Looking Ahead to December and Beyond

1. A linkage between weather changes occurring in widely separated regions of the globe. 
2. A significant positive or negative correlation in the fluctuations of a field at widely separated points. 

Most commonly applied to variability on monthly and longer timescales, the name refers to the fact that such correlations suggest that information is propagating between the distant points through the atmosphere.

Basically these are large scale patterns in the atmosphere that all have an inter-related effect on each other. Some are short term variations like the NAO and AO, others are longer term like the PNA, MJO and Nino, while others, like the PDO, are measured in decades. When added together, they all have some level of influence over the weather that we see here in the southeast, and there are combinations of those different teleconnections that can greatly enhance (or take away) our prospects for winter weather here in our neck of the woods. Keep in mind, global patterns do not occur independently; they occur on overlapping timescales, and have important interactions that can magnify or diminish the effects of an individual pattern. 
So let's take a look at some of these indices and see where they've been, where they are now, and what direction they might be headed, as well as how the can effect our winter weather.

Arctic Oscillation - AO
First, let's look at the AO or Arctic Oscillation. Right now we're in a positive phase of the AO (+AO). When the AO is in the positive phase, the bulk of the cold air stays mostly north of the US, locked up in Canada and the Arctic, with only brief cold air outbreaks in the central to eastern CONUS that don’t last very long. Notice the mild temperatures we're having right now. This is more of a zonal flow where systems that do come in don't last very long, and we're currently in one of those phases. The last cold shot we just had fits into that scenario nicely, where we had a sharp cool down, but then it quickly moved on. However, when the AO goes negative (-AO), there can be numerous prolonged cold air outbreaks into the central and eastern CONUS, and the chances of having a big winter storm is much higher. A negative AO pumps up a western ridge that causes the jet stream to ride up and over the ridge, bringing cold air back down the other side, and into the central part of the country. 

The current outlook for the AO is for it to take a dive into negative territory as we move toward the end of the month, and it has already started a downward trend. 
Remember: -AO is what we want during the winter.

North Atlantic Oscillation - NAO
The NAO, or the North Atlantic Oscillation, is a block that occurs to our east and helps to prevent the cold air from shooting west to east at more northern latitudes. This block forms near Greenland as a large area of high pressure. This block works the same way as the AO in that it doesn't allow the jet stream to shoot due east, but instead causes it to turn north, and up and over the block.  (See graphic here)  Additional research at the Southeast Climate Office has revealed that the NAO appears to go through a full-phase cycle approximately every 20 days, and that the NAO had the most significant impact of all the teleconnections on snow days in the southeast. Even in winter months that featured La Niña conditions (typically warm and dry), combined with a negative NAO, only February saw a decrease in snow days, which suggests that the NAO has a more direct influence on NC snowfall than ENSO.
 
During a negative NAO event, a strong upper-level low pressure system (in weather terms, a 500 mb polar vortex) is often located near the northeast US or Newfoundland. The position of this cyclone helps drain very cold air from Canada into the eastern U.S. As the polar vortex pulls out (and the NAO transitions toward neutral), if jet stream interactions have resulted in surface cyclogenesis (often along the Gulf Coast/southern U.S. region), the resultant surface low is able to track northward along the east coast of the U.S., often intensifying along the Gulf Stream. 
(Some from the Data State Climate Office of NC)
Remember: -NAO is what we want during the winter.

Combined with the -AO, the -NAO forces the jet stream to buckle and dip south, before returning north and east over the block. It creates a large trough in the center of the country where cold air pools. Shortwaves follow the jet stream as it dips and turns, and they will occasionally dip far enough south to tap into Gulf moisture and bring winter weather to the southeast. Lately both indexes have been sitting in positive territory (hence the mild temps and fair weather), but they've both started a downward trend and are forecast to go into negative territory by the end of the month. Again, a good sign for potential winter weather in the southeast, and a sign that some blocking may be in our future.

Pacific Decadal Oscillation Index - PDO | Pacific-North American Index - PNA
We're also looking  at a pretty strong +PNA pattern and that also helps to prop up a western ridge that will also help to funnel the cold air southward, as well as a PDO that may be headed back up from it's current negative state, to a more neutral/positive state as the winter goes on. Couple that with a neutral ENSO, that also may become a weak El Nino in the February/March time frame, and things may get more interesting as the winter progresses.  
Remember: +PNA is what we want during the winter.
Remember: +PDO is what we want during the winter.
Remember: a neutral to weak El Nino is what we want during the winter.

Sudden Stratospheric Warming - SSW
As the winter progresses, we'll be watching for events like Sudden Stratospheric Warming (SSW), as they can have drastic effects on winter weather. A SSW is an event where the polar vortex of westerly winds in the winter hemisphere slows down or even reverses direction over the course of a few days. The change is accompanied by a very sudden rise of stratospheric temperature by several tens of kelvins. There are several classifications of SSW events, with the Major having the most effect on the weather at the surface, with a major event occurring roughly every two years. These Major events occur when the westerly winds at 60N and 10hPa (geopotential height) reverse, i.e. become easterly. A complete disruption of the polar vortex is observed and the vortex will either be split into daughter vortices, or displaced from its normal location over the pole. When this happens, cold air can get forced south and deep arctic outbreaks can occur.
Last year we had a SSW event during the middle of January, and temps fell in February in response. Go to this page and you can see from the various years, spikes in the strat temps, generally in the January through the March timeframes. 
I have several pages related to the stratosphere:
Stratosphere Details
Stratospheric Time Series

Madden-Julian Oscillation - MJO
One last item to cover, and that's the Madden-Julian Oscillation or the MJO. No, it has nothing to do with John Madden, but instead has to do with a traveling pattern of of anomalous rainfall. This pattern is intraseasonal and has a period of about 30-60 days. Right now the pattern is in the "circle of death", but hopefully will begin moving to a particular phase (we want Phase 1,2,3,8 for winter weather), so this is also something we'll be watching.
Remember: Phase 1,2,3,8 is what we want for winter

Lots of weather to keep track of in the winter, lots of indices to follow, and lots of computer maps to watch. With so many variables and things going on, it is by far the best season for weather watching.

Here is a winter weather forecast put out by WXRisk.com, that outlines their winter outlook. It's about 70 pages or so (very light reading, many images), so it's very detailed and with great examples to refer too. I converted it to a PDF from a PowerPoint since some of you may not have Microsoft Office. This forecast most closely matches my thoughts (not that I have any great insight! :-)) 
WXRisk Winter 2013-2014 Forecast
UPDATE - 5:44am Tuesday
This is from this morning's AFD, goes along with what I was talking about above...

"HIGH PRESSURE WEDGE SETS UP AGAIN ON MONDAY... WITH ISENTROPIC LIFT BEGINNING MONDAY NIGHT AS LOW PRESSURE FORMS IN THE WESTERN GULF. THE GFS IS HINTING AT A POSSIBLE WINTRY MIX IN THE FAR NE ZONES FOR MONDAY NIGHT... BUT THE ECMWF REMAINS DRIER. FOR NOW... THERE IS STILL ONLY A SLIGHT CHANCE OF PRECIP... AND MODELS WILL HAVE TO BE MONITORED FOR CONTINUED TRENDS BUT WILL GO AHEAD AND INCLUDE A RAIN/SLEET MIX IN THE FAR NE MTNS. HOWEVER... PTYPE AND POP CHANCES WILL DEPEND ON THE STRENGTH OF THE ISENTROPIC LIFT AND IF THE ATMOSPHERIC COLUMN WILL BE ABLE TO MOISTEN ENOUGH TO PRODUCE PRECIP."

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