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Posted at 01:26 PM ET, 12/21/2012

December 26 winter storm chances grow; wintry precipitation Christmas Eve?


Two weather systems are expected to impact the area next week, a weak one late Christmas Eve into early Christmas morning with a stronger one expected on Boxing Day, December 26. Each has the potential to bring winter weather to portions of the Washington, D.C. metro area but each also could end up primarily as rain.

Only light precipitation is expected with the Christmas Eve event. Right now that precipitation is more likely to fall as rain than some type of frozen precipitation (ice or snow). However, it is still worth monitoring if you need to travel on Christmas Eve especially if you will be driving north or northwest.

The more significant storm slated for Boxing Day is more complex as the models have yo-yoed on its track and on how much cold air will remain over our area and for how long. It looks like there is a good chance that most of the area will start out with some type of frozen precipitation either as snow, sleet or freezing rain but that the precipitation will probably change to rain along and east I-95.

However, cold air east of the mountains can be tough to dislodge so areas west of I-95 have the potential to end up with significant icing if some of the colder model solutions are correct. By contrast, if the warmer solutions win out, the precipitation could change to rain fairly quickly. This is one of those cases where we probably won’t really have a good feel for the precipitation type forecast until we’re within 24-48 hours of the onset of precipitation.

Why is there so much uncertainty?

Earlier this week, I discussed that there was potential for a storm shortly after Christmas. The commentary noted that the models were showing tremendous amount of uncertainty about the pattern and forecast for the December 25-31 period. That uncertainty remains as the models continue to display significant run-to-run differences on the track of the storms and on how quickly the cold air over the area will will erode.

Less than 24 hours ago, the GFS was forecasting the Boxing Day storm to stay so far south that it would have given Raleigh, North Carolina a major snowstorm while a little light snow at most.

The next run of the model took the low to the Kentucky/Virginia border with a mix of precipitation. By contrast, 48 hours ago, the European model had the low tracking to near Illinois favoring mostly rain. The next run had it tracking across Kentucky well east of its previous run with more of an icy look.

Now today’s GFS run has jumped the low back west to central Kentucky well west of its coastal track from yesterday. The new track is not one that is favorable for snow but is one that can produce an ice storm especially for locations west of I-95. Similarly, the latest European model shows a track favorable for an icy mess west of I-95.


The GFS forecast surface map at 7 a.m. December 26 showing the possibility of freezing rain along and west of I-95.
At 7 a.m. December 26 the GFS model shows the freezing line almost bisecting the city (see figure to the right). It implies that areas out towards the mountains could have a considerable period of freezing rain before any changeover to rain occurs. Remember that today’s GFS is only one deterministic forecast and that the pattern is one fraught with uncertainty.

The constant jumping around of the track from run to run indicates that the pattern is one in which small differences in the initial conditions at the start of each model cycle can end up having a huge impact on the eventual track and evolution of the storm. Below we’ll look at the ensembles to see how much impact those differences in initial conditions can have.

A demonstration of the uncertainty from an ensemble modeling system

The various ensemble member forecasts of the 1000 mb isobar - an approximation of the location of the storm’s low pressure - are shown below. Some of the contours display a closed off circulation considerably west of the mountains while others are along or even off the East Coast. Remember that the only differences between the various ensemble members are that each member starts with a slightly different initial condition. Those small initial differences grow with time.


Diagram showing the GEFS ensemble member forecasts for low pressure locations at 1 p.m. on December 26

The lows with tracks inland from the coast are ones that generally cause enough flow from the south or off the ocean to warm us enough to eventually produce rain though they can sometimes be preceded by a period of snow or mixed precipitation. The tracks west of the mountains almost always give us a rain storm.

Those model ensemble members with a track to the east of the mountains and to our south tend to keep our low level flow coming out of the north which prevents warming. Such tracks would be much more favorable for snow. The large variation in the ensemble representations of the 1000 mb contour suggests that we are far from being able to pin down a final track with enough certainty to completely rule out any type of precipitation with the Boxing Day storm.

Plume diagram analysis for the Christmas Eve and Boxing Day (December 26) events

A plume diagram is shown below. It illustrates how precipitation evolves over time (both the type and amount) for Sterling ,Virginia from the various GEFS ensemble members starting on Christmas Eve and ending on December 29. The precipitation type is indicated at each moment by the color of the line. Green indicates rain, red freezing rain and blue snow.


Diagram showing the precipitation over time for the various members of the GEFS ensemble from last night. Blue=snow, red=freezing rain, green-rain between Christmas Eve and December 29.

Looking at the diagram and all the green lines that are displayed Christmas Eve into Christmas morning suggests that the period of light precipitation with the first system is more likely to be rain than snow or freezing rain. This morning’s GFS and NAM runs support this idea with both now forecasting light rainfall amounts from the first weaker storm system.

More importantly, the diagram suggests the second storm on Boxing Day is likely to produce significant precipitation with most members predicting over an inch of liquid equivalent. Note that the lines climb rapidly upward on the 26th indicating a period of heavy precipitation.

Most ensemble members are still calling for the bulk of the precipitation to fall as rain (green lines). However, a few of the forecasts indicate significant icing (red) while a few more are predicting accumulating snow (blue line segments). While I still favor Washington D.C. ending up with mostly rain, it is way too early to rule out the potential for snow or ice, especially west of the city. The same type of plume diagram for Oakland, Maryland displayed a much higher percentage of members with freezing rain or snow than shown on the above diagram.

The bottom line

A very unsettle period of weather is in store for the area from Christmas Eve through Boxing Day as two potential systems affect the area.

The first on Christmas Eve is expected to produce only light rain across the area but it will also help pull down a little cooler air which is helping to complicate the forecast for the second stronger storm. The latter is expected to produce significant precipitation across the area.

However, there is an inordinate amount of uncertainty about the storm track which makes it extremely difficult to make a deterministic forecast of how much, if any, snow, sleet or freezing rain we might get from the storm. There is still an outside chance that the area receives accumulating snow.

My own feeling is that areas inside the beltway are more likely to see mostly rain than mostly snow or ice but I wouldn’t bet much on such a call. West of the city may be a different story as the potential for a significant ice storm is growing.

The Boxing Day event is worth monitoring closely as someone nearby will probably end up with adverse winter weather and it still could be us.

By  |  01:26 PM ET, 12/21/2012

 
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