2300 EST 24 NOV 2012 …. STAR DATE 201211.24
I am reading and seeing a lot of posts on various FB weather pages and weather forms (American wx for one) regarding o the strong signals from some of the weather models that the Arctic Oscillation ( AO) is about to turn severely negative. And in doing so this means there is a high probability that December 2012 …at some point …is likely to turn much colder which is what happened in December of 2009.
Or so the argument goes.
This is one of my pet peeves with regarding forecasting by Teleconnection or specific climate /weather patterns. You simply cannot look at ONE particular Teleconnection or weather pattern and develop a long range / seasonal forecast or even monthly forecast. The atmosphere does NOT work based upon ONE particular atmospheric pattern . It does not matter if the one particular atmospheric pattern or Teleconnection ..such as the AO or NAO… or PNA… or EPO.. etc etc … is powerful and dominant. There are other things that one MUST take into consideration and fit all the different aspects of the atmosphere and “mesh” them into a coherent pattern.
But because so many do not GRASP this … when they see a model develop a very strong signal for particular atmospheric pattern or Teleconnection… they make the decision that such and such is likely to happen. The misplacement and over for emphasis on some of these atmospheric teleconnections can lead to some serious screw ups. One of the more common misunderstandings with these atmospheric patterns are Teleconnections has to do with the Arctic Oscillation (AO). In fact all of these to various important large scale weather patterns have a certain subtleties which must be taken into consideration but with the Arctic Oscillation … there is an awful lot of misunderstanding regarding its importance.
Don’t misunderstand me. I am NOT at all saying there the phase of AO is not important in figuring out long range weather patterns for several weeks or for next month. But you must look at the AO with other atmospheric patterns. Yet for some reason there are an awful lot of forecasters and weather hobbyists out there that get awfully excited when they see a weather Model TANK the AO into severely negative values.
For example this is the latest CPC GFS Ensemble forecast for the AO as of November 24. As you can see there are 4 plots and all of them show the AO moving steadily and strongly into negative territory by December 1 and continuing into negative territory by the middle of December.
And if you don’t want to look at just the operational GFS this next image shows you the GFS ensemble mean plots. Not surprisingly there not as severe has the operational GFS but they do show a prolonged and persistent interval of a strongly negative AO feature.
So of course it only makes sense to forecast a colder and potentially stormy are pattern for the central and eastern U.S. as you move towards the middle of of December 2012… right ?
NO!!!!!!!!!!!!!! It does not and let me explain why.
In my winter forecast and especially in the final winter forecast which I issued last week,,, I pointed out in the Winters of 2009-10 and in 2010-11 that it was the conjunction of both the AO and NAO operating at the same phase and both severely negative which is what help produce the stormy in cold pattern
There is gross misunderstanding that simply because the Arctic Oscillation has gone negative that it follows that
1) the NAO has also turned negative
2) or that it has to turn negative…
3) that the overall pattern is now a much colder and stormy are one for the central and eastern US. None of this is remotely true.
There are numerous cases of the Arctic Oscillation turning negative but the cold pattern sets up over the Pacific… or say Western Canada …or Europe or Central Russia for example. Of course .. as I stated above… if you have BOTH the -AO and -NAO at the same time and with roughly same intensity… then the overall pattern over the central and eastern U.S. does favor below normal temperatures and above normal precipitation.
Let’s take a look at some of the weather models as we move into early December and we can see there a lot of other very important features to consider besides a strongly -AO. This next image shows comparison between the operational European model at DAY 9 on the left side and the operational GFS model on the right side. We can see several very important features but the most dominating one happens to be the BERING SEA OMEGA RIDGE . (for those of you that do not know the Bering Sea is the body of war to the separates Alaska and far eastern Siberia). It is this feature which has been persistent now for about two weeks which is the dominating or driving feature across the western hemisphere and not the AO. The placement of the OMEGA RIDGE over the Bering sea means that you have to by definition have a trough in the jet stream over Alaska and Northwest Canada. That trough constitutes a POSITIVE PHASE of the EPO and therefore -PNA pattern ( West coast Trough in the Jet stream).
Remembering our basic high school physics for reaction as an equal and opposite reaction…. So the place another trough in the jet stream on the West Coast means the atmosphere HAS to counter it by developing a RIDGE over the eastern U.S. A pattern like this allows for the buildup of a lot of very cold air over western and Central Canada but there is simply no mechanism or path for that cold air to come into the U.S. east the Rockies. In addition I have highlighted three other important LARGE upper Lows or vortices in the Jet stream. #1 is the Siberian PV polar vortex)… #2 id the Northeast Canada PV AND #3 is a large and persistent upper low which develops over western and Central Europe including the United Kingdom.
The three positions of these large vortexes constitutes a three wave pattern… which is a rather stable pattern…. and not one which is likely to change anytime soon.
In this next image we can see the Day 10 European ensembles on the LEFT and the Day 10 GFS ensemble on the right. As you can see the models are in strong agreement in the first week of December as to how the overall pattern is going to develop. There is simply no mechanism for delivering cold air into the country east the rocky mountains during the first week or two weeks of December… and possibly throughout most of the month.
This is the latest November 24 CFS Model runb….( 16 CFS runs each day going back over the last 10 days) . The map on the left hand side represents precipitation…. and we can notice of a lot of rather dry conditions over Texas the Gulf coast and up into the Middle Atlantic region and above normal precipitation over the Midwest and on the West Coast. This sort of dryness is symptomatic of a model showing a strong ridge over the southeast states.
The map on the right hand side see it shows a pretty warm December for much of the country but especially between the Rockies and the Appalachians.
Indeed the new 10 day group of CFS ensemble mean show a very mild December for most of the country. There probably is some skepticism with regard to this much warmer December CFS Model runs… Especially given the development of a strongly negative AO. But as I showed above …it is quite possible to have a-AO and still have a Mild pattern over the central and eastern US
In order to get this pattern to change the strong OMEGA Ridge in the Jet stream over the Bering sea HAS to move to move.. HAS to. In doing so it will force the EPO to switch from +EPO to -EPO… and that in turn will allow for a strong west coast Ridge ( -PNA to +PNA) which sets up a much colder pattern and allows the trough to form of the eastern U.S.