Is Nashville About to Experience a "Very Warm" Winter?
Local Research suggests the current cold snap may be an anomaly.
La Niña is associated with a periodic cooling of tropical sea surface temperatures (SST) in the eastern Pacific. It is the counterpart to El Niño, which is a characterized by periodic warming of eastern Pacific waters. Both La Niña and El Niño are known to affect weather patterns around the world.
During a recent research project conducted at the National Weather Serivce in Old Hickory, TN, a unique La Niña climate signal was found in the weather records that correlated with very warm weather at Nashville during the winter and early-to-mid spring months. Based on historical records, if one is confident that the La Niña is approaching maximum strength (i.e., nearing an "SST max") and two successive three-month running temperature means show up during the three periods preceding the maximum that are "very warm," there is a good chance that a climatologically "very warm" weather is in the early stages of unfolding at Nashville. This unique climate signal has, indeed, shown up this year, as revealed in the temperature records from August through November.
The three-month period of August-September-October saw a temperature departure of +2.5o above normal. the following three-month period of September-October-November had an average temperature departure of +1.7o. Based on local research, this unique La Niña climate signal indicates the likelihood for frequent significant wam spells continuing for the next 3 to 6 months in the Nashville area--and, climatologically-speaking, a very warm winter.
There have been five La Niña events at Nashville where climate records show this unique signal of significant warmth leading up to the La Niña maximum:
within the SST Max - 3
back periods with
"very warm" weather
following SST Max
|La Niña 1973-1974||NDJ 1973||3.0o (ASO)||5.3o (SON)||4 (DJF - MAM)||+2.9o|
|La Niña 1974-1976||NDJ 1975||2.5o (OND)||1.5o (NDJ)||3 (DJF - FMA)||+4.5o|
La Niña 1998-2000
|3.6o (SON)||2.6o (OND)||1(JFM)||+2.3o|
La Niña 1998-2000
|2.2o (OND)||3.4o (NDJ)||3 (DJF - FMA)||+3.4o|
|La Niña 2007||DJF 2008||3.5o (SON)||3.9o (OND)||0||-|
Table Key: ASO= August through October;
SON= September through November;
OND= October through December;
NDJ= November through January;
DJF= December through February;
JFM= January through March;
FMA= February through April;
MAM= March through May
When this unique signal was observed, there were often three to four subsequent back-to-back periods after the La Niña maximum where the three-month running means were "very warm" (i.e., departure from normal > 1.2o), generally extending from early winter (the DJF period) all the way through early-to-mid spring (the FMA and MAM periods). Even though this signal did not give advance indication of the onset of "very warm" conditions, it usually showed up during the late summer and fall and indicated that frequent and significant periods of "very warm" conditions are on the way for a good part of the upcoming winter and much of the following spring. In addition, there is some evidence to suggest that the likelihood for significant warmth increases if the La Niña anomaly (over the Pacific Ocean) is less than -0.5o (which it currently is). To find out the strength of the current La Niña, go to the latest La Niña Update at http://www.cpc.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.pdf.
Based on the previous four La Niñas that were accompanied by extended periods of significant warmth, the La Niña maximum typically occurred during the NDJ or DJF period. As a result, the earliest period in the "SST Max - 3" window was the ASO period. Only the 2007-2008 La Niña failed to produce very warm weather past the period of the SST Max. However, during that particular event, the La Niña maximum occurred somewhat late (during the DJF period). The average temperature for that period was 2.5o above normal -- and included a December that was 5.8o above normal, a January that was 0.2o above normal, and a February that was 1.2o above normal. Although the temperature average for the DJF period indicated "very warm" conditions, the variable monthly averages (ranging from +0.2o to +5.8o) make an important point, and remind us that even during a "very warm" winter there are always intervening cold spells. For instance, in January, 2008 there was a 6-day cold snap, right in the middle of an otherwise "very warm" winter, when high temperatures struggled to get out of the 20s and 30s, and lows fell into the teens! Also, the Decembers, leading into "very warm" La Niñas winters, were often associated with intermittent cold snaps, where the highs were in the 40s and sometimes even in the 20s and 30s, and lows dipped into the teens.
This local research is in agreement with the latest winter temperature forecast from NOAA's Climate Prediction Center. (See http://www.noaanews.noaa.gov/stories2010/images/winteroutlooktemp_2_20102.jpg.)
As far as precipitation is concerned, the climate record for La Niña events at Nashville is highly ambiguous. There are no unique precipitation signals that show up. However, an interesting feature of the La Niña precipitation record is that whenever observed precipitation averages veered away from normal, they usually veered toward the extreme. For instance, of seven cases that were not "near normal," six of them were either "very wet" or "very dry." The most recent La Niña record, including data from 1973 through 2008, indicates ten times when La Niña Max anomalies were observed. Five of those events exhibited winter rainfall that was above normal and five saw winters that were below normal.
Based on this research, of previous La Niña events, we conclude that the odds are in favor of the upcoming winter at Nashville to average out warm -- and perhaps, very warm. All bets are off, however, when it comes to a forecast of the average precipitation.
As previously stated, even during "very warm" winters, there will be periods of cold weather when freezing or frozen precipitation can occur. Therefore, you can never rule out the possibility of cold snaps and/or winter storms during any Middle Tennessee winter -- even a very warm one! After all, it only takes a single major ice storm or a single significant snowstorm to make it seem like a "rough" winter.
Finally, it should be recognized that the number of historical La Niñas that have produced very warm weather at Nashville are relatively few in number (i.e., only five). Thus, from a statistical point of view, the amount of confidence one can put in a forecast based on such a small set of data is rather limited. However, as more data is collected from future La Niña events, the confidence for such forecasts will increase--provided, of course, the data continue to show the same type correlations. That's part of the joy of research--the more data a person collects the more confident he can be in his conclusions!
Note: To view the entire research paper, written by Mark. A. Rose and Darrell Massie, go to, http://www.srh.noaa.gov/ssd/techmemo/sr231.pdf.