The past couple of weeks brought cold and snowy weather to Alaska and mild, dry weather to the Northern Plains. This is likely to flip-flop soon, meaning cold weather is coming. The forecast change is due to a developing switch to the negative phase of the Arctic Oscillation (AO).
When the AO is in the positive phase, there is high pressure near the North Pole and lower pressure at generally our latitude. This pattern locks Arctic air in the Arctic, particularly in winter.
During the negative phase of the AO, pressure lowers near the pole and increases farther south. More significantly, the general polar circulation, known as the Polar Vortex, develops a very wavy pattern that allows large areas of Arctic air to drop southward. Over the next couple of weeks, the first of these cold surges is expected in North America. Much of the United States east of the Rockies will likely become colder than average.
