The Big Picture
Many people are up in arms over the latest CFS model forecasts for this winter trending from a brutally cold US winter to a rather average, or even slightly above average winter in the temperature department. These are the times when people tend to show a different side- the side that tells a forecaster he/she was wrong about the CFS forecast when they posted how it was advertising a cold winter. People can get particularly nasty towards forecasters who don't deserve it. I posted about the CFS cold trend; luckily, all of you are polite folk who haven't thrown around vulgar comments like I've seen other forecasters get as a result of this sort of situation, when a model suddenly changes its forecasts. But I'm going off on a tangent now. The big picture here is that the CFS really has not changed its forecast as far as the atmospheric pattern goes. While temperature forecasts may have changed, it's pretty apparent that the CFS remains steadfast on a favorable mid/upper level atmospheric flow for the upcoming winter. You will see in the image above that I have five areas of interest numbered from 1 to 5. Each area of interest will be described below.
1. Active East Asian Jet Stream
You will see a tightening of the contour lines in eastern Asia on the image above, under the number 1. This symbolizes the jet stream is under stress, thus increasing its strength in that area. From looking solely at the image above, it would appear pressure anomalies are below normal in East Asia, meaning more storms in that region. If you have been with this blog for a while now, you may recall the times I have brought up a correlation between storms in East Asia and storms in the East US 6-10 days later. If the tightening of the jet stream in E. Asia is due to a stormier pattern in that area, it is entirely possible we see a stormier East US. Let the record show that the CFS has been adamant with this particular item for some time now: November-December-January 200mb forecast from June 2013
2. Ridging Pattern in Northern Pacific
It is well established that the Bering Sea is a favorite place for the origination of sudden stratospheric warmings (SSW's), a phenomenon that involves rapid warming in the stratosphere that then allows unusually cold air to reach the surface and lower latitudes just a couple of weeks later. Last winter, we saw a magnificent SSW originate from the Bering Sea, and other warmings attempt to take place in that same vicinity. With positive height anomalies stationed over the Bering Sea, intrusions of warm air by these positive height anomalies into the Arctic are more easily accomplished, thus increasing the likelihood of SSW events. Once again, this feature has been displayed on CFS forecasts for multiple months now, as the above link depicts. Additionally, bear in mind that these features were in the forecast when the CFS was projecting a very cold winter. In my opinion, it could very well be that the CFS is flopping around with its temperature forecast, as is customary of long range models, while the upper level atmospheric flow really tells the story (i.e. the pattern really is favorable for cold weather, even though temperature forecasts do not reflect it).
3. Ridging Pattern in Western North America
The image at the top of this post displays a widening of contour lines under the number 3. When you get the expansion of these lines, it is typically demonstrative of high pressure anomalies present in the area. The presence of high pressure anomalies in Western Nor. America is defined as a positive Pacific-North American index (PNA) pattern. In response to high pressure in the West US, low pressure (and thus cold air) is favored in the Central and East US. It is also known that a positive PNA results in winter storms taking a track favorable for the Midwest/Ohio Valley to receive snowfall. Yet again, the June CFS forecast identified the +PNA pattern for this winter, just as the current forecast shows.
4. Low Pressure in Eastern North America
This is kind of an extension of Number 3, in the sense of a low pressure response to positive height anomalies across the West US due to the positive PNA pattern. The CFS is picking up on those low pressure anomalies, though the model is reluctant to allow the anomalies to stretch into the United States. My theory on this is that, because the model is showing a forecast averaged out over 3 months, low pressure anomalies/cold weather DOES make it into the US due to the +PNA pattern. The reason why the model may be reluctant to do that on the image above is because we could see high pressure in the Bering Sea work against development of the +PNA pattern, thus preventing low pressure anomalies from reaching the US.
5. High-Latitude Blocking Pattern in Greenland
The image above shows an expansion of contour lines towards Greenland, an indicator that high pressure anomalies are favored in that area. Positive height anomalies in/near Greenland results in the negative phase of the North Atlantic Oscillation. The North Atlantic Oscillation (NAO) is a phenomenon that involves height anomalies across Greenland. In the negative NAO, high pressure is stationed over the land mass in a move that permits cold air and Nor'easter's to affect the Eastern US. On the other hand, a wintertime positive NAO results in a warmer United States with lower probabilities of big snowstorms for the nation as a whole. I brought about the theory that averaging out three months in a forecast may not project the real picture, and that theory also applies here. It applies here in the sense that the NAO may be very negative at times, but may also be less negative during some times. The tendency for a more negative NAO over a less negative/more positive NAO for this three month period in winter may then lead to pressure anomalies averaging out positive over Greenland (a sign of the negative NAO), thus showing an expansion of contour lines towards that region.
As the title of this post says, don't worry, be happy. Concerns over the increasingly-warmer CFS forecast for this winter are unwarranted on the basis that the mid/upper level air pattern has been steadfast throughout the last few months, including the time when the CFS was calling for a brutally cold US. The five points I discussed above have been steadfast in the last few months, and those same five points, when in place during the winter, do tend to set up a favorable pattern for cold and snow in the US area east of the Front Range.