Students and teachers love them, but in Middle Tennessee, snow days aren't very frequent
Winter temperature outlook from the Climate Prediction Center
Winter's precipitation outlook from the Climate Prediction Center
Tennessee's scorching summer days have already faded to cold Autumn mornings. This time of year, weather enthusiasts always ask me the same question: "is it going to be a snowy winter?"
I'm a winter weather fan, so this is a question I often think about.
The odds for lots of snow are not stacked in Nashville's favor. Middle Tennessee, especially the lower elevations that form the "Nashville bowl" where the majority of our metropolitan population lives, isn't positioned to receive reliably big snowfalls. You know the deal: a typical winter system in Tennessee usually offers cold rain at the onset, then a mix of rain and snow, before finishing with the possibility of snow showers as cold air rushes in from the northwest as the system departs. In these situations, surface temperatures usually begin above
32 degrees, prohibiting the chance of an "all snow" event. The reason that most winter systems end up as cool and wet instead of cold and white is that very often, the moisture and cold air don't phase together. Promising-sounding predictions of rain changing to snow rarely result in consequential snow totals for the Mid-South.
Nashville's average annual snowfall is about 6". That's based on the observed weather from 1980 through 2010. Before 2010, when the "average" was computed using the observed weather from
1970 to 2000, Nashville's typical snowfall was a more-impressive 9" per winter. The very snowy 1970s (five winters with double-digit snowfall that decade) skewed those results.
If you look at the observed snowfall from the past 20 winters in Nashville, you'll be disappointed. Only three of those winters had above-average snowfall (1995-96, 2002-03, and 2010-11).
Three winters were near-average (1996-97, 2001-02, and 2009-10). A whopping 14 of the past 20 winters (70%) saw below-average snowfall. You can (correctly) argue that in an average winter, Nashville will only encounter a few opportunities for accumulating snow.
So what about this winter?
You've probably heard about El Nino and La Nina, which happen when water temperature in the equatorial Pacific Ocean are warmer or cooler than average. Generally speaking, an El Nino winter offers a better opportunity for snowfall in the Mid-South because the southern branch of the jet stream (which steers large storms from west to east) becomes more active. This results in enhanced precipitation across the southern United States. Additionally, cooler-than-average winter temperatures are usually found across the southeastern United States during an El Nino.
During a La Nina winter, the equatorial Pacific is cooler than average, leading to less storminess there, resulting in a more suppressed southern branch of the jet stream. This typically results in less precipitation for the southern United States. Temperatures often end up being warmer-than-average in the southeastern United States during a La Nina pattern.
The forecast this fall and winter call for neither an El Nino or a La Nina. We affectionately call this pattern "La Nada."
Six of the past 20 winters have featured a La Nada pattern, but that doesn't appear to have a clear-cut correlation on our snowfall. Of the six, three brought below-average snowfall (1993-94, 2003-04, and 2012-13), two brought near-average snowfall (1996-97 and 2001-02), and one featured above-average snowfall (1992-93).
With respect to temperatures during a La Nada winter, the results are equally non-specific. Two of the six La Nada winters were near-average (1992-93 and 2003-04) and two were warmer-than-average (2001-02 and 2003-04). Only one winter was colder-than-average (1993-94).
We aren't able to make any concrete guesses on this winter based on our La Nada pattern, but if we had to draw any conclusions, it would be this: the odds for colder-than-average temperatures and the odds for snowier-than-average conditions do not look good.
The Climate Prediction Center has arrived at a similar conclusion. Their mid-October-issued forecast for the months of December, January and February show an equal chance of temperatures being either above-or-below average in the Mid-South. A large swath of the southern United States from Arizona to Louisiana is expected to see above-average temperatures.
The Climate Prediction Center's precipitation outlook is also on the fence for Middle Tennessee, showing an equal chance of precipitation being above-or-below average. Meanwhile, less-than-average precipitation is expected across the southern United States.
There are other seasonal climate drivers that meteorologists study to try to predict the upcoming season. You may have heard of the North Atlantic Oscillation. Basically, when that value is negative, the jet stream is slower-than-usual across eastern North America, due to a ridge of blocking high pressure over Greenland. This "stopped-up" flow usually spills cold air into the eastern United States. If you like snow, ideally you want a negative NAO that's transitioning back to neutral. That usually means cold air across the eastern United States giving way to a more active jet stream, setting the stage for potential winter weather.
The past three winters with above-average snowfall (15.1" in 2002-03, 7.1" in 2009-10, and 12.5" in 2010-11) all featured a negative NAO trending toward neutral. The North Atlantic Oscillation is better correlated to winter weather predictions than the presence of El Niño, La Niña, or La Nada. Unfortunately for us, accurate predictions for the North Atlantic Oscillation only go out about two weeks!
To sum it up, one of the only reliable long range weather indicators (the lack of an El Niño or La Niña) appears to suggest that Tennessee temperatures will be average-to-above-average between December and February. Snow chances are also looking below-average-to-average this winter. We'll just have to cross our fingers and hope!