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Is winter over?

Posted by David Epstein  February 1, 2012 06:22 AM

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Ah, February. Welcome to the month of sunshine, temperatures in the 50s, buds opening, birds chirping, crocus and daffodils. I know it's a bit scary out there lately wondering what happened to our winter but, I am here to assure you it's all normal. Admittedly, as much as I love the bare ground, the ability to do some gardening and the savings on my heating bill, this weather is a little freaky. So what's next?

First, this winter, like all winters, is part of the normal cycle of highly variable temperatures and snowfall. As I have said many times to folks when asked, "normal weather is just the average of extremes". Remember, it was 12 months ago everyone was in a panic about roofs collapsing, where they could find a place to put the snow, and fears of spring flooding from rapid melting. Don't worry, our climate hasn't done a 180 degree turn in a year.

The question on many of your minds may be what will the next 8 weeks bring? February is traditionally our snowiest month and March can be just as snowy too. This winter the cold air has been locked up in Canada and Alaska. Fairbanks, Alaska saw temperatures stay below zero for almost the entire month of January. This week the town of Jim River, Alaska recorded a temperature of -79°F when the temperature gauge broke. The all-time record for the United States is -80°F.

There are years when we have seen virtually no snow in the first two months of winter, December and January, and then gotten hammered with snow during February and March. In the winter of 1955-56 we only had about 12" of snow up until this part of winter. That year, nearly 50 inches of snow fell in the next 12 weeks. The winter of 1968-69 saw the short month of February measure over 41 inches of snow and that wasn't a leap year. These are of course extreme examples, but snowy winters have started very late.

Much more likely is that we will have two to three more episodes of snow and short-lived cold outbreaks. Even if we were to get a storm with 6 inches or more of snow, I suspect it would melt fairly fast. The current state of the atmosphere is just not conducive to cold and snow in this part of the world . Mid -Febsmall.jpgFurther the models which forecast out to the middle of February are not showing any major signs of change.

The North Atlantic Oscillation

There are all sorts of patterns that the computer models use to help forecast our weather. One of these patterns or indices is called the North Atlantic Oscillation or NAO. This index is a measure of the difference in pressure systems between a semi-permanent low over Iceland and a semi-permanent high over the Azores. As those systems strengthen and weaken, the index fluctuates between positive and negative values. We have data that goes back for decades showing the NAO value. nao.timeseries.jpg Snowy years tend to have negative values (blue) and years with less snow have positive values (red).

Arctic Oscillation

Another index that has an effect of winter weather in the United States is the Arctic Oscillation. This index fluctuated between positive and negative values. Positive values tend to keep much of the cold air locked across Canada. The index has turned negative recently and this could mean cold air will have an easier time moving southward. AO large.jpg There are all sorts of combinations of both the NAO and AO that affect our weather. Last year both indices were negative and most likely heavily contributed to our rough winter. While we are gaining a better understanding of these indices, there is much more we can learn. There are other oscillations as well like the Pacific Decadal Oscillation (PDO) and the Madden Julian Oscillation (MJO) each playing a role in our weather. With so many combinations of the various indices you can quickly see why it can be hard to predict the future. Then there are El Nino and La Nina both of which have tremendous impacts on our weather. We have seen a double El Nino over the past few years. These phenomena are cycles of warming and cooling of ocean water in the Equatorial Pacific.

Past Winters
In the winter of 1979-80 the NAO index was mostly positive. We had very little snow and very mild temperatures. I believe this winter is a repeat of sorts, of that cycle of a very positive NAO. There is research that shows when the NAO index flips from one extreme to the other big storms can develop. The blizzards of 1978, 1993, and 2006 each came around the time the index was flipping.

Predicting The Future
There are computer models that make an attempt to predict how the NOA index will behave over the next couple of weeks. We can see that we have been in a mostly positive NAO phase since October. Right now, the NAO index is forecast to remain positive through the middle of the month. The various forecasts are the collection of red lines at the end of the time series. Each of the red lines represent a separate model prediction for the NAO. NAO predicting.jpg Since most are in the positive range there is a good chance the prediction will be validated. (if the models all disagree, confidence is much lower in what will happen) If the NAO is positive for two more weeks, we are less likely to see any major storms or arctic cold during that time. Postives.jpg Once we get beyond mid-February the chances of arctic cold diminish each day. As long as the global patterns are locked in the mode they have been all winter we have a very good shot of having one of the top 5 least snowy winters on record.

Follow me on Twitter at @growingwisdom and check out my latest videos at GrowingWisdom.com

This blog is not written or edited by Boston.com or the Boston Globe.
The author is solely responsible for the content.

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About the author

David Epstein has been a professional meteorologist and horticulturalist for three decades. David spent 16 years at WCVB in Boston and currently freelances for WGME in Portland, ME. In 2006, More »
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