It’s the question on the mind of anyone who steps outside during this so-called winter: Where’s the snow?
Boston typically receives 10 inches of snow in November and December. Worcester usually gets about 16 inches. Last winter, the region was socked by repeated storms, leading some to speculate that we were entering a vicious snowmageddon cycle. The October snowfall this year reinforced that notion, but meteorologists are using a word, not a number, to describe the snowfall amount since then: trace.
It’s risky to make long-term predictions about the weather, and forecasters warn that the lack of snow so far does not mean winter won’t deliver a punch this year.
The scientific explanation for the unseasonably warm and snowless holiday season stems from a phenomenon called the North Atlantic Oscillation, which can be in either a positive or negative phase. When there is higher pressure in the Arctic, the oscillation is negative, as it was the last two years. The jet stream that moves from west to east dips southward, bringing cold air into New England and conditions that favor a snowy winter. When the oscillation is positive, as it is now, it tends to set up a fast-moving river of air that flows from west to east and keeps the cold air north in the Arctic, and the warmer air below.
“In a positive phase, everything is really locked up and there’s very little mixing,’’ said Judah Cohen, director of seasonal forecasting at Atmospheric and Environmental Research, a Verisk Analytics company in Lexington. “Everything’s confined to different regions: The cold air is locked up in the high latitudes.’’
Cohen has also found a relationship between Siberian snow cover in October and the wintry conditions the Northeast receives. Snow in Siberia, he said, builds up a cold dome of air, which creates a disturbance that he compares to “opening the refrigerator door.’’ This year, he said, the snow cover in Siberia was low - until the very end of October.
Michael Rawlins, assistant professor of geosciences and manager of the Climate System Research Center at the University of Massachusetts Amherst, said the National Oceanic and Atmospheric Administration’s forecast for the winter stated there was an equal chance for temperatures and precipitation in the Northeast to be normal, or above or below normal. He noted that another oddity of this year’s winter has been the lack of a phenomenon called blocking, when a high-pressure system sits over Greenland and causes weather systems traveling from the west to dip southward.
“Think about a river of air. . . . When you have a blocking high [pressure system] that sets up in winter, it slows down the whole area, like putting a rock in a stream,’’ Rawlins said. But without blocking, weather systems are traveling rapidly and moving on. He said that is what is expected to happen to the cold front expected to arrive today: It will chill things down to convincingly wintry temperatures but then move on.
Matt Noyes, chief meteorologist at New England Cable News, said the warm and snow-free winter has not been much of a surprise. La Niña, a phenomenon that results in cooler water temperatures in the Pacific Ocean, usually means that winters here will be warmer than normal with more mixed precipitation, he said. In addition, the droughts in the South meant that there was more residual heat.
“There comes a point where you exhaust a lot of the heat energy, and we’re just starting to reach that tipping point,’’ Noyes said.
He said the region will almost certainly have below-average snowfall this winter and there are currently no storms that he is tracking. But predicting that the entire winter would follow the early-season pattern would be foolhardy, he added.
Because the North Atlantic Oscillation can flip back and forth, it is difficult to make long-term predictions in the Northeast, and most weather specialists think it has a good chance of changing before the winter is over.
“These types of slow winters to start are not unusual; it’s just unusual it’s lasting this long,’’ said Bill Simpson, a meteorologist at the National Weather Service in Taunton. “There is some hint that it might be changing, but not until the end of the month.’’