It’s been a very tranquil late November, and this tranquility will continue into the first part of the 12th month, which of course begins later this week. Temperatures have been about where we would expect this time of year, mainly in the 40s and occasionally into the 50s for an afternoon or two. Wednesday in Portland was actually just shy of 60.

Thus far there hasn’t been any truly cold air that’s lasted. We did have one record-breaking cold outbreak in early November, but since then things have been pretty quiet. That may be about to change. Many of us in the forecasting game are looking to the second part of December for a more extreme period of cold to enter the United States.

This time of the year, all across the Arctic regions of the planet, there’s no sunlight and the ground is white. This combination radiates any summertime warmth, what little there was, up into the atmosphere. In turn, cold air build, becoming more intense on a daily basis. This cold air helps the ice to continue to expand across the Arctic. Even though we’ve seen less ice over the past five years than in previous decades, there still is a lot of ice building.

The cold air is very heavy and dense and actually would like to move south. However, much of the time the winds up in the Arctic are spinning quite quickly and those fast winds lock the cold air in place. However, if the winds slacken, the cold will travel south and enter the United States, parts of Europe or perhaps parts of Asia. Think of it like this: When your washing machine is in the spin cycle, the clothes stay tight to the drum, but as the spinning slows down, the clothes flop.

The negative phase of the Arctic oscillation often brings colder weather to the United States.

Meteorologists measure this spinning using an index called the Arctic oscillation. When the winds are moving quickly where you are in a positive phase and when they slow down, we turn negative. During the second week of December, the Arctic oscillation is forecast to become negative, which in layman’s terms means that the winds around the poles are going to slow down and allow that cold air to flop southward.

A negative Arctic Oscillation is forecast later this month.

The average of the models brings a negative AO in early December.

This is not unusual. It happens from time to time every winter. However, each winter is different and some years we get only a brief blast of Arctic air. Other years, we get continued shots of cold. When making a long-range forecast it’s difficult to know whether the core of the cold will end up going across Chicago or whether it will come across parts of New England, including the Portland area. What I do have some confidence in is that it is going to turn markedly colder for the second part of December, not just because it’s getting later in the year but because of this Arctic invasion.

Blasts of cold air move south from the Arctic during the second week of December.

The process takes time. When the AO becomes negative the cold air can take a week or more to actually then move south. As we get closer to the middle of the month it will become clear exactly how cold it’s going to get but you can be sure temperatures are going to be below normal for an extended period of time perhaps as long as two weeks to close out the year.

The coldest of the air is likely to miss Maine at first, but could arrive here late in December.

Now that I’m expecting more significant cold, the next question is likely going to be will there be any snow with the cold? In order for us to see a significant snowstorm, we will actually need the boundary between the cold and warm air to be relatively close to New England. That helps create storms. If we end up with just a lot of cold air and the warm air is shunted well out to sea, the lack of a temperature contrast will mean that we just remain quite dry. This is definitely possible. One thing is for sure there are lots of forecasting challenges ahead and winter is just getting started.