Cold weather has been hard to find so far this season, but that is about to change and one of the factors that will lead to the change is the North Atlantic Oscillation (NAO). The NAO index is a measure of the atmospheric variability in the Northern Hemisphere, and it can be particularly helpful to monitor during the winter. The NAO is measured by the difference in sea level pressure between Gibraltar (an area of high pressure) and Southwest Iceland (an area of low pressure). This is a picture of the Atlantic Ocean where we look for these features.
The NAO will fluctuate between a positive and negative phase based on the strength of the low and high pressure centers. The positive phase is characterized by a larger than normal difference in pressure between the high and low. That sets up a strong pressure gradient between the two centers of pressure, and therefore a strong jet stream between the high and low. Below is a picture of that.
When the jet stream is positioned in such a way, it's tougher for the cold air to push south out of Canada, leading to above normal temperatures.
But with the negative phase, and it looks like that's where we are trending for the middle part of December, there is not a big pressure difference between the high and the low. That helps to set up a blocking pattern in the jet stream over the Atlantic Ocean. Picture below.
That blocking pattern to the east of United States, means there will often be a dip in the jet stream over the lower 48, and the coldest air can come south out of Canada.
The graph you see below is the measurement of the NAO since the start of August.
You can see the values will fluctuate up and down quiet a bit. Past values is where the line is black, and the forecast portion is in red. The red lines on the graph indicate that the NAO is going negative into the middle part of December. Again, when the index is negative, especially as much as the forecast is suggesting, cold air is likely coming for parts of the country.
The NAO is just one of the factors to look at when determining the forecast, but it is good to look at it for the short to medium range forecast (one to three weeks). Since the index fluctuates a lot, like over a one to two week time span, it may not be the best indicator of the long range forecast, such as, two or three months out in advance.