Winter 2011-2012: What will the weather be like in Middle Tennessee?
There are differing ideas within the scientific community.
By this time of year, many people are wanting to know what forecasters are thinking about the weather for the upcoming winter. Understanding that the accuracy for long term forecasts is considerably less than for short term forecasts, people nevertheless want to know what the experts' "best guess" is for the upcoming season.
The most recent temperature and precipitation forecasts by the Climate Prediction Center (CPC) are shown below, indicating above-normal temperatures and above-normal precipitation for Middle Tennessee.
Long term climate forecasting is similar to shorter-term weather forecasting insofar that both depend on a large number of variables. The problem that is faced by all forecasters is trying to determine the amount of "weight" that each variable will carry in the forecast. For instance, if a weather forecaster expects heavy rainfall to move across the area, he or she must determine how rapidly it will move. If it moves very slowly, he might expect an enhanced potential for flooding. If it is expected to move swiftly, flooding potential will be more limited. Thus, it is important to provide a great deal of weight to both the amount of rainfall expected and the speed at which it moves across the area. Climate forecasting is similar, but is more statistical in nature--depending not only on the variables in play, but also the statistical probability that a certain combination of variables will produce a particular type of outcome (i.e., above- or below-normal temperatures, above- or below-normal precipitation, etc.)
Scientists are constantly gaining new insight into both climate and weather forecasting. New theories are constantly being advanced and tested and new computer models developed.
One of the most interesting aspects of this upcoming winter is that a La Nina event is expected to play a dominant role. This was also the case during the winter of 2010-2011. The CPC graphics above indicate the statistical probabilities for certain types of weather for the winter of 2011-2012. For Middle Tennessee the forecast is for above-normal temperatures and above-normal precipitation. This is similar to the forecast issued last year. However, as we all remember, a large part of last winter in the Mid State was brutally cold and snowy. From mid-December through mid-February, the weather was highlighted by a negative Arctic Oscillation and consistently deep upper level low pressure systems over eastern parts of Canada and the United States. The result was a "pipeline" from the Arctic, sending wave after wave of frigid air sliding southward into Middle Tennessee. Even though temperatures warmed up pretty well for the last third of the winter, the dominant part of last winter was considerably colder than what was indicated by most pre-season climate forecasts.
Climate researchers Judah Cohen and Christopher Fletcher have found evidence that average winter temperatures over the eastern United States are correlated to the extent of Siberian snow cover in the preceding October (see, "Improved Skill of Northern Hemisphere Winter Surface Temperature Predictions Based on Land-Atmosphere Fall Anomalies, Journal of Climate, August 2007, American Meteorological Society). Just prior to last winter they pointed to the fact the amount of Siberian snow cover in October 2010 pointed to a colder-than-normal winter over the eastern United States. As things turned out, their forecast was certainly in the ball park. There's no way to know, of course, if the Siberian snow cover was the dominant variable that led to those conditions. Nevertheless, it is an interesting thing to consider.
Below is a NOAA map that shows the extent of Siberian snow cover on November 1, 2010:
Below is the snow cover in Siberia that existed a few weeks ago, on November 1, 2011:
As you will notice, when comparing the two graphics above, the amount of Siberian snow cover at the end of October this year was actually greater than what was observed at the end of October last year. The big question now becomes: What will be the degree of impact of this feature be on the weather in Middle Tennessee for this upcoming winter? Just like last winter, there are differing ideas within the scientific community on what the dominant types of weather might be. However, if we're headed for similar weather as last winter, it's probably a good idea to at least keep the snow shovel and ice scraper handy.