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Posted at 04:13 PM ET, 12/17/2012

White Christmas odds fading, but more winter-like temperatures loom in long range


A white Christmas looks unlikely this year and odds are probably below the historical average of 10 percent.

Jason outlined the forecast for the coming week and indicated that a front will be crossing the area Thursday ushering in colder weather. This colder air mass is expected remain over the area during the days leading up to Christmas (December 22-24) as flow from the northwest aloft dominates our weather.


The GFS model shows no snow on ground in the Washington, D.C. metro region December 25. Most snow cover is concentrated in the interior Northeast and north and west of Chicago. (WeatherBell.com)

But the northwest flow will keep us dry (mostly snow-free) east of the mountains while maybe leading to snow showers over the usual upslope (mountainous) areas of western Maryland and West Virginia.

High temperatures across the Washington-Baltimore area leading up to Christmas are expected to be in the upper 30s to low 40s with lows in the upper 20s to low 30s except for in the usual colder suburban locations where it could drop a little lower.

Christmas Day looks like it will also probably be dry but chilly.

The week after Christmas looks very unsettled with an unusual amount of uncertainty about the pattern. Such uncertainty argues for leaning heavily towards climatology (average) for temperature forecasts for the December 25-31 period (that means highs in the mid-40s, on average).

However, even with the uncertainty about the evolution of the pattern, precipitation looks like it will be above normal. That is the big forecast bugaboo as the extended model guidance is predicting the potential for a storm sometime in the December 26-29 period. Unfortunately, the various models differ significantly on the track and timing with most (but not all) models taking the low to our north. While most solutions suggest rain is more likely than snow, the current uncertainty about the track make it impossible to completely dismiss the possibility of snow sometime between Boxing Day and New Year’s Eve.

December 22-24 technical discussion


180 hour GEFS ensemble mean 500 mb height and anomaly forecast forecast valid 1 p.m. December 24. Red areas indicated above normal heights, blue shades represent below normal heights.
Last night’s GEFS ensemble mean 180 hour forecast valid at 1 p.m. December 24 (see to the right) illustrates why the period leading up to Christmas is more likely to be cold. Remember that the ensemble mean is simply an averaging of all the various ensemble members and that those members have had there initial conditions tweaked to get a feel for the predictability of the pattern. In this case, the various ensemble members are pretty emphatic that there will be a strong 500 mb low with an area of below normal heights (blue shading) located somewhere near the Canadian Maritimes and an area of above normal heights (red area) just to its north.

Such a pattern with a 500 mb high just north of a low serves as a block and this block will dominate our pattern leading up to Christmas. The upper level flow on the map below is approximately parallel to the black lines and moves from west to east. The low near the Canadian Maritimes helps hold northwest flow across our area as the flow is forced around the southern portion of the low which should keep cooler than normal temperatures our area.

The feature also promotes upper level confluence (the dashed area) from the Great Lakes to our area. Note that within the dashed circle the spacing between the lines narrows as you move from west to east. Such confluence helps build surface pressures across the region while also making it hard for any upper level divergence and lift to get established. You need lifting to get precipitation and upper level divergence to help maintain areas of lower pressure. Therefore, any weak low pressure system embedded in the northwest flow should weaken and not have much impact on our area in the days leading up to Christmas. However, I can’t rule out a few stray flurries making it over the mountains as the northwest flow could lead to snow in the upslope areas to our west. However, accumulating snow is unlikely.

How fast the southern end of the block shifts eastward will play a huge role in what happens during the post Christmas period (see below). The slower the upper low ends up moving eastward in relationship to any approaching upper level system, the more chance there is for the December 26-28 low pressure system to track to our south. The faster it moves, the more likely for the low to track towards the Great Lakes region leaving us on the warm side of the storm.

December 25-31 technical discussion


240 hour ensemble mean forecast of 500 mb heights and anomaly valid at 7 p.m. December 26 for the GEFS (top panel) and European (bottom panel) ensemble. Blue areas represent below normal heights and warm colors above normal.
The model differences are unusually large for this period making this an even lower confidence forecast than most 8-14 day outlooks. The 240 hour ensemble mean forecasts from the GEFS (top) and European (bottom) models illustrate the chasm between the two general model camps during the period.

A comparison between the two model ensemble means (to the right) have important differences. The GEFS ensemble mean (top) has the mean trough and below normal heights still over the western U.S. while the European model has a small area of above normal heights in almost the same place where the GEFS has below normal heights. By contrast, the European ensemble mean has its trough and below normal heights already east of the Mississippi River. That faster movement would give the storm a better chance of tracking more to the south and east than indicated by the bulk of the GEFS ensemble members as the block would have more impact on the trough position. Note: the ensemble means don’t even begin to portray the range of trough/ridge positions portrayed by the various members.

The same can be said about the track of any individual low during the period. Four GEFS ensemble members valid at 1 a.m. December 27 are shown below. They illustrate only a portion of the various model solutions but do give a feel for how the various members differ on handling the low. Some members take the low towards the Great Lakes like last night’s and today’s GFS deterministic forecasts (top left below). Some track a strong low towards the East Coast but far enough inland that the precipitation would be mainly rain (bottom left), some only depict a weak wave that might offer some snow (top right) while still others are predicting more than one wave (bottom left).


Example of the differences in 4 individual ensemble members for the same 240 hour forecast

The only thing that can be said is that there is potential for a storm sometime in the December 26-29 time range while also stating it is too early to have any confidence about the storm track. My own feeling is that we are more likely to see the track end up to our north than south and are more likely to be plagued by rain than snow. However, because of the uncertainty it is also impossible to rule out the latter.

Conclusion

Christmas and Christmas Eve day are likely to be chilly and dry as a front ushers in cold dry air sometime late Thursday. The chance of 1” or more of snow on the ground Christmas Day is just 5 percent.


Four runs of the CFS model all show maximum snow depth in the days leading up to Christmas is around zero in Washington, D.C. (WeatherBell.com)

Beyond Christmas the picture is very muddled. The period looks to be on the wet side with near normal temperatures. However some days will probably end up being below normal and others above as there is no real certainty about the evolution of the pattern.

Most models are suggesting there will be a storm sometime between Christmas and New Years Eve but the track and timing of the storm are very uncertain.

Behind that storm there is likely to be another push of cooler air. However, timing such a push into our area is impossible with such uncertainty about the evolution of the pattern. People travelling the week after Christmas need to keep abreast about the potential storm. A number of different scenarios are still on the table.

By  |  04:13 PM ET, 12/17/2012

Categories:  Latest, Winter Storms, Holidays

 
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