Cohen’s forecasting research and methods, which has been supported by the National Science Foundation, are heavily focused on the behavior of Siberian snow cover in October.
Related: Siberian snowfall may help improve U.S. weather forecasts, meteorologist says (from 2011)
When Siberian snow cover extent is prosperous and increases rapidly in October, it provides a strong signal that a weather pattern known as the Arctic Oscillation (AO) will tend towards its negative phase during the winter months (December through March).
When the AO is negative, frigid air spills south from the Arctic and temperatures are usually colder than average over eastern North America and western Europe. In the D.C. area, the overwhelming majority of our big snowstorms have occurred in winters in which the AO averaged negative.
Conversely, when there’s little October Siberian snow cover and it advances slowly, the AO is typically positive during winter.
A positive AO usually means cold air is bottled up in the Arctic and it’s mild in the mid-latitudes. In the D.C. area, snow is typically scarce in AO positive winters.
The statistical correlation between the rate of October snow advance in Siberia and the phase of the AO, demonstrated by Cohen, is striking.
This October, Cohen says, the snow in Siberia advanced slowly, a sign that the positive phase of the AO may well dominate this winter – which would favor above normal temperatures on the East Coast and lower odds for big snows.
But there’s one wrinkle this year, Cohen says. Even though Siberian snow advanced much more slowly than normal this October, its overall extent was substantially above normal. Such a mixed signal makes this year’s AO forecast more uncertain, Cohen stresses.
Cohen offers additional insights and detail about his winter outlook in the interview below, which we conducted via email.
Q: Describe the state of Siberia snow in October. How abundant was it and how quickly did it advance?
A: This October was highly anomalous in that the snow cover extent [SCE] and the snow advance index [SAI] strongly diverged. The SCE was well above normal, which is a robust prediction of a negative winter AO while the SAI was well below normal, which is a robust prediction of a positive winter AO. This has never happened over the forty years that we have calculated both indices. I view the SAI as the more reliable predictor but I also think it would be a mistake to dismiss the high Siberian SCE from this past October. How the two predictions evolve this winter should be interesting.
Q: What are your thoughts on the AO through the winter?
A: I have to admit that I am struggling with how to interpret the conflicting predictions based on Siberian snow cover. . . . Based on what I have seen so far the hemispheric circulation of the atmosphere has evolved more consistently as if the Siberian snow cover was below normal or closer to the prediction derived from the SAI. Therefore, this favors an overall positive winter AO. However I do see signs, based on the large-scale energy propagation of the atmosphere, that favor building of high pressure in the mid-high latitudes in the near term and that should help to support colder temperatures and even a negative AO right at the start of winter. Eventually I would expect atmospheric conditions to favor a more positive AO but that is no guarantee.
Q: What are your thoughts on weather and temperatures in the Eastern U.S., Mid-Atlantic region this winter?
A: It is looking as if the winter will get off to a relatively cold start in the Eastern U.S. and Mid-Atlantic. If that cold air can generate an extensive snow cover across the U.S. the cold can become self-perpetuating and persist longer.
Still, based on the SAI prediction of an overall positive winter AO, I would expect temperatures to eventually turn milder.
However, it is important to recognize the limitation of the forecast. Using snow cover to predict winter temperatures is dependent on the assumption that the AO is a reliable predictor of temperatures in the Eastern US. …the relationship between the AO and surface temperatures is stronger across northern Eurasia than eastern North America. The AO has been strongly positive this month so far; temperatures across northern Eurasia are well above normal yet in the Eastern US temperatures will average below normal for the month. The positive AO is associated with low pressure across the Arctic, which favors stronger westerly winds that has drawn milder air deep into Eurasia. At the same time, the positive AO is also often associated with ridging in the Central Pacific that then allows a trough along the West Coast that drives mild Pacific air across the US. However, this month the Pacific ridging is further east than its average position, closer to the West Coast, so instead that ridging has allowed cold air to slide out of Canada into the Eastern US. It is those kind of regional subtleties that are hard to predict ahead of time but yet can make or break the forecast.
Q: Can you offer some thoughts on snowfall in D.C. this winter?
A: If the forecast holds up and it is a warm winter in D.C., I think it would be going out too much on a limb to predict above normal snowfall. However, I do think that high SCE value does tilt the probability to better than climatology that there is at least one large snowstorm (12 inches or more) in the Northeast this winter, somewhere between D.C. and Boston.