Every October, seasonal forecasters pay close attention to the trends in snow cover in Siberia, as they have shown to have some relationship with winter weather conditions in the eastern United States. When Siberian autumn snow is expansive and increases quickly, it tends to favor a cold winter in the East; whereas, when it’s scarce, a mild winter is more likely.
The meteorologist who discovered this relationship, Judah Cohen of Atmospheric and Environmental Research, says this October’s Siberian snow cover is off to a fast start, which may portend another cold winter for the East.
“I think that [the Siberian snow cover] will be above normal,” Cohen said in an e-mail. “[But] it is lagging the two blockbuster Octobers of the past two years.”
In technical terms, above normal Siberian snow cover in the fall is strongly linked to the negative phase of the Arctic Oscillation (AO) during winter. A negative AO is associated with a weaker polar jet stream that tends to stray more into the mid-latitudes, unleashing blasts of Arctic air.
“The predictors that I look at all seem to be indicating to me a negative bias for the upcoming winter AO,” Cohen said. “Of course, a negative AO increases the chance of a cold winter in the eastern U.S.”
Last October, snow cover over Siberia increased at a furious clip and spanned the second greatest area since records began in 1972. “The signal from the snow cover was both strong and consistent,” Cohen said heading into last winter.
Based on the snow cover behavior, Cohen forecast a colder than normal winter for the eastern U.S. He was not only right that the winter would be a cold one, but also accurately forecast that the second half of winter would be significantly colder than the first half.
Cohen shared his 2014-2015 winter temperature forecast (below right) compared to what actually happened (below left) and the match is remarkable. “As far as seasonal forecasts go, that is incredibly accurate with the gross features correct and differences only in the details,” Cohen said.
Cohen also correctly predicted above-normal snow for the large northeastern cities, including Washington. He called for 24 inches in the District, whereas Reagan National received 18.3 inches and Washington Dulles 36.9 inches. Notably, he said the heaviest snow would fall between New York City and Boston. “That turned out to be truly prescient and will likely be a career forecast,” Cohen said.
While Cohen’s forecast was close to spot-on and Siberia’s fall snow boom preceded a brutal winter in the eastern United States, some critics wonder whether he got the right forecast for the wrong reason.
The AO, which Cohen forecast to be strongly negative last winter, was actually positive. Cohen counters the AO is just a statistic and that his model based on Siberian snow cover correctly pegged the evolution of atmospheric features that led to the harsh winter in the East.
“I feel confident in saying that the atmospheric physics or dynamics unfolded this winter consistent with the dynamics we have argued for how extensive snow cover influences the atmospheric circulation in late winter. It just didn’t project on to the negative phase of the AO,” he said.
Cohen has making winter predictions for 15 years. “In the scientific literature our model remains the most accurate to date,” he told the Boston Globe last spring. The Globe said his winter outlooks have a 75 percent accuracy rate, better than the National Weather Service.
While Cohen’s early comments about the 2015-2016 winter suggest he’s leaning cold in the East, he cautioned: “It is very early and I have not issued an updated winter forecast.”
He stressed he’s not only considering Siberian snow cover for his outlook. “I am also watching El Niño like everyone else,” he said. “I usually don’t give much weight to El Niño in making a temperature forecast but this is a big one.”
Cohen will produce his detailed winter outlook in November and we will share it here at the Capital Weather Gang. Our own winter outlook will be issued between Halloween and Veterans Day.
More from the Capital Weather Gang on Judah Cohen’s work: