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Posted at 03:07 PM ET, 12/10/2012

White Christmas tracker, December 10

Today, I’m introducing the first day of the White Christmas tracker. We’ll run this every week day through Christmas Eve. It’s a daily update on the potential for snow on the ground Christmas Day using computer models.

For today’s debut and until around December 18 or so, it’s important everyone understand model forecasts more than a week into the future are very unreliable. They have not demonstrated the ability to accurately predict the weather this far out in time.

We’re showing you the models so we can see how they evolve (i.e. do they shift snowier, less snowy, or constantly flip flop?) and then, in hindsight, evaluate how well or poorly they simulated reality.


Predicted snow depth Christmas Day at 7 p.m. EDT from GFS model (WeatherBell.com)
First up is a simulation of snow cover at the end of Christmas Day (7 p.m. EST or 0 UTC, December 26) from the GFS model.

It shows substantial snow cover in places you’d expect a lot of snow: near the Canadian border, in the Rockies and around the Great Lakes. The I-95 corridor is snow-free, including Washington, D.C. and New York City, as are the South and the Pacific coast. (Strangely, Minneapolis - which just got over 10 inches of snow - seems to be in a snow hole.)

Next up, we have the CFS version 2 - this is a long-range or seasonal forecasting model. It has a product which shows you the 5-day maximum snow depth over a given time period. It includes four simulations that are generated using slightly different inputs.

For the 5 days leading up to Christmas, the four runs have a similar look to the GFS but push the snow farther south and east.


Predicted maximum snow depth between late December 19 and late December 24 by four runs of the CFS version 2 model. (WeatherBell.com)
All four of the model runs show snow in the D.C. area, but we’re on the edge of appreciable snowfall in 3 of the runs, with less than 1 inch forecast (areas not far to the north and west have substantially more). And as the CFS is showing maximum snow depth over a 5-day period (not the amount on the ground per the GFS above), consider some if not all (where totals are low) snow shown may melt before Christmas Day arrives.


Probability of a White Christmas (defined as 1” of snow on the ground Christmas Day) based on historical data. (NOAA)

Considering available information now, I would not place the odds of having snow on the ground Christmas Day in D.C. above or below the historical average (see above) of just over 10 percent (stay tuned for a post by Ian Livingston that will provide many more details about the odds of a White Christmas in D.C.)

By  |  03:07 PM ET, 12/10/2012

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