Spring will begin officially next week and there are already signs for very warm air to develop across the southwest US over the next few days with high temperatures flirting with the 90 degree mark in some desert locations. That warm air will, however, have a very difficult time of advancing anywhere near our part of the country over the next couple of weeks as an impressive “omega-shaped” blocking pattern will develop in the upper atmosphere to our north across Canada. This upper atmosphere blocking pattern will affect our temperature pattern in a couple of ways. First, it will act to force additional cold air masses into the central and eastern US from northern Canada and secondly, it will prevent any of the warm air building up in the southwest US from expanding into the northeastern US – at least not on any kind of consistent basis – through the rest of the month of March.
Two indices that can provide clues about the developing “omega-shaped” blocking pattern in the upper atmosphere include the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO). These index values are related to the “oscillation” between the pressure differences in the North Atlantic and when they are in negative territory, blocking high pressure systems often develop around the Hudson Bay region of Canada. While this type of pattern allows for warming in that part of Canada, it also aids in pushing cold air masses into the northeastern part of the US. Both the European and GFS computer forecast models predict that the AO and NAO indices will remain in negative territory consistently through the remainder of March. In fact, the AO is about to tank from “slightly” to “sharply” negative values over the next week or so.
This overall colder-than-normal pattern will also be quite active and fast-moving over the next couple of weeks with a series of storms to potentially deal with, and snow cannot be ruled out for the Mid-Atlantic region given the expected cold weather. The first system will be a “clipper” that will quickly drop southeast from the Upper Midwest early this weekend and it’ll spread rain and/or snow showers into the I-95 corridor late Friday night and Saturday. A second system will then slide to our south early next week and it too can bring rain and/or snow showers to the Mid-Atlantic region.