Why Has It Been So Cold In The Southeast?
Winter has been a cold one east of the Mississippi River, especially in the southeastern United States. Below is a map of the U.S. showing the departure from normal temperature over the past 30 days. Normal temperature is the average taken from the normal high and low temperatures.
Why has it been so cold in the Southeast? - - By now, most people are aware of the current La Nina conditions in the Pacific Ocean. La Nina is the negative or cold phase of a large-scale circulation pattern known as the El Nino / Southern Oscillation cycle (ENSO). La Nina has been the reason most of the western two-thirds of the U.S. have been warmer than average over the past 30 days. However the cold weather in the southeastern U.S. is being caused by a different cycle known as the Arctic Oscillation.
How does the Arctic Oscillation affect the weather? - - Like ENSO, the Arctic Oscillation (AO) is another large-scale circulation pattern. There are literally dozens of these large scale circulation patterns that cycle between positive and negative phases, each with their own resulting weather patterns. As the name would suggest, the AO refers to atmospheric conditions circulating around the arctic. The positive phase causes arctic air to remain “locked up” in the Arctic Circle while the negative phase allows that arctic air to be released southward into the eastern half of the United States. The more strongly negative the AO is, the further south that cold air is able to go. Below is a graph showing the previously observed values of the AO the past four months and the associated forecasts for the next 14 days.
Graph showing the observed value of the AO (black squiggly line) and several 14 day forecasts (red lines). Source: Climate Prediction Center.
Notice how the AO has been in a negative phase since the middle of November and strongly negative since the middle of December. This has allowed for more cold air outbreaks than usual across the southeastern U.S. as well as the upper Ohio River valley.
Forecasting for the AO - - The red forecast lines in the graph above are several different computer model forecasts showing where the AO could possibly head to in the next 14 days. Large-scale circulation patterns are extremely difficult to predict more than a few days out, which is why they aren’t well suited for making daily forecasts. However, they are useful for explaining why particular weather events have occurred and if it’s likely for the same weather patterns to continue over the next couple of weeks.