By Terry Eliasen, WBZ-TV Executive Weather Producer

BOSTON (CBS) – Are you dreaming of a White Christmas? Like it or not, you have to admit that a little bit of snow around the holidays certainly helps to get you in the spirit.

Last year was one of the more bizarre holiday seasons: we actually had a white Halloween and a brown Christmas! In fact, after the freak Halloween storm, we didn’t have any accumulating snowfall in Boston until mid-January.

But, just two years ago, it was the exact opposite: a snow blitz for the ages began in mid-December and didn’t let up for six straight weeks. It wasn’t just a white Christmas that year, it was a whiteout. By the end of December, 2010 Boston had nearly two feet of snow on the ground.

So, it can be hard to predict. But that is nothing you haven’t heard before when it comes to New England weather.

So let’s take a look back, historically speaking, at the statistical probability of a white Christmas in Boston.. By the way, a white Christmas is defined as at least 1” of snow on the ground on Christmas day. Believe it or not, studies have been done by numerous researchers from The National Weather Service to Cornell University on this very topic. NOAA came out with this map several years ago, depicting the chances across the entire U.S.

Cornell University in New York recently came out with a study based on 50 years of data, listing several Northeast cities and their chances.

Snow probability

Snow probability

Some interesting cities to note…

Caribou, ME: 90%

Concord, NH: 73%

Worcester, MA: 60%

Providence, RI: 30%

Boston, MA: 20%

New York City, NY: 12%

Obviously the percentages increase quickly the farther north you go and with more elevation.

Snow probability

Snow probability

So what about this year?

Statistically speaking, the numbers are the numbers. Boston has about a 1-in-5 chance and Worcester about 3-in-5.

But as we noted, each winter has its own unique “footprint,” and this one is no different.

So far, Boston has remained snow-free mainly for two reasons. First, we had a very dry November; very little precipitation fell at all. It’s hard to get snow if nothing is falling from the sky.

Second, most of the storms systems have tracked to our west, and when this happens we are on the mild side of the storm and see mainly rain. This has been the case for all of December thus far. Now here is the technical part. I won’t bore you too much, but something called the NAO (North Atlantic Oscillation) becomes key this time of year in forecasting rain vs snow. Essentially, when the NAO goes “negative” it means that a block sets up in the northern latitudes near Greenland and this forces cold air southward from Canada. Often that comes with significant storminess along the East Coast. So far the NAO has been “positive” for the most part this month, but there are signs it is about to take a dip in the next week or so.

This has us watching a few storm possibilities over the next 7-10 days — the first of which is Sunday night and Monday — and then another one may follow by the middle of next week.

Track and precipitation type is still unclear, but it does appear that we have at least 2 chances for snow before the holidays.

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