Monday, December 14, 2009

Chances of a Mid-South White Christmas

With Christmas now less than two weeks away, I am starting to get more questions about the possibilities of a white Christmas in Memphis this year. What is a White Christmas? There are two definitions that are used most commonly: measurable snow on Christmas Day (this means flurries don't count) and measurable snow on the ground Christmas morning.

First let's take a look at history. This is a recap of last year's post on the subject. Memphis' weather records date back to 1872 - an impressive 137 years. The only time that measurable snow (0.1" or more) has fallen on Christmas Day was in 1913, when 3.5" fell on the 25th. There have been 3 other years that a trace (flurries) of snow fell on Christmas Day: 1975, 1980, and 1992.

As for the latter definition, measurable snow on the ground Christmas morning, the following years could be considered "white Christmases": 1962 (2"), 1998 (1"), and 2004 (2"). A trace of snow was on the ground in 1966, 1980, and 1990. So, no surprise to anyone, history is not on our side - 4 Christmases out of the past 137 have had snow (2.9%). However, according to the National Climatic Data Report report "White Christmas?", published in 1995, Memphis has a 7% chance of 1" of snow on the ground on Christmas morning. I like their odds better. See the map below for the NCDC odds of a White Christmas for the continental U.S.

Next we factor in El Nino. If you haven't heard or read, we are in an El Nino winter. See my previous posts on the winter El Nino pattern and the 2009-2010 winter outlook, and a late November El Nino post. The first of those outlines what the typical weather pattern is during a moderate El Nino, such as exists currently and is forecast to continue. The general pattern is for an active southern (subtropical) jet stream over the southern tier of the U.S., escorting weather systems to our south, and cold air filtering down from the north.

Occasionally, the timing is such that these two factors will mesh into a winter weather-producer, but the timing and placement of the cold air and low pressure system have to be just right. Fortunately for us, El Nino winters tend to provide more opportunities for having "just the right timing." It does not mean our chances are any higher, just that there are more low chance events.

Moving away from climatology and into meteorology, the long-range forecasts now have the period leading up to and including Christmas in their sights. I don't have to tell you however, that computer model accuracy at 10-14 days is not the greatest, but it will give us a look at trends. The trend seems to be for more of the same as we have been getting - southern stream systems with cold air poised just to our north and occasionally infiltrating the Mid-South. Again, it's all about timing.

For comparison purposes down the road, I've posted an image below from the GFS model showing its forecast for precipitation total for the 48 hours leading up to Christmas morning. It does not show precip type, amount of cold air, etc., but other data that I have examined indicate that the precip shown is another rainy system with cold air coming in on the backside of it, similar to what we have seen the past couple of weeks. In addition, the Climate Prediction Center's 8-14 day forecast covering the period around Christmas continues a trend of below normal temperatures for the southeast U.S., but also somewhat below normal precipitation for the Mid-South between December 21-27.

So, my opinion? (That is, after all, why you read this in the first place!) I am fairly confident that the current pattern continues and we will see a precip-producing low move by to our south within a couple of days of Christmas. I also think that, given the track record and the fact that we are still pretty early in the winter season, any precip that falls will be a cold rain. I believe the chances are slightly higher that we could see a little ice vs. snow around Christmas, but I still hold the chances of either of these things happening at less than 10%. I believe climatology will once again win out.

I'll continue to monitor in the days ahead, update you with any new information or major shifts in my thinking, and will post the results of my long-range forecast right after Christmas. Let's hope I'm wrong! I also want to thank Eddie Holmes of Jackson-West Tenn Weather and Paul Yeager of for their previous entries (Christmas Weather in Jackson and White Christmas in the South, respectively) which prompted me to put this post together. It's a fun exercise!

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