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Posted at 01:00 PM ET, 03/21/2011

Cold pattern coming back to East


The North Atlantic Oscillation (NAO) is forecast to go negative this week, which signals cold air from the Arctic spilling into eastern North America from late March into early April. (NOAA)

Later this week, there is a high likelihood of a winter-like regime settling in over the Eastern U.S. for at least a short stretch of time (five or so days). Beginning Thursday, the high around 80 degrees (in the D.C. metro region) just six days earlier may not seem real. You may ask yourself, what spring?

Signaling the pattern change is a flip in the North Atlantic Oscillation (NAO). The NAO is forecast to switch phases from positive to negative . Remember, when the NAO is negative, it signifies areas of blocking high pressure over the Arctic in the vicinity of Greenland which help drive cold air over eastern North America. In the positive phase of the NAO, pressure tends to be low over the Arctic and that’s where the heart of the cold air stays. For the past six weeks, the NAO has remained positive for the most part. Not surprisingly, temperatures have been much warmer than average.

But NOAA’s Global Forecasting System (GFS) model (and most of its ensembles) predicts the NAO index to tank this week, to levels not seen since December and early January when temperatures were much colder than average.


Temperature relative to average this Saturday. The relatively warm (10 to 20 degrees above average, shades of yellow and red) air over the Arctic and colder than average temperatures (by 6 to 12 degrees, blues and greens) over Eastern North America are highly characteristic of a negative NAO pattern. (Source: Florida State, Ryan Maue )
The European model also suggests the NAO will trend negative. NOAA’s Climate Prediction Center is on board with the idea of a cold pattern, forecasting below average conditions for the eastern U.S. in both its 6-10 and 8-14 day outlooks.

Over at Weather Bell, chief forecaster Joe Bastardi writes he believes this will be the biggest late March cold outbreak since 2001. Bastardi thinks this year’s cold shot will have longer duration than 2001.

“The cold did not hold as strong in April as I think it will this year, as I have been warning folks about this going through mid month in April,” Bastardi wrote.

So how cold will it be the D.C. metro region? Early indications are that temperatures will probably average 5-15 degrees below averages during the heart of the cold snap. Instead of highs near 60, we’ll probably be looking at highs averaging around 50. Some days may not see highs escaping the 40s (or even the 30s if there are clouds and precipitation). But because it’s spring and the sun is strong, we still may fit in a mild day or two day in the cold pattern. While the negative NAO favors colder winds from the north and northwest , a brief relaxation in the pattern could bring back winds from the south boosting temps to average or above average levels.

Could it snow? There’s a chance. The combination of colder than average conditions and an active spring storm pattern may bring the necessary set of ingredients for some spring snowflakes. But because of the strong March sun angle and warm ground, accumulating snow isn’t all that likely, not to say it hasn’t happened before. On March 28, 1942, a foot of snow fell in the metro region. A nighttime storm coupled with heavy precipitation rates would give us the best odds of seeing flakes stick.

Generally speaking, though, the odds of snow are low despite the cold pattern given how late in the season it is. However, as you head north toward the Mason Dixon line, and west into the mountains, the odds increase to better than 50/50 pretty quickly.

We’ll be closely following both the progress of the cold air and any storm systems that could interact with it starting late this week on into next week.

By  |  01:00 PM ET, 03/21/2011

 
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