1) Mid and upper level temperatures. Very cold mid and upper level temperatures like we saw last winter will force the bundling of the jet northwards(a stronger polar vortex) toward the poles and force the positive AO that we saw last winter! This is also why alaska and greenland experienced a very cold winter. This contributed to record amounts of sea ice in the bering sea. The reason for last winters very positive AO and NAO was the very cold mid and upper level temperatures that were present before and during the winter time. We need the mid and upper level temperatures to warm up towards next winter or at least level off if we want to see the cold penetrating north america. Most people just said its the positive AO that caused the warm winter or its the jet stream position that caused the warm winter but it is what made that occur that actually caused the warm winter pattern.
Look at the huge drop in temperatures from 2010(tan) to 2012 (green)
At 600mb:
And at 400mb:
This caused the 500mb pattern to look like this last winter. When stratospheric warming finally occurred last winter the cold air bundled itself over the larger continents (which it naturally does in this type of pattern where the PDO has recently switched phases). When you turn down a thermostat in your house the air in the room cools before the surface does and it is the same with regards to the atmosphere where the PDO is the thermostat of the earth. When you cool the PDO then you are lowering the energy budget of the earth and start rapidly cooling the mid and upper levels of the atmosphere. It takes a while before the surface can begin to cool as well. So as long as we are in this pattern and the upper levels stay cold then the cold air will always bundle towards the poles. If forced to come down due to a stratospheric warming ETC then it will bundle over europe and asia and cause blocking there and not in north america.
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This is what europe looked like at 500mb with blocking across the north and cold air funnelling in for good reason.
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2) If blocking is found in the southern hemisphere during summer:
we can see big swings right now in the AAO from very negative to very positive and perhaps in the tank again. I did a blog post on the CFS forecast of this summer in the southern hemisphere and how that has effected us in north america int he past.
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3) If we see a pronounced atlantic tripole (warm, cold, warm SST's)
We can see that in the year of 2009-10 contributing to the strong persistent blocking that showed up.
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Also in the winter of 1952-3 we can see the warm, cold then warm pattern setting up. This winter had consistent blocking for most of the fall, winter and spring.
1952 10 -0.28
1952 11 -1.32
1952 12 -0.49
1953 1 -0.12
1953 2 -1.00
1953 3 -0.45
1953 4 -1.96
SST's
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There are more examples...
4) Solar activity
We know that years with low solar had strong blocking showing up. This year we are heading to wards our maximum although it is not very high when compared to other years. I don't feel as though there is a strong signal in the effect that solar activity has on this winter.
5) Volcanic acitvity
Years where arctic volcanoes erupted had very strong persistent blocking such as 2009-10. This also contributes to a big drop in global temperatures the following years. Due to the volcanos that recently arrested we may see some strong cooling showing up this winter or the following winters due to this.
In the year of 1912-13 we saw mount novarupta erupt and this produce major cooling 3,4 and 5 years later.
In the winter of 2009-10 we saw Mt. Redoubt in Alaska, Sarychev in Russia and Eyjafjallajökull in Iceland. If we do the math we can see that as history is shown to repeat itself we will see a lot of cold showing up in north america in the winters of 2012-13, 2013-14 and 2014-15.
6) A negative QBO
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