A potent Christmas Eve Blizzard remains in the forecast cards for next week, projected by multiple runs of different forecast models. The American GFS computer forecast model is the most robust of the bunch, calling for a blockbuster snowstorm that would not only bring heavy snow to the heavily populated I-95 corridor in the northeast, but it would also put down fresh snow deep into the south.
The GFS and ECMWF are among many computer models meteorologists use to assist in weather forecasting. While meteorologists have many tools at their disposal to create weather forecasts, two primary global forecast models they do use are the ECMWF from Europe and the GFS from the United States. While the models share a lot of the same initial data, they differ with how they digest that data and compute possible outcomes. One is better than the other in some scenarios, while the opposite is true in others. No model is “right” all the time. Beyond the ECMWF and GFS models, there are numerous other models from other countries, other academic institutions, and private industry that are also considered when making a forecast.
Yesterday afternoon, both the GFS and ECMWF forecast were in good agreement with their forecast solution, showing a pre-Christmas low coming together during the middle and later part of next week. However, this afternoon, they are diverging from their solutions, with each forecasting a radically different outcome. While both still show a very strong area of low pressure moving through the northeast, the position, structure, intensity, and movement are very different with each other.
While the GFS and ECMWF forecast models now differ with their solutions, the GFS has been providing consistent run-to-run agreement on the evolution of this storm for many model runs now. This isn’t true with the ECMWF model, which is now suggesting a scenario structurally different from prior runs.
Very interesting seeing how the 12z GFS wants to paint a wide area of snow across the eastern U.S. in the days leading up to Christmas. pic.twitter.com/lSkVm1sOD7
— the Weatherboy (@theWeatherboy) December 16, 2022
Forecast model accuracy gets better within 10 days; beyond 10 days, accuracy isn’t great. But inside 10 days, and especially inside 5 days, model accuracy and performance improve. Because we are within 10 days to Christmas and because both models are suggesting similar solutions, confidence is growing in the meteorological community that this storm will take shape. While there is model agreement and some run to run consistency with their output, it is still too early to lock in any one forecast storm track, intensity, and precipitation types and amounts. Getting into such forecast specifics will likely come early next week in the days leading up to Christmas weekend.
If the American GFS model solution suggested today comes to fruition, it would produce significant snowfall across a large part of the eastern United States. Noting that this remains a big “if”, if it were to verify, there’d be at least an inch of snow across northern parts of Texas and Oklahoma, Arkansas, northern Mississippi, Alabama, Georgia, and South Carolina; accumulating snow would be possible to the coast from North Carolina north to Maine. There would be more than 6″ of snow along the entire I-95 corridor, with more than a foot of snow stretching from the mountains of western North Carolina and Virginia north and east into New England.
The GFS also shows packed isobars wrapped around an intense low pressure system in the northeast. Isobars on a weather map reflect lines of equal atmospheric pressure; the closer they are, generally the more windy it becomes. Because of how tightly packed they are in the modeled forecast, it suggests the presence of very strong winds. With falling and/or blowing snow and high winds, the definition of a blizzard could be reached in some areas.
While most people associate a blizzard with a heavy snow event, a blizzard is officially defined by wind and visibility –and not by accumulation. To reach official blizzard criteria, you must have blowing and/or falling snow with winds of at least 35 mph, reducing visibilities to a quarter of a mile or less, and such conditions must last for at least three hours. Winds lofting the current snow pack and reducing visibilities without any falling snow is called a ground blizzard and is just as hazardous as a blizzard with freshly falling snow.
If the latest GFS forecast model run were to verify, most of the snow would fall before Christmas Eve. It suggests snow will move into the mid Mississippi River Valley around the evening of Thursday, December 22. During the day of Friday, December 23, snow would spread from southwest to northeast into the eastern U.S. If the GFS were correct with its solution, blizzard conditions could be possible over portions of Virginia, Maryland, Delaware, New Jersey, Pennsylvania, and New York during the early morning hours of Christmas Eve, with snow pushing north and east into New England throughout the day. By midnight Christmas Eve / early Christmas Day, the GFS has only lingering snow showers left in the northeast, mainly from lake effect snow coming off the Great Lakes.
While the concept of a White Christmas is romanticized in pop culture, climatologically, they are more rare than they are common for much of the United States.
While some may be dreaming for a white Christmas, many are wondering if their dreams will turn into forecast reality. Beyond what the forecast guidance is suggesting will unfold later this week, one can also look to climatological norms to see how likely snow for Christmas is in any given area.
The map here shows the historic probability of there being at least 1 inch of snow on the ground in the Lower 48 states on December 25 based on the latest (1981-2010) US Climate Normals from NOAA’s National Centers for Environmental Information (NCEI).
The 1981–2010 Climate Normals are the latest three-decade averages of several climatological measurements. This collection contains daily and monthly normals of temperature, precipitation, snowfall, heating and cooling degree days, frost/freeze dates, and growing-degree days calculated from observations at approximately 9,800 stations operated by NOAA’s National Weather Service (NWS).
While the map shows the historical probability that a snow depth of at least one inch will be observed on December 25, the actual conditions in any year likely vary from the norm. Different weather patterns and storm systems each December set the stage for whether or not there will be snow on the ground for Christmas Day.
— the Weatherboy (@theWeatherboy) December 16, 2022