Map Updated: White Christmas vs. Global Warming

December 20, 2011; 1:25 PM ET

UPDATE: We've added a video about the stats I talked about last week.

The National Weather Service has issued an updated version of their White Christmas Probabilities map, based on the newer 1981-2010 climate averages, instead of the 1961-1990 data we had before. I had blogged about calculating the change in percentages for major cities, but we didn't have the original tools or methodology to make the map. Hover over the image below to see the change between the old and new map:

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Problems with comparing the two maps:

- The two maps may not be statistically comparable because of the difference in isoplething techniques which smooth the data (there are only about half a dozen points in each state, at least in the old data).
- Because the new map isn't as detailed, and averages out data, it's fairly useless west of the Plains, where mountains are high and data changes frequently.
- The new map only goes down to 10%, while the old one went to 5%, so the northward "jump" that appears to happen is not valid.
- The maps have different color scales, which aren't consistent, even on the same map (anything between 11% and 39% can't be compared because the color spacing was 15% in the old map but 10% in the new).
Areas on the southern fringe may be less statistically relevant because of the number of snow events (I'm not sure).

RELATED: Will the U.S. get a White Christmas this year?
RELATED VIDEO: Do The Math: Historical Probabilities of a White Christmas

So what can we safely see based on eyeballing?

Two 30-year sets of data later, the places that had a 90% chance (the whitest color) seem to have suffered the most. Originally, that area included half of Wisconsin, Minnesota, and Maine. Now very little of those states have the chance. This may be the most compelling evidence that global warming is decreasing White Christmas chances in the U.S.

Elsewhere, it's harder to tell, and chances have increased in some areas. Take a look at the 10% line, which is the dark blue color in the new map, and the end of the green color in the old map (the blue color in the old map starts at 11%, not 10%). It's really not that different in the East (see central Virginia, southern New Jersey) and is ever farther south (more of a chance) in the new map for Missouri, Oklahoma and Texas. The 40% line seems to be roughly in the same area. Since this doesn't cover the globe, and shows both warming and cooling, I can't pin it on Global Warming.

See something else? Leave me a Comment below. For U.S. city, Canada and United Kingdom chances of a White Christmas, please read my blog from last week.

Why do we dream of a White Christmas anyway? Because all Christmas Lore was invented during the Little Ice Age.

The views expressed are those of the author and not necessarily those of AccuWeather, Inc. or AccuWeather.com