Tom’s previous blog points to some features in the computer models that have caught the attention of East Coast meteorologists … the potential for Christmas Day cyclogenesis, better known as storm development.
Those models continue to come together on a forecast that would point to the possible development of a significant coastal storm on Christmas Day.
The chart below is the forecast for late Christmas Day from the European Center for Medium-Range Forecasts.
This forecast map shows a strengthening storm off the mouth of the Chesapeake Bay. In the scenario the storm had spent the previous w24 hours moving eastward out of Tennessee and across North Carolina, generating snow in Maryland. Two National Weather Service surface forecast maps show the progress.
There is a slight difference in the position of the storm center between the National Weather Service maps and the European Center maps due to computer model interpretations. These differences are what make forecasting difficult beyond a couple of days.
The Model Output Statistics (MOS) for one global model generated by the National Weather Service Sunday morning (December 19) gives a rough numerical sketch for the week ahead, including Christmas Day.
The columns outlined in red are Christmas Eve and Day.
The dark blue outline highlights the precipitation probabilities the computer model has assigned for those two days for 12 and 24 hour periods.
The green outline highlights the quantitative precipitation forecast (liquid) for the 12 and 24 hour periods. The “4” in the 24 hour row indicates half to one inches of liquid precipitation is possible. At a ten to one ratio, this would yield 10 inches of snow. The high temperature for the day in this model is pegged at 37 degrees and all snow or a rain/snow mix is possible.
The light blue outline highlights the type of precipitation expected in that time frame. “S” indicates snow. The line immediately below the light blue box indicate the snowfall amount forecast. In this case the “6” indicates a 6-8 inch snowfall range is possible.
A comparison of the snow and liquid numbers in this particular computer run would seem to indicate a somewhat wet snow with a possibility of a rain/snow mix somewhere nearby. These numbers came from the 7am Sunday computer model run. The 1pm Sunday computer model run comes up with a slightly different storm track and lends some support to the 7am model’s hint at a rain/snow mix by bringing the storm center and warmer air farther north.
The forecast map above shows the storm center over the Baltimore/Washington area at 1pm Christmas Day. The blue line through the center of the storm is the freezing line at 5,000 feet. South and east of that line, temperatures at that altitude are above freezing and precipitation at that altitude would be rain. Temperatures at the surface south and east of that line would determine weather rain, freezing rain or sleet is falling. North and west of that line, snow would be the most likely type of precipitation.
The bottom line … we are 5-6 days away from Christmas and computer models are indicating some sort of storm development. Every 6 to 12 hours those model come out with slightly different ideas as to how the storm will behave. The models will continue to swing back and forth over the next few days with different solutions for Christmas Day weather. At this time, the final outcome is far from a sure thing. Stay tuned.
Check out Tom’s latest blog entry for a look at the setup for the week’s weather developments.