very interesting times. With the NAO/AO now positive, we’re seeing the atmosphere do what I’ve been hoping for and that is the building of cold over the source region during November. Given the negative trend in the NAO ensembles and the forecast of a return to negative later this month, this reload at a time when cold air production is of a much more mature level, given the sun angle is very low, sea ice coverage is growing and the overall conditions are much more conducive for rapid building, the building blocks are now being set for a potentially wild start to December across the UK and parts of Europe. The polar vortex is strong during positive AO phases but I want you to pay close attention to Greenland over the next week to 10 days.

As pressures rise over Europe and mild air builds, we continue to see pressures lower over Greenland and the arctic and at this stage in the season, that means thickness values tank and the cold air intensifies, this is the atmosphere casting as expected. The setup now and over the next 2 weeks, could mean the difference between a tamely cold NW flow and a true arctic outbreak during the first part of December. An outbreak which could last. The stronger the push of arctic air south and the greater expanse in blocking over top, the longer this cold spell may last. Remember that cold air is denser than warm and so weaker pushes of cold can and will get pushed out by Atlantic air easily. A no show in the +AO would mean less cold air over the arctic. What amazes me is the people who don’t get that. Arctic air doesn’t just develop, conditions have to be conducive for it to build and conditions are not conducive when the NAO/AO is negative.

Had the NAO/AO remained negative through November, like some ‘forecasters’ previously advertised, then there would have been a limited reservoir of arctic air available for heading south. However, the pressure field is now set for a generally mild west-east Atlantic flow over the next 10+days across much of the mid-latitudes while low heights over Greenland and the arctic will see the cold intensify significantly. This happened back in 2009 also.

As stated in my previous post, it’s not if there’s a flip but when, regarding the NAO/AO and the evidence is backed up by the trend we’ve seen all year, particularly during the summer and now autumn months. The wetter than normal conditions, the mean trough, brought by the -NAO are all opposite to last year. The excessive rainfall has been aided by the warming of the equatorial Pacific (El Nino).

The North Atlantic tripole is still very evident and should support plenty of blocking through much of this winter season but the cold must build first and it cannot do that if the NAO/AO signal holds in negative territory. You will probably get tired of me stating this but it’s important to understand what must happen in order for us to get to the cold signal. It’s all very well having a persistent negative but if there’s no cold air available then it’s meaningless. Once you understand the physical drivers and what must go into the equation, then you can better understand the final result. This is reality, not wishcasting or simply me telling you what you want to hear.

What’s exciting me is the fact that the ideas I had back in July and August and what I believe was needed for a cold start to December, is in fact happening. I believed a positive NAO/AO during particularly November was vital. Why November, because it was close enough to winter that the conditions over the polar region would be just right for an arctic reload and, once the hemispheric pattern flipped, it would be at a time, when the arctic air would moderate less as it dove south, ie. the sun angle is plenty low and with snow, it can get very cold with little weakening as it heads for the mid-latitudes where the sun angle in naturally higher compared to the polar region where it’s absent now.

Folks, the December we are approaching is looking colder and colder. It would not surprise me if many parts of Britain are dealing with heavy snow during the first week of the month, followed by bitter cold. As I right this, the temperature at Summit Camp in Greenland is at it’s lowest so far, -56C/-69F and over the next 7 days, the trough will remain deep with thickness values continuing to fall, a temporary moderation comes Sunday as a low sweeps in blizzard conditions and as this week progresses modelling shows a frigid high building, we could well see temperatures sink into the -80sF. The all-time record low for Greenland is a bone-chilling -87F or -66C. Given what I am seeing, this record may be challenged late in the week.

It’s the fact that we’re seeing this level of cold developing over Greenland and indeed across the arctic which is worth paying very close attention to, the evidence of a return to negative, without model guidance was thanks to what we’ve observed and the likely feedback from the NA tripole, but the evidence is now showing in modeling. Believe me, once heights rise over the high latitudes and this magnitude of cold over Greenland and the arctic splits and gets transfered into southbound troughs, we will be thrown into the freezer!

Take a look at the latest NAO ensemble.

Tags: cold december 2012, cold december forecast for u, cold december looms for US