WRAL WeatherCenter Blog

Winter outlooks maintain "Equal Chance"

Posted November 19, 2012

NOAA's Climate Prediction Center (CPC) issued an updated set of seasonal climate outlooks last week, including the first two maps shown here, which show their assessments of temperature and precipitation trends for the 3-month period December through February. The first map shows that the southwestern United States is expected to have a good chance of above normal temperatures, but much of the country, owing to a lack of strong El Nino or La Nina temperature signals over the equatorial Pacific, is in a rather uncertain state and the temperature forecast can only show an "EC" indicating and equal chance that seasonal temperatures end up above, below or near-normal.

The precipitation outlook in the second image largely shares that "Equal Chance" forecast, although there is a notable area across the Tennessee Valley and into the mountains and foothills of our state, where there is a weak tilt toward an enhanced probability of above normal precipitation, based on a consensus of results from some long-term dynamic models used by CPC, taken together with the "La Nada" pattern in the Pacific.

These, of course, do not offer up much to go on regarding winter conditions across central NC. It does seem reasonable to think that we are unlikely to repeat the especially warm temperatures of last winter, but of course an "EC" forecast doesn't rule that out. One item of interest I have noticed is that snow cover across Eurasia as of the end of October was considerably greater than the same time last year. The departure of Fall snow cover there from normal has been found to correlate inversely with the following winter temperatures across the northeastern U.S., so perhaps the snow cover departure for Eurasia this year (shown in the third image, and amounting to about 1.22 million square kilometers above normal) could tilt the odds toward a colder winter. Historically the strongest correlation with these figures has been across the Great Lakes and into New England, but with a somewhat weaker signal down into our state. It's a very brief snapshot, of course, and offers up no guarantees about the winter to come, but a peek back at the past two winters shows that October 2010 featured Eurasian snow cover 730,000 square km above normal. This was followed by Dec-Feb temperatures across the northeast U.S. that ran about 1-3 degrees C below normal , and temperatures across central NC that were about 4-6 degrees C below normal that season. Last year, October snow cover across Eurasia was about 600,000 square km below normal, and was followed by temperatures across the northeastern U.S. around 2.5 to 3.5 degrees C above normal, and temperatures over central NC that ran 2-3 degrees C above normal for the period. It will be interesting to see how this all plays out in the next three-four months...


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  • Mike Moss Nov 20, 2012

    SaveEnergyMan, That's a reasonable question, but in a case like this I look at it as a situation where the CPC forecasters aren't exactly saying they don't know. What they are saying is that based on the patterns in place and how those have historically played out, along with the range of results from modeling, application of long-term trends and seasonal analogs, etc, what they "do know" is that the signals in place give an about equal chance to the above, below and near-normal outcomes for temp or precip in the areas that are listed as such. It isn't all that unusual to see that result in these long-range, seasonal outlooks, although less of the country tends to receive that rating when there is a strong La Nina or El Nino pattern in play, for example.

  • SaveEnergyMan Nov 19, 2012

    Weather forecasting is still as much art as it is science - owing to the complexities of fluid mechanics, heat & mass transfer, and a myriad of other parameters. But, forecasters aren't generally in the business of saying - "I don't know", are they?