Published: 2010-11-15 08:26:10
Updated: 2010-11-15 08:26:10
Posted November 15, 2010
By Mike Moss
It's been out for a couple of weeks now, but in case you haven't seen the winter outlook from NOAA and its Climate Prediction Center, here are a couple of maps that illustrate the forecast as it stands based on current projections of temperature and precipitation tendencies.
Long-range outlooks like this are quite general, of course, and simply attempt to give a sense of where temperatures and precipitation will, on average through the winter, tend toward higher or lower than normal values. This year, the major influence used by the climate forecasters is the La Nina pattern in the Atlantic, which is expected to remain at moderate to strong level through the winter and at least into early Spring of 2011. There is a well-known correlation of La Nina to above-normal temperatures across the southern U.S., for example, and also a correlation to below normal precipitation across the south and parts of the southeast, and you'll see both of those tendencies reflected in the location of the orange shading in the maps.
Other long-observed correlations with La Nina include areas of above-normal precipitation across parts of the Midwest and eastern Great Lakes region and the Pacific Northwest, and below normal temperatures along the west coast and the northern plains, and these are reflected in the CPC outlook as well.
What this means for central North Carolina is a little tougher to nail down with confidence. While the maps, and the historical influence of La Nina, argue toward a tendency for the winter to average drier and warmer than normal, it is not a very strong signal for our area. You'll notice that in both cases, we are just within the orange shading indicating about a 35 percent chance of ending up in the "below-normal" category, with that chance increasing off to our southwest (for temperature) and our southeast (for precipitation).
The manner in which CPC constructs these outlooks means that we therefore have about a 35 percent chance of above normal temperature and below normal precipitation (that is, falling into the range of the warmest or driest 10 years of the 30 used to compute "normal"), about a 33 percent chance of near-normal (in the range of the middle 10 years), and about a 32 percent chance of below normal temperature and above normal precipitation (in the range of the coolest and wettest 10 years). When you look at it that way, you realize that it is not a super high-confidence outlook! It is worth noting that if the forecasters were simply predicting whether the winter averages would be above or below the specific long-term "normal" values rather than above or below a normal "range" of values, the confidence in drier and warmer than normal levels would appear a little higher.
The question also comes up - what does this mean regarding our chances of snow? Not much, as there are just no reliable methods of forecasting on a seasonal basis whether snow will run above, near or below normal, since snow is so dependent on very detailed combinations of temperature, moisture and lift-producing weather features that make it hard to predict a few days (and sometimes hours!) in advance, much less weeks or months ahead. I took a look at our last 5 winters with a moderate or strong La Nina in place, and the winter snowfall totals at RDU were as follows (average is 7.1 inches): just one-half inch in 2007-08, 25.8" in 1999-2000, 12.0" in 1988-89, 4.1" in 1984-85 and 3.0" in 1975-76. It's a small sample, but the variability speaks for itself!
Some of you may have heard about another climate influence that appears to inversely correlate with winter temperatures in the northeastern U.S., and that is the Fall snow cover over Siberia, or Eurasia. This can be an influence that magnifies or counters that of La Nina or El Nino. The general observation is that the greater extent and depth of snow cover in Eurasia in the October to December time frame, the colder winter temperatures are likely to be in the northeast U.S, with the strongest relationship over the Great lakes area into New England, but perhaps a lesser impact farther down the eastern seaboard as well. This year, October snowfall in Eurasia is a bit of a mixed bag as seen in the third image, which shows a map of snow versus normal from the Rutgers University Global Snow Lab. The blue areas in Eurasia indicate above normal snow, while the orange shading indicates below normal. Overall, the measured snow extent of 10.6 million square kilometers there was somewhat above normal, but only by about half as much as the same time last year. It will be interesting to see how this snow cover trends through November and December, and whether temperatures in the northeast follow the historical relationship I mentioned. Right now, the CPC outlook for northeast temperatures is for an equal chance of above, below or near normal values.