Analyzing pre-Christmas snow odds
A D.C. white Christmas is rare, what about this year?
Some readers may have heard rumors of a possible snowstorm this Thursday and another around December 19th from reading various weather chat forums. Others may be wondering: what are chances of a white Christmas?
I'll start off by discussing the odds of getting a white Christmas from a strictly climatological perspective (using data provided to me by Ian Livingston) and then will discuss what the models are suggesting about the upcoming period leading up to Christmas.
The odds of seeing snow any time during the week prior to Christmas (Dec 19-Dec 25 inclusive) based strictly on climatology varies depending on the amount you analyze. For the snow-starved who want any amount, snow has accumulated at least one tenth of an inch 57 times in the past 123 years (a 46% probability) between Dec 19 and 25.
However, on any given day in the week leading up to Christmas, snow fell an average of just eight times in 123 years. So there is about a 6.6% chance of seeing a few flakes on either Christmas Eve or Christmas Day (or any one of the days leading up to it) based on climatology.
The frequency of events drops off quite a bit when looking at one inch or greater accumulations. Only 25 such events have occurred during the week prior to Christmas in the past 123 years (about once every five years). The average number of one inch or better snowfalls per day during that week is only three or four giving a probability of getting an inch or more on Christmas Eve or Christmas Day (or any particular prior day) of just 2.9 percent.
Santa's reindeers want more snow than that but they know the probability of getting a snowstorm of over four inches during Christmas week is only around seven percent. Moreover, the odds of getting a such a snowstorm on either Christmas Eve or Christmas Day is only 1 in 100 based on climatology.
So a major snowstorm during the week ahead of Christmas is rare and getting one on Christmas Eve or Day is even rarer. Last year's Christmas snow cover from the December 18-19 storm was the exception not the rule.
Prior to Christmas week this year, there is a chance of light snow on Thursday as a weak low scoots by to the south. The NAM model gives us almost .25" liquid and the GFS model around 0.08" which would equate to under an inch to a couple inches of snow. The system has very little upper level energy with most of its lift being supplied by warm advection (winds from the south). The upper system will also be shearing apart and the low will be weakening as it comes east so its snowfall potential is pretty limited. Right now I prefer the less robust GFS as this is the type of system the NAM typically overdoes and I'm not impressed with the upper level support.
The December 19th threat has more potential though the probability of getting a storm still is relatively low. The models have been alternating between having a storm close enough to the coast to give us snow and one too suppressed to give us any as shown in the figure below. The first model to identify the threat was the European model. Now it has drifted away from the snowstorm solution. Two of the last three runs of the GFS have shown a storm. But the latest GFS model has trended back to a flatter look suggesting the odds are better for a miss than a hit.
Two other global models are split in their take of the storm suggesting the pattern is again a fairly chaotic one making any definitive call about the storm dubious.
The crux of whether a storm develops close enough to shore to produce snow really is dependent on what happens in the steering pattern aloft. The figure to the right below shows a "spaghetti diagram" which can help us analyze the upper level pattern (at about 18,000 feet) and range of possible storm tracks.
The white line (amidst the green lines) with the big dip (trough) in the East is the operational GFS model from last night - a run that supported snow as it simulates a storm moving up the coast with cold air in place. Note that there are a number of green lines near it representing additional runs of the same model (with slightly different inputs) known as ensemble members. Many have less of a dip than in the operational model (i.e. the white line). These ensemble members have a weaker, flatter trough and end up with the surface low being suppressed and well out to sea. That's essentially the European solution. The green lines that are east of the white lines also are members that have the low too far east for snow.
The important thing to notice is that the differences between the white line and the other more amplified green lines and the flatter less amplified green lines are relatively small. The differences in this case between a storm and a non-storm are relatively small suggesting that the potential for snow on December 19th is higher than normal.
I think the flatter idea (i.e. no storm) is probably right because the timing and the evolution of the northern (in green) and southern jet (in red) streams needs to be almost perfect in this case to get a storm. That can happen but is sort of like threading a needle. You often miss the eye. However, the evolution of the pattern is certainly worth watching and the probability of snow on the 19th while probably only being in the 20 to 30 percent range is still above normal for the day.
The probability of getting any significant snow decreases and pretty much reverts towards climatology (i.e. average) beyond the 19th. The pattern will be colder than normal suggesting that any precipitation that occurred would have a better chance of falling as snow than normal. If it is to snow prior to Christmas this year, it will likely stick around through the holiday as cold air should be in abundant supply. But the pattern should be drier than normal as the upper flow looks like it will primarily be from the northwest. The Climate Prediction Center precipitation 8-14 day forecast ending on December 26 is for drier than normal weather.
With respect to the weekend (Dec 19) storm, we'll be watching it closely over the next several days. In this pattern, we expect the models to go through several more flips before converging on a solution. The game is far from over. I'll offer updates about the models in the comments section during the next couple of days unless another full-scale article is warranted.
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