D.C.’s White Christmas chances slim as usual this year; 60s this weekend

cwg_JunkerThis year a white Christmas is unlikely. The coming weekend temperatures are expected torch into the 60s as two lows track towards the Great Lakes region and pump southwesterly winds across the area starting on Friday but lasting through Sunday.

Sunday’s maximum temperatures could even rise into the mid- to upper 60s.  And I can’t rule out a few spots ending up near 70 if enough sun is available to provide warming.

A cold front is expected to arrive sometime on Monday marking the transition to the colder than normal temperatures that are expected by Christmas Eve Day. The threat of showers exists for the weekend into Monday as the front will be accompanied by rain and there should be plenty of moisture around. However, once the front sinks far enough south to usher in sufficiently cold temperatures for snow on Tuesday, it is expected to be too far south to bring precipitation into the area. The chances of a White Christmas are thus low, probably no better than climatology.

Related: What are Washington, D.C.’s historical chances for a White Christmas?

Christmas Day looks chilly but not frigid, a pretty typical winter day. Remember that, by Christmas, the average maximum temperature is 44 degrees and the minimum is 30.

The pattern from Christmas to New Years is a bit uncertain, with differing ideas in the models. Temperatures during that week will probably average near to a little below normal but the lack of blocking high pressure across Greenland and eastern Canada will make it hard to hold in cold air should any storm show up on the horizon.

How confident are you that the temperatures will be well above normal this weekend?

I’m very confident that temperatures by Friday will be running well above normal.

Below is a box and whisker diagram showing last night’s operational European temperature forecasts (black line), European ensemble mean temperature (green line) and the spread of the European ensembles. The rectangular boxes span one standard deviation from the mean. Essentially those boxes combined with the thinner blue lines (whiskers) give you a sense of the range of forecasts. If the distribution resembles a normal distribution, around 68% of the forecasts would lie within the rectangular boxes, whereas the blue whiskers extending outside the boxes provide a sense of outlier temperature forecasts.

Temperature forecast from European model and its ensemble members through Christmas (WeatherBell.com)

Temperature forecast from European model and its ensemble members through Christmas (WeatherBell.com)

Note that all the European ensemble members have temperatures on Friday well up in the 50s. They exhibit more spread as the weekend progresses as they struggle with how cloudy it might get and on the timing of the frontal passage. Still, the mean of all 50 European ensemble members has the temperature rising into the 60s on Saturday and Sunday. Today’s GFS model run is calling for max temperatures in the low 60s on Monday and mid- to upper 60s on Sunday.

How confident are you about Christmas being on the chilly side?

The ensemble temperature spread around Christmas is unusually small for such a long range projection providing some confidence that Christmas Eve and Christmas Day will end up with temperatures at or below normal. That means highs running in the 30s Christmas Eve Day and in the upper 30s to mid-40s on Christmas. At least it should feel a little like Christmas even without having any snow.

Why do you think snow is unlikely for Christmas?

Below is a plume diagram of the precipitation being forecast by the various GFS ensemble members (GEFS). The members start showing a chance of showers as early as Friday (December 20) and hold onto the idea of the possibility of rain until Christmas Eve. The ensemble are not saying that rain will be constant during that 4-day stretch but that the average rainfall during that  period will end up being well above normal.

The details of which day will end up with the most rainfall are still up in the air. My guess is Saturday and Monday but that’s just a guess.

plume-xmas

For snow lover’s, the diagram is bleak. Of the few ensemble members that give some precipitation on Christmas, they only show light precipitation amounts and are indicating either rain or freezing rain rather than snow. Those three members are outliers compared the other ensembles or the latest GFS.

If you’re expecting chilly weather after Christmas, why aren’t you more excited about the prospects for snow?

The Arctic and North Atlantic Oscillations remain strongly positive on the latest CPC super ensemble mean Day 11 forecast (for December 28). Heights (or pressures) across Iceland, Greenland and Baffin Bay remain below normal (blue on the chart below) where they need to be above normal (red) to supply blocking for holding in cold air if there is a storm. Without blocking, the tendency is for impulses to dive southeast from Canada but usually track across the Great Lakes region or Ohio Valley region to our north. That tends to limit how cold we get but more importantly makes it very difficult to get a significant snowstorm in the DC area except occasionally out towards the Shenandoah Valley.

The upper level height pattern averaged over the 5 days centered on December 28.  The 10 dates on the right side of the image are past dates (or analogs) with similar patterns.

The upper level height pattern averaged over the 5 days centered on December 28. The 10 dates on the right side of the image are past dates (or analogs) with similar patterns.

The 10 analog dates – based on past patterns similar to the one forecast – play scrooge concerning our snow chances. Not one 1″ or greater snowstorm was found within 3 days of December 28 in these past patterns, suggesting there is not a lot of snow potential between Christmas and New Years Eve. The lack of any blocking and these analogs are my reason for feeling the snowfall potential during the period between Christmas and New Years is no better than climatology (the long-term average – which is about a 20 percent chance of 1″).

Below I’ve composited (averaged) the temperatures across the U.S. for the 10 analog dates starting two days prior to December 28 and ending two days after it. The temperatures averaged a little below normal but the coldest air during those analog years was centered to our north. The analogs also suggest that not every day in the 5-day period will end up colder than normal. When a low passes to our north, our temperatures are likely to creep up. When I looked at precipitation anomalies for the same days, I got mixed signal suggesting precipitation between December 26 and December 30th would be near normal.

wes-day11-newyears

For snow lovers, the latest run of the European model 10 days into future shows a set up that may be a bit more promising, but at this point it’s an outlier.

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