Published: 2007-12-15 13:25:32
Updated: 2007-12-15 13:25:32
Posted December 15, 2007
By Jonathan Griffin
MIKE MOSS SAYS: Jonathan (and a few others who have written with the same question), I wish could give you some kind of quantitative answer that would be helpful, but all we have to go on for a seasonal snow forecast is some very large-scale trends and the way those trends tend to play out in terms of organizing the weather features that control our temperatures and precipitation. Unfortunately, while those large-scale influences do have some detectable tendencies that work out reasonably well when averaged over several years and across an entire season, they aren't as helpful in predicting specifics down to the level of a few days or a couple of weeks.
For our part of the country, those specifics are crucial when it comes to snowfall, and significant snow events here, being rather sporadic and fairly rare in the first place, can occur given the right combination of ingredients despite whatever overall seasonal trend happens to be in place.
With all those big caveats out of the way, a primary factor in seasonal forecasts is the state of the El Nino/Southern Oscillation pattern in the Pacific, that is are we expecting the winter to be characterized by an El Nino, La Nina or "La Nada" (near neutral) event? During the latter half of this year, a La Nina (cooler than normal sea surface temperatures across the eastern equatorial Pacific) has developed, gradually strengthening from weak to moderate intensity, and now forecast to be moderate to strong for the winter season. Historically, this tends to produce a pattern in which some upper level ridging is fairly common across the southeastern U.S. while upper level troughing and surface storm tracks are diverted a bit to our northwest. This leads to a seasonal tendency for temperatures to run warmer than normal in NC (especially central and eastern parts of the state) and for precipitation to trend below normal, while precipitation tends to be enhanced from about the mid-Mississippi Valley into the Ohio valley area. That is the current outlook from the Climate Prediction Center for the Dec-Feb winter period this season.
As a first approximation, then, it would appear our chances of significant snow are below average, and this may very well turn out to be the case. However, it is worth noting that La Nina winters are known for rather unstable large scale patterns in which the overall average pattern tends to be strongly disrupted two or three times for a few days to a week or so, allowing very cold outbreaks into the central and eastern Unites States. Some years, this just prevents the seasonal temperatures from being even higher than they would be, but they remain above normal and precipitation remains below normal. In some years, though, other ingredients (moisture, surface fronts, surface low pressure centers and upper level troughs, along with surface high pressure to our north) combine just so during these outbreaks to produce a few snowstorms in our state. An often-cited example is January 2000, at which time a quite strong La Nina was in place in the Pacific, and yet we experienced the greatest daily and monthly snowfall totals on record for the Raleigh area. If you subtract out that week and its associated cold and precipitation, the average over the remainder of that winter was warmer than normal with below normal precipitation, as one would expect with La Nina, but in that case the specifics of a particular combination of features during one of those La Nina cold snaps was just right to produce a major snowstorm anyway.
Those kinds of short-term details are impossible to forecast more than a few days in advance, so we're left with the somewhat unsatisfying answer to your question. Our chances of significant snow this year are probably lower than normal, but in spite of that a big winter storm or two simply can't be ruled out and we should be prepared to deal with them just in case.