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Saturday, January 5, 2013

2012 sets record in Nashua for mild temps

Doug Webster

For those who think there’s a lot of snow on the ground, how quickly you forget.

Just five years ago, 2007, was the snowiest December on record in southern New Hampshire, with nearly 40 inches on the ground. ...

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For those who think there’s a lot of snow on the ground, how quickly you forget.

Just five years ago, 2007, was the snowiest December on record in southern New Hampshire, with nearly 40 inches on the ground.

But compared with the beginning of the month, you’re right, there’s a lot more snow than we started with.

Winter got off to a slow start in December, leaving many of us wondering if a repeat of last winter’s lack of snow and mild temperatures was on the way. Christmas was brown, but then back-to-back snowstorms ushered in a white New Year.

Similarly, temperatures were quite mild during the first half of December, but settled back down to a little closer to the seasonal norm for the second half of the month.

After a dry November, we saw several precipitation events during December to help to erase a bit of the dry anomaly for the calendar year.

The position of the jet stream can be blamed for the relative lack of winter weather through mid-December, but it also was responsible for the arrival of cooler temperatures and snow late in the month.

The polar jet stream is generally the path through which storm systems travel and also separates warm air to the south from cold air to the north. Through most of the first half of December, the ribbon of high-speed air about 10 miles above the Earth’s surface flowed from the Pacific Northwest states through the north-central Rockies and then turned east-northeast toward southeast Canada.

This path left New England mostly on the warm side of the main storm track, bringing our region mainly rain during storms prior to Christmas. Changes began to take place near Christmas, as high-latitude blocking in the area of northeast Canada and Greenland developed.

High-latitude blocking is a roadblock in the upper atmosphere that can occur in the Arctic that can deflect the jet stream farther south than normal, delivering cold well to the south.

The easiest analogy would be a stone in a stream that forces the water to go around it. In our case, the stone is the high-level high-pressure area near Greenland and the jet stream is the water flowing around it being deflected to the south.

Periods of blocking normally will come and go during any given winter and vary with duration and location from year to year. Last season saw virtually no blocking after October’s snowstorm until later in April and May.

After Christmas, storms were forced to take more of a southern track, depositing snow across our area and bringing temperatures down to levels more in keeping with the season. Northern portions of New England saw a couple of sizable snows earlier in December, and with the addition of late-month snows, brought about a great start to the ski and snowmobile season.

So now that winter has begun to make its mark, will wintry conditions continue or take a break? Given that a neutral El Nino – Southern Oscillation (ENSO) continues, the idea of changeable weather is the best route for now. More than likely, we’ll see periods of wintry weather for a week or two at a time, followed by mild weather with no real set pattern lasting through the winter.

Plenty of information concerning ENSO and how it affects the world’s climate can be found at www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/enso.shtml.

Temperatures during December were above normal after a colder than normal November. The average temperature at Nashua’s Pennichuck Water Works was 33.2 degrees, or 4.3 degrees above normal, making December the 10th warmest on record since 1885.

No record high or low temperatures were recorded, and we saw no blasts of Arctic air like we can see at this time of year. All of the Arctic air remained bottled up across Alaska and western Canada, where harshly cold temperatures that started during November carried on during December.

The mild December helped establish a record mild annual temperature for Nashua. The average temperature during 2012 was 51.3 degrees, 1.1 degrees warmer than 2010 and 3.8 degrees above normal. The majority of the warmth last year was because of January through May, after which temperatures were closer to normal.

Precipitation, including melted snowfall, for December totaled 5.24 inches for the Gate City, making it the 20th wettest since 1884. The surplus of 1.39 inches helped erase some of the deficit for 2012.

The 2012 precipitation total of 42.00 inches was 5.97 inches below normal, putting the year in the middle of the pack at 64th driest since records began in 1884. Much of the dry departure was because of January through April. From May through December, precipitation totaled almost exactly normal for Nashua.

The late-December snows helped push the snow total to above normal levels. The 13.8 inches of snow that fell last month was 2.2 inches above normal. The seasonal snowfall now stands at 14.3 inches, or 0.6 inches above normal.

Seasonal snow totals at this time last year were also a bit above normal, but we ended up with the 10th least snowy season on record. That’s merely an observation, not a forecast.

Temperatures don’t necessarily relate to how much snow we can see during the winter. As December showed us, we can have very mild temperatures and above-normal snowfall. There have been cold months with little snow, warm ones with little snow and everything else in between through Nashua’s more than 100 years of data.

Last winter’s lack of snow was much more about the lack of storms and dry weather than temperature. It’s important to note that during a New Hampshire winter, it can still be cold enough to snow even when it’s milder than normal.

Christmas was officially a brown one this year. While a dusting of snow was on the ground on Christmas morning, we need an inch or more to qualify for a white Christmas. Four of the last six years have seen a white Christmas, which is a bit above the long-term average of 59 percent for Nashua.

Another three months or more remain during which we can get snowed upon. Even though few flakes are foreseen in the short term, that doesn’t mean we won’t be complaining about all of the snow a month from now. Only time will tell.

Weather & Climate appears the first or second Saturday of the month, depending on when final weather data are available. Doug Webster, of Hudson, is senior meteorologist at Telvent DTN in Woburn, Mass.