A chilly, active pattern brought winter storm after winter storm to much of the Midwest, Northeast, and Mid Atlantic from late January through much of February. Then, March hit, and for most of us the snow shut off rather abruptly. While some enjoyed high temperatures in the 60s and 70s on multiple occasions through in the month of March, many were left wondering what caused the proverbial switch to flip.
The strength of the polar vortex from the ground up through the stratosphere, from late January through early April. Reds denote a weaker vortex, favorable for colder weather. Courtesy stratobserve.com
What changed was the “polar vortex” got its act together in late February (meaning it strengthened near the North Pole), allowing much of March to run mild across the central and eastern United States. The vortex was quite weak in January and much of February, bringing us our colder and more active pattern. It went from weak to rather strong by early March, corresponding nicely with our sudden warm-up.
The “Arctic Oscillation” or AO from December through March, courtesy of the Climate Prediction Center
Another index that signals this change is the Arctic Oscillation or AO. When the index is negative, it favors generally colder and stormier weather across the U.S. It was negative for much of the winter through the middle of February, and was especially negative in late January and early February, corresponding with our coldest and snowiest weather. This abruptly flipped to positive for late February and much of March, signaling a substantial pattern change in the milder direction.
