Potential Cold December
In our winter outlook, we discussed the potential for a cold December. That would be much different than most Decembers over the past several years, especially last year.
This does not mean it will stay cold the entire month, however, the month as a whole may end up below average. Let me explain.
I like to look at 3 teleconnective indicies: EPO (Eastern Pacific Oscillation), AO (Arctic Oscillation), and the NAO (North Atlantic Oscillation). Knowing the state of these indices help predict weather patterns across the northern hemisphere. Generally, when they are negative, it indicates blocking over the higher latitudes. That blocking disrupts the pattern and sends the cold air south. Getting all 3 of these indices to turn negative during the same time period is a recipe for arctic intrusions.
Follow the green line which is the model ensemble mean. They are all going negative.
A birds eye view of the northern hemisphere shows the ridging in red and orange over the polar areas. Look at the blue showing up over Canada and the northern United States. That's a trough and it's filled with cold air pressing south.
IF this happens, expect the cold to dive south towards the end of the first week of December. How long will it last? Tough to say. Once these patterns break, they break big time and the warmth rushes back in. Let's just get the cold air in here first, then worry about that later.