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Weather forecasting and analysis, space and historic events, climate information

12:15 PM | It Ain't Over Till Its Over - winter that is

Paul Dorian


A very interesting late winter weather pattern is forecast to evolve in the US over the next week or so. We often talk about “teleconnections” here at and how atmospheric and oceanic patterns in one part of the world can have an effect on the weather pattern here in the Mid-Atlantic region. For example, the wintertime atmospheric pressure and sea surface temperature pattern in the North Atlantic can dictate whether cold air masses will penetrate into the central and eastern US from Canada and, if so, can they be sustained. One way to track the overall pattern in the North Atlantic is through an index called the North Atlantic Oscillation (NAO). If the NAO is negative, for example, a blocking pattern can set up in the northern latitudes (such as in the region between Hudson Bay, Canada and Greenland), and this typically results in cold air intrusions into the central and eastern US. In addition, the pressure pattern in the eastern Pacific can determine whether Canadian cold air masses can drop southward into the central and eastern US. One way to track the pressure pattern in the eastern Pacific is through an index called the Pacific-North American (PNA) pattern. If, for example, the PNA is positive then high pressure ridge is likely to set up along the US west coast and this typically pushes cold air masses into the US from Canada due to the circulation (clockwise) around high pressure.

Experienced snow lovers and “weather weenies” in the Mid-Atlantic region know that while there are no guarantees, the best pattern for cold and snow usually involves a couplet with a positive PNA and a negative NAO. We really haven’t had that couplet combination all winter, at least not more than for a couple of days at a time, but that may be about to change. All indications are that this type of “teleconnections” pattern will develop over the next week or so enhancing the chances for cold and snow in the Mid-Atlantic region by early March. While we have had a negative NAO reading at times this winter, the change to a positive PNA is quite a change in the eastern Pacific. It should result in a trough of low pressure to develop in the Gulf of Alaska and a corresponding downstream ridge of high pressure to form along the US west coast. This, in turn, should help to bring cold air southward into the US from Canada. One final note, experienced snow lovers and “weather weenies” also know that when the calendar flips to March, time is running short for snow as climatological temperatures begin to climb quite rapidly.

Stay tuned.