The cold air coming to the Midwest and Northeast over the next 10 days will come in two or three surges with a spell of modestly milder air coming after each surge. This will result from the NAO going negative (please see our first blog in December for terminology explanations) during a developing +PNA pattern. This will allow Siberian air to cross over the North Pole, rush through Canada and then enter the United States via potent arctic fronts.
The –NAO appears likely to have resulted from the stratospheric warming event that we detailed several weeks ago. The result is going to be more typical winter conditions across the northern third of the country over the next one to two weeks, including air that is colder than normal more times than not and enhanced risks of accumulating snow. For Rochester and the Finger Lakes this means more snow and more cold than we have seen so far this winter, not that that is saying much.