Arctic Blast Coming Next Week…Christmas Forecast Remains Uncertain

There has been some talk over the last few days regarding a storm around Christmas, which has the potential to bring nothing, rain, ice, or snow to the area. The fact of the matter is that there are still major differences in the computer models about this forecast (as is usually the case a week out). The European model is much slower with the system’s evolution which keeps us much warmer and slightly drier, compared to the faster GFS/American model which is much colder and a little wetter. Once this system moves through, it will turn COLD next week – all the models show this to be the case giving much more confidence in that forecast. It won’t be record cold, but highs may struggle to get above freezing a couple days next week.

Before we dive into this year, here’s a look at some of the past Christmas’s in the area.

A quick note, the “years with at least 1″ on ground” includes snow that fell in the days before Dec 25, and not only snow that fell on Dec 25.

Christmas Storm…Major Timing Differences

As I mentioned above, there are major differences between the GFS and European models on what to expect for Christmas Eve and Christmas Day. The following image is the upper level pattern at 7AM Christmas morning, comparing the GFS (left) and the European (right) models. There’s a lot going on, but don’t worry, I’ll explain what you need to know.

The European model is running about 200 mi slower than the GFS at the same time. If you look closely, the dark red dashed line that I labeled ‘trough axis’ on the GFS is already into Arkansas and Missouri and moving into the Mid-West, while on the Euro it still is across Central Oklahoma, Kansas and the Great Plains. 200 Miles may not seem like a lot when looking at large areas like the United States, but when it comes to weather & forecasting, 200 miles is a night and day difference.

The result of the faster/GFS solution is that the warm air to the east of the system is already moving out, the surface cold front is farther east, and the precipitation stretches from the Appalachians to the East Coast. While the solution is colder, the GFS also does not bring the cold air in time to deliver a white Christmas to the entire region, only the mountains/areas in higher elevation. The I95 Corridor/east would see chilly rain with temps in the 30s.

GFS Model 7AM Dec 25. Blue/Purple = Snow, Green = Rain

The result of the slower European solution is essentially the same, except everything is shifted about 200 miles west. The means it is still very mild over the entire East Coast. In terms of precipitation, per the Euro, our area is dry, and the Mid-West is getting the snow/frozen precipitation and rain is beginning to push into the Appalachians. If we continue into the future, rain arrives late in the day, and it possibly ends as snow early on Dec 26 before it all moves out.

European Model valid 7AM Dec 25. Blue = Snow, Green=Rain, Pink = Mix/Sleet/Freezing Rain

And here’s a look at the temperatures at 7AM Christmas morning from the GFS and Euro models. The Euro is running 10-20 degrees warmer than the GFS!! That would be a very WARM Christmas day!

In fact, the European has afternoon highs in the 60s, with some spots pushing 70°, while the GFS has 30s and low 40s.

Ensemble Models

I’ve mentioned them before in previous forecast discussion, the Ensemble models. The images above come from what we call the operational GFS and Euro models, but we also have the GFS and Euro Ensembles. Remember than and Ensemble is taking one computer model, altering the initial conditions and running the model again. If you do this enough times, you can average them together and compare to the operational model to see if its solution makes sense. The GFS has 21 ensemble members, and the Euro has 51.

In the upper levels, the Ensemble means are very similar to the operation runs. The GFS Ensemble also runs slightly faster than the Euro Ensemble, though the distance is not as great. Both ensemble means have precipitation reaching the coast by Christmas morning, but neither is cold enough for snow.

If we look at the European Ensemble Temperatures (and I’m just using BWI as an example), there is a very large spread among the individual ensemble members. The mean temperature is 43° for a high on Christmas day, but the range is from about 31° to 64°. A range this large indicates a high amount of uncertainty in the data, and tells me as a forecaster that the model doesn’t yet have a good handle on the system. Though not shown, the GFS Ensemble has a slightly smaller range (mid 30s to low 50s).

European Ensemble Max Temperatures. Image via Ryan Maue / weatherdotus

So, the take away is that this matter is still unsettled. Much of the forecast will depend on the evolution of the upper level pattern over the next week, which is becoming highly amplified (stronger troughs/rigdes in jet), and the models have trouble when the upper levels are amplified.

The Cold Next Week!

European Ensemble 5 day Temperature anomaly at 850mb / ~5,000 feet above ground. Valid 7PM Dec 25 to 7PM Dec 30. Blues/greens/Purples show colder than normal expected conditions

If you look at the BWI graph above beyond Christmas, you’ll notice the temperatures dipping way down next week. In fact, taken verbatim, the European Ensemble has high temperatures at or below freezing Wednesday through Saturday. The GFS Ensemble is similar for the most part, though a couple degrees warmer, especially on Saturday. Regardless, it’s going to be cold next week, which is good news if you’re hoping Santa brings you some new winter gear for Christmas! Though, this is just model output and NOT a forecast. This is also just using BWI as an example, and temperatures do vary by location.

Lot’s to watch over the next week as the holiday’s approach. If you’re traveling, stay tuned as details become more clear and the forecast refines itself over the next week.

-Andrew