Most Read: Local

Posted at 12:48 PM ET, 12/13/2011

What about Santa, the elves, and snow? Assessing white Christmas odds in Washington, D.C.

Santa and his elves have commissioned the Capital Weather Gang to take an early look at the snow potential for the week leading up to Christmas. We tried warning Santa that any forecast this early is only marginally better than using an Ouija Board, but he insisted that Rudolph and the gang needed to know what type of runners were needed for his magical sled.

The CWG team doesn’t want to disappoint Santa so here are our thoughts about the Christmas snow potential in Washington, D.C. this year. Santa: you may need rollers, the probability of snow is probably even a little lower than most years.

Historical odds (climatology)

We begin by reminding Santa the following about snow chances, based on historical data, during the week of Christmas, Christmas Eve and Christmas day:

* the chance of getting at least a dusting some time Christmas week is about 46%
* the chance of 1” or more Christmas week (enough to possibly stick around) is about 20%
* the chance of at least a dusting of snow on Christmas Eve or Christmas day itself is only about 6 or 7% each day
* the chance of getting at least 1” snow on Christmas Eve or Christmas day is only about 3 percent each day.
* the chance of a snowstorm producing more than 4 inches of snow on Christmas Eve or Christmas day is about 1 in 50.

To summarize, snow of consequence during the week of Christmas any year is an uphill battle and getting snow on either Christmas Eve or Christmas day is rare. A significant snowstorm on Christmas day is extremely rare even with a relatively favorable pattern for snow.

Pattern overview

Previously I discussed why the atmospheric circulation pattern probably favored below normal snowfall this December . The area averages below normal snowfall during La Nina winters because the storm track is usually too far west to hold us in the cold air. So this year Old Man Winter has even more work cut out for him than normal to get us a white Christmas.

A second problem is the lack of high pressures over Canada to our north. Air moves from regions of higher to lower pressure. When the pressures are lower than normal across eastern Canada into Greenland, cold air has a hard time settling southward. We can get transitory shots of colder than normal air but they usually only last a few days.

The persistently positive Arctic Oscillation (AO) indicates a lack of high pressure (or blocking) over Canada. Such a pattern typically produces less than half the snowfall compared to when it’s negative. The positive AO and bad luck prevented us from getting any meaningful snow during the first half of December.

Unfortunately for Santa, the La Nina is still in place across the tropical Pacific and the AO remains positive and is forecast to remain positive through Christmas day. Therefore, the pattern remains an unfavorable one for snow potential.

Technical discussion and forecast

Let’s take a look ahead. Below is a five day mean 500mb (18,000 feet) map centered on December 23 that shows areas of above (warm shades) or below (cold shades) normal heights (or pressures) across the northern hemisphere. In general, the heights (pressure) are lower than normal across the high latitudes and above normal to the south except across portions of Europe. Such a pattern is indicative of a positive AO suggesting it will probably remain a force through the period.


Five day mean map of 500mb (18,000 feet) pattern centered on December 23. It shows areas of above (warm shades) or below (cold shades) normal heights (or pressures) across the northern hemisphere (NOAA Climate Prediction Center)
The map also shows lower than normal heights (pressures) developing across Alaska with higher than normal heights (pressure) to the south, a look typical of a positive East Pacific Oscillation index. Indeed, NOAA ensemble models are now predicting that the index will switch from negative to strongly positive during Christmas week. Such a pattern usually floods most of the continental U.S. with warmer than normal air.

At the bottom right corner of the above graphic, NOAA’s Climate Prediction Center lists the best historical analogs to match the pattern shown (the list is partially cutoff). If you were to composite those analogs, you would get a map very similar to the 8-14 day temperature forecast from the Climate Prediction Center shown below.


NOAA’s Climate Prediction Center is predicting warmer than average temepratures over the eastern half of the country leading up to Christmas. (NOAA Climate Prediciton Center)
The combination of the Pacific trending towards a warmer look and the lack of a negative AO does argue that the seven day period temperatures will average above normal across most if not all of the eastern half of the nation. However, that does not preclude a chilly post frontal day or two.

Santa probably almost had a coronary looking at such a warm looking forecast for his one very special week of the year. Such a forecast certainly does nothing to improve the already low climatological probabilities of snow. Despite the horrid look (to snow lovers) of the map above, a couple of analog years identified by CPC did have and inch or two of snow within a couple of days of the centered mean suggesting there is enough uncertainty to keep from saying unequivocally that our area won’t see snow sometime around the Christmas week.

Essentially, the analogs support the climatological 20% chance of snow falling sometime in the 7 day period. Personally, I think the probability is less than that but I’ve sometimes suffered from hubris in making such statements.

While, I personally would bet against snow during Christmas week, most ensemble members do suggest that the coolest weather during Christmas week will be Christmas eve day and Christmas. And there is a model ensemble member that would bring a storm up the East Coast.

Below are various ensemble member forecasts of the surface and thickness pattern for 7 p.m. on Christmas Eve. Note that one member has a weak low pressure system along the North Carolina coast (top right panel).


Different model simulations (ensemble) for December 24 at 7 p.m. One of the 12 shows an East Coast storm (top right panel) (Penn State)
The point of showing the ensembles is not to show that one member is implying we might get snow, but that they all look different. That variability indicates how much uncertainty there is in any forecast that extends out so far in time. And even with all these different solutions, the real answer may be different from any of the ensemble members. The fact that one member is showing a snow storm keeps a glimmer alive for snow lovers. While I don’t think that snowy member is very likely, I’m sometimes wrong.

Summary

The bottom line is that during Christmas week the pattern favors warmer than normal temperatures until Christmas Eve day when temperatures will probably drop to normal or slightly below providing the models are correct (a big assumption). The probability of a major snowstorm is very low. The probably of an inch or two is probably a little below the already low normal probabilities.

By  |  12:48 PM ET, 12/13/2011

Categories:  Latest, Local Climate, Holidays, Winter Storms

 
Read what others are saying
     

    © 2011 The Washington Post Company