Published: 2011-11-14 08:58:35
Updated: 2011-11-14 08:58:35
Posted November 14, 2011
By Mike Moss
Some of you may have seen a post last year around this time in which I mentioned some research that indicates a typically inverse relationship between winter temperatures across the northeastern U.S. and the departure from normal of snow cover across Eurasia in the October-November time frame. If not, you can review that blog item at www.wral.com/weather/blogpost/8625051/.
While large-scale correlations of this sort don't provide us with guarantees as to future seasonal patterns in a specific year, it is another clue in the toolbox for longer range outlooks. The data for Eurasian snow cover for October 2011 is now available from the Rutgers University Global Snow Lab, and I've included an image here. Compared to last year, a significantly larger part of that region has normal or below normal snow cover, while a smaller portion is above normal or, especially, much above. Putting numbers to it, we find that at the end of October 2010, snow cover was about 754,000 square kilometers above normal overall, while at the end of this October, the coverage was around 568,000 kilometers below normal. That means this particular climate influence could contribute to a warmer than normal temperatures, on average, across the northeastern U.S. this winter.
Of course, such an influence can't be taken in isolation, and the winter outlook from the U.S. Climate Prediction Center also takes into account some other long-term trends and even more so more immediate factors such as the state of the El Nino-Southern Oscillation in the Pacific. This year, that appears to be headed for a weak-moderate La Nina pattern involving somewhat cooler than normal ocean temperatures across the central and eastern equatorial Pacific. The resulting outlook from the CPC follows fairly closely to the historical correlations associated with La Nina, but perhaps modified a bit to reflect the expected weakness of the pattern this year. The second and third images above show the winter outlooks for temperature and precipitation, and are in many ways similar to last year's outlook, when a stronger La Nina was anticipated.
For our state and the northeast, CPC projects an "equal chance" forecast for temperatures above, near or below normal, while above normal precipitation is most likely across the Ohio Valley and the northwestern U.S., with a dry pattern expected across the deep south and much of the southeast. Overall, for our local area it simply doesn't give us much to go in terms of how winter will turn out, and none of the larger scale climate patterns has shown much predictive power when it comes to winter snow and ice. We typically have to "wait and see" with forecasts for wintry weather only becoming apparent a week or two in advance (and often less), and that remains the case for the winter to come. Of course, not far to our northeast, the winter weather season got off to a fast start, with the heavy snow and high winds that recently impacted that region.