What is the Chance of a White Christmas?
Many WA residents feel the same as Bing Crosby, our native son crooner who incidentally was born in Tacoma and raised in Spokane, and dream of a white Christmas. Others are not so keen on the idea, with memories of the treacherous streets and roads across most of Washington State during the holiday season of 2008. Here we take a look back at the record and see just how often we get to delight in, or suffer through, snow on Christmas.
There are two ways to define a snowy day. Arguably, a snowy day must include actual snow falling, but conceivably, measurable snow on the ground is what actually matters. In the colder parts of the state snow on the ground often persists over long periods, and just because there is the ammunition for a snowball fight, from some perspectives that is not enough to make it a snowy day. On the other hand, especially in the lowlands west of the Cascades, it often snows at temperatures slightly above freezing and while lovely to watch, the snow does not accumulate and cause the attendant joy and angst. This is a matter of dictum on which OWSC is not inclined to make a stand. Instead, we will examine the data from both perspectives. Specifically, for 12 cities around the state we have considered daily weather summaries going back to 1950 and computed the frequency of years for which snowfall was recorded on either 24 or 25 December, and separately the frequency of years for which a snowdepth of 1 inch or greater was recorded on 25 December.
The results of this exercise are summarized in Figure 1. As expected, the prospects of a white Christmas are higher in the eastern part of the state, especially in Spokane and Omak, along with places like Wenatchee and Yakima in the foothills of the east slopes of the Cascades. It is a bit surprising how low the numbers turned out for Vancouver, and that Hoquiam has been completely shut out. It is possible that the cold-air outbreaks at Christmas time for the Pacific Northwest have brought more freezing rain than snow to Vancouver, WA but further analysis is beyond our present scope. It is also noted that snow can certainly fall at the Washington coast; at that time of year for Hoquiam the overall frequency of snow on the ground, and snowfall is 4% and 6%, respectively, with slightly higher odds farther north at Quillayute. It just has not happened to occur right on Christmas Day for Hoquiam in its record extending back to 1953.
A weak-moderate La Niña is occurring this winter, which tilts the odds in favor of wetter and cooler weather in Washington. This also generally means a better chance of low elevation snowfall, but the timing of these events within the winter season is basically random. This randomness due to the “noise” in the daily to weekly weather, along with the small number of La Niña events in the historical record, hampers statistical analysis. Essentially, the statistics of snow on Christmas Day in La Niña years lack sufficient robustness to be meaningful. Recent La Niña winters have been accompanied by both snowy (e.g., 2008) and calm (e.g., 2010) weather at Christmas time. It will be interesting to see how the upcoming holiday season plays out.
