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Don Paul: Prognosticating the probability of a White Christmas

Don Paul

The long-advertised moderating trend has developed as forecast, and on schedule. This doesn’t mean every day in the next two weeks is going to be above-average, which is in the mid 30s for high temps this time of the year. For example, on Tuesday we’ll likely be back in the 20s … for all of one day.

But much of the time between now and around Dec. 22, the lower 48 states will be flooded with Pacific air rather than arctic air. Tuesday represents a “kink” in that flow. It comes quickly and exits just as quickly. This pattern and its probabilities has led the National Weather Service Climate Prediction Center to assign a 4 out of 5 confidence factor for this 6-to-10 day outlook:

Notice the probabilities for above-average temperatures are higher to the west of us. That’s because a little troughing in the west to east flow keeps the east from having a “blowtorch” warmup (say, in the 50s) with that sharp drop Tuesday. The CPC 8-14 day outlook is another matter. The probabilities in the east shift more toward seasonable/near normal temperatures over that period:

Truth be told, that trend toward normal is mired in uncertainty. CPC has the lowest possible 1 out of 5 confidence level for this period. The American ensemble mean does show a western ridge over North America toward Christmas, which forces an eastern trough into eastern North America not far from the Great Lakes. Such a pattern would allow more frequent delivery of colder air from Canada, if not true polar air.

The same American ensemble mean shows near-surface temps cold enough for snow. The average high would be 32 and the average low 20 on that day. There’s nothing extremely cold indicated, but a seasonable chill is, again, “cold enough.”

The Canadian ensemble mean is just a tad milder, but still “cold enough.”

Also, in the all-important air aloft, the American ensemble shows below-average temperatures at ,5000 feet, which would favor snow…IF there were a system in the vicinity capable of producing snow, or if we had a low-level flow which could produce lake effect. Lake Erie was at 40 degrees on Dec. 14. There is zero probability for the lake starting to freeze by Christmas, so that’s not an issue.

The fly in the ointment for a seasonably cold Christmas is in the European ensemble. The European does not have the western ridge and eastern trough. It shows a nearly flat west-to-east flow, which would allow Pacific air to continue to cut any real arctic air off at the pass.

Moreover, there is an inordinately complex oscillation out over the Indian and western Pacific Oceans called the Madden-Julian Oscillation/MJO. There is no nice and neat graphic I can use to explain its complexity. The European forecast for the MJO which just ran Friday morning predicts a phase which would keep the East mild. Many meteorologists like me weight the European more heavily because it has quite a bit more computer crunch power behind it than other models and ensembles, and possibly superior physics underlying its calculations. So the European may be the cheese which stands alone against a seasonably cold Christmas. Its superior underpinnings, however, do not mean it is the best model all the time. There have been quite a few exceptions to its superiority.

As for precipitation probabilities, they are notably unreliable 10-12 days in advance in all models and ensembles. There are no current signs of big-deal storms between now and Dec. 22. Beyond that, it’s bad science to even speculate on snow chances for Christmas Eve or Christmas so far out in time. My friends, when I do bad science it’s by accident, not by design.

Here is the climatology probability for a White Christmas in and near Buffalo, based on three decades of past holidays’ records.

Past probabilities don’t tell you much about how the atmosphere is going to be set up this year. I’m factoring in the European into my early probability guesstimate: Still a decent chance of a White Christmas, but somewhat below the climatological norm of 56 percent this year; better than a remote chance, but somewhat worse than usual. Obviously, when we get closer to the holiday, we can bracket in the target more effectively, pardon the artillery talk.

In the meantime, please, no wagering.

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