White Christmas Countdown

White Christmas? What are the odds? Around central Indiana, it’s around 25-60 percent depending on you define it.

The National Climatic Data Center produced the above map, which shows the statistical probability (in percent) that a snow depth of at least 1 inch will be observed on December 25th. The probability was computed using snow depth observations for December 25th for the full period of record for a given station. Around here that chance is about 25 percent.

There are several ways to define a White Christmas. For our purposes, we’ll include a White Christmas as any day that has either at least a trace of snow on the ground, or experiences at least a trace of snow falling that day. In more general terms, we’ll define a White Christmas as any Christmas day that Indianapolis sees snow.

So what is the chance of at least a trace of snow falling at Indianapolis on Christmas day? Well, snowfall records on Christmas day have been kept in the Indianapolis area since 1871. And in that 140 year period of record, a trace of snow (or more) has fallen 64 times. This could be equated to a 46% chance of at least a trace of snow falling on Christmas Day.

Our other possibility for a White Christmas would be if there was at least a trace of snow already on the ground. This could be old snow from a week ago, or just the day before. Snow depth records don’t go quite as far back, and these were started in 1896. And in that 115 year span, there has been at least a trace of snow on the ground on Christmas Day a total of 57 times. This could be equated to a 50% chance of at least a trace of snow on the ground on Christmas Day. The last Christmas that saw snow on the ground in Indianapolis was last year, with five inches of snow on the ground Christmas morning.

With both of our possibilities for a White Christmas explored, what then is our final chance? What would our probability be of having either a Trace or more of snow falling, or a Trace or more already on the ground? Since 1871, this has happened in a total of 82 times, where either the snow has fallen or already been on the ground. This gives us an overall probability of just under 60%! Which means that 6 out of every 10 years, on average, will see either some snow falling or some snow already on the ground for Christmas Day. So, much more often than not, there will be at least a few flakes around to greet us come Christmas.

This year, however, not so much. Here’s a look at the chart depicting current snow cover across the country: (Click for larger version.)