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Posted at 03:17 PM ET, 12/23/2012

Stormy but snow hard to come by Christmas week


Three rounds of precipitation are on tap for Christmas week but meaningful snow may prove to be elusive.

In the the first two rounds, falling on Christmas Eve (Monday) and Boxing Day (Wednesday), rain is most likely but some frozen mix is possible, especially when the precipitation starts. For the Christmas Eve event, snow or sleet may mix with rain (no accumulation). For the Boxing Day storm, a period of sleet and/or freezing rain is possible in the far west and northwest suburbs before a changover to plain rain mainly.

How the third round, possible on December 29, plays out is fraught with uncertainty as some models are forecasting rain and others snow. My own feeling is there is a slightly higher chance that the storm is more of a rainmaker. But all scenarios ranging from a rain storm to a snow storm or even a miss are still in play.

Technical discussion

The forecast from the various modeling centers and their ensembles have finally come into close agreement about the first two events.

Christmas Eve


The weak low pressure system associated with the first event is now forecast to track to our west. The diagram to the right shows how tightly clustered the various ensemble members are on the timing and amount of precipitation now being forecast to start early afternoon on December 24 with the precipitation ending in the evening. A few members start the precipitation as snow or sleet but they change precipitation to rain and would imply no snow accumulation.

The latest European model also suggests the possibility of sleet and/or snow Monday afternoon, but temperatures too warm for accumulation.

Boxing Day event

Rain is favored for the Boxing Day event as indicated by the green lines rising sharply in the diagram above right. The ensemble members from the GEFS are pretty much unanimous in taking the surface low to Kentucky by Wednesday morning to a position almost identical to the one shown on today’s GFS (see to the right below). This allows mild air to stream into the immediate D.C. metro region from the south. The GFS has temperatures in the city and in points to the south and east climbing to near 50.


However, on the GFS simulation shown here (which I prefer for simulating surface temperatures because it has higher resolution than the ensemble members) note that western Maryland and parts of the Shenandoah Valley are below freezing when the precipitation starts Wednesday morning. A period of freezing rain is possible across that area, but icing should recede northward during the day. One caveat about low level cold air especially in the valley: it sometimes holds on longer than forecast.

December 29 and 30 event

The ensembles clearly show the 3rd event probably starting sometime on the 29th. Quite a few of the GEFS members simulate snow with this third event but some are now forecasting rain.

Complicating the forecast for this storm are the tremendous differences between recent operational European model runs and today’s GFS. The European runs track the low to our west giving us a rain storm and the GFS suppresses the storm so far to the south and east it mostly misses us.

The ensemble mean forecasts (below) from the European model (left) and GEFS (right) give some idea of the complexity of the forecast for the December 29 or 30th storm.


The ensemble means from both models imply some type of low tracking towards the Lakes before reforming somewhere east of the mountains. The European model and its ensemble mean (left) suggest that the track of the primary low would be far enough to the north to produce enough warming across our area to favor rain more than snow. By contrast, the GEFS mean (right) shifts the low off the coast and keeps us cold enough for snow explaining why so many of its members are forecasting snow at that time period on the plume diagram above (top right image). Either scenario is possible.

Personally, if I’m forecasting snow, I don’t like seeing a surface trough that extends northward from a coastal low towards the Great Lakes. The dashed magenta line shows that both ensemble means have such a feature. Usually, such events do not remain all snow. Therefore, I’m leaning slightly more towards the bulk of the next event ending up as rain or some type of mix rather than all snow. However, if a low holds across the Maritimes like the GEFS mean is forecasting, the low would be forced east off the coast offering a snowier picture.

The models struggled resolving a similar blocking pattern for the Boxing Day storm during the past week. Despite my inclination noted above, it is way too early to lock into a deterministic forecast for December 29 and 30 It’s another event that warrants monitoring during the next several days.

Conclusions

Three storms will threaten the area between now and the New Year. The first one is expected to be a minor event that is expected only to produce around 0.1-0.25” of rain which should arrive during the afternoon. The precipitation could start off as a few messy wet flakes but then should change to rain as temperatures climb to near 40 across the region. The warm surface temperatures would preclude any accumulation even if we do see snow at the onset of the event. The precipitation should be gone by Christmas Day.

The second more significant storm is expected on Boxing Day (December 26). However, current forecasts now track the storm well to our north and west placing us in the warm southerly flow on the east side of the storm. Enough cold air may be in place when the precipitation first arrives to produce freezing rain in the far western suburbs but the current storm track suggests even in those areas the precipitation would rapidly change to rain as temperatures rise on the strength of the deep southerly flow. Temperatures around the city especially to the south and east could rise to close to 50 during the late afternoon or evening on Wednesday.

The third storm slated to arrive during the December 29 or 30 timeframe is much trickier as the models differ significantly on the track and on what type of precipitation might occur. The most that can be said about that time range is there is likely to be a storm somewhere from the Ohio valley region to off the Carolina coast and that the probability of precipitation is higher than normal somewhere in the 40 to 50 percent range.

By  |  03:17 PM ET, 12/23/2012

Categories:  Latest, Winter Storms

 
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