DC December Forecast: Winter cold is coming, but will December 'snow-prise' the District?
WASHINGTON - We told you in our Fox 5 Winter Outlook back in October that we expected temperatures in the latter half of fall and into the early winter to feature lots of ups and downs, periods of both warm and cold.
Here at the very end of November, we are certainly dealing with an "up". Average temperatures have been running above seasonal since the weekend. The mercury hit 70 degrees on Wednesday for the first time since Nov. 7 and is expected to hit 60 degrees on this final afternoon in November.
It's hard to believe that December begins on Friday and the first full week of December should feature a good amount of seasonal variability, a chilly day here and a mild day there, but nothing too atypical. However, as any "Game of Thrones" fan will tell you, "winter is coming," and for the eastern half of the county, winter looks to roar in during the latter half of next week.
While November brought its fair share of chilly temperatures to the region, the coming cold blast by the second week of December has all the signs of the regions first real taste of an extended winter chill. For December, this means the potential for overnight lows in the teens and low 20s while afternoon highs stay in the 30s for many locations.
While the details about which days will be the coldest will still need to be worked out in the weather modeling world in the coming days, there is a good amount of confidence in the following things:
First, the cold is coming late next week. Model agreement across the board is strong, and they have been consistently showing it for the better part of the past five days.
Second, this will be a longer-lived cold pattern as opposed to the shorter-lived events of cold that our region saw at times in November. Strong blocking patterns setting up over the high latitude Arctic region will be slow to break down. There is a decent chance that colder than normal conditions will persist right up to and perhaps through the Christmas holiday as well.
Now to the reason you clicked on this article: let us talk snow.
The pattern that weather models are forecasting is an impressive one from a cold standpoint, but it also shares similarities with some of the snowier Decembers we have had in the Washington, D.C. area.
Compare the image with the forecast pattern with the pattern seen during the snowiest Decembers since 1950. You will see a lot of similarities, but also some differences as well.
Contrary to popular belief, December is actually not typically a snow-filled month for our region. In the 132 years of recorded snowfall records for the District dating back to 1884, only 23 of them have seen December receive more than 6 inches of snow.
In fact, over that 132-year stretch, Washington, D.C. averaged just 3 inches of snow. Even by that margin, recent Decembers have been lackluster. Our region has not seen above normal snowfall in the month of December since the blockbuster year of 2009, which featured one of the worst blizzards in the city's history just six days before Christmas.
However, it has also been about that long since a pattern similar to the one forecasted has overtaken the region in December with the same forecasted longevity.
Despite sharing some pattern similarities with 2009, there is one glaring difference: El Nino.
Driven by ocean temperatures in the equatorial Pacific Ocean, El Nino patterns tend to produce more storminess than their La Nina counterparts, and unfortunately for blizzard lovers we happen to be in the midst of a weak La Nina pattern. Now that does not mean they cannot happen, it just means they tend to be more difficult to do.
There is only one La Nina case where the District received more than 10 inches of snow from a single storm in December and that was all the way back in 1973. So you can see why it would be difficult for us to favor such a scenario. Could it happen? Sure, and in fact, I may have even suggested there would be a decent chance of it if this was an El Nino winter.
However, just on the basis of statistics, I think it would be a stretch to favor such a scenario at this point at this point in time. Perhaps this will change as we get deeper into December. We'll see.
A more favorable possibility is some smaller scale events. Ones that may allow our region to squeeze out a few inches of snow before the month is out. After each run of our weather models, the computer picks 10 years in the past that have a similar pattern to the one that is being forecasted.
These are called analog years, and the good news is that every single one of the top 10 cases for December had some form of measurable snow. Half of them even had above normal snowfall throughout the month, which is something that has only happened twice in the past decade.
So the question then becomes, if we are going to see any snow, when would it be?
Well, the cold pattern enters our region heading into the next weekend, likely Dec. 7 and Dec. 8. While many have shared with me some whispers of snow on social media around this time, I do not like it. Here in the District, it is pretty tough for us to get measurable snow as we break a warm pattern and head for cold.
I think if we are going to see snow, it is going to be beyond the next weekend time frame. While predominately a cold pattern during the second half of the month, the cold will at times be reinforced. It is during these periods of cold air reinforcement that we will have the best chances of squeezing out some snow.
While not favored, if we were to get a bigger storm it would probably be as the larger scale pattern begins to break down across the polar region, which is not likely until later in the month.
The final, quick point I will make is to be careful what you see on social media when it comes to snowfall. As forecasters, except in rare cases (January 2016 blizzard), we usually do not feel confident enough in a forecast to make the call whether or not our region is looking at rain or snow until three to five days out in the best of cases.
Getting snow to fall is like baking a cake. You need all the right ingredients to come together in just the right amounts, otherwise, you will ruin the cake. While I can confidently say that the pattern is more favorable for snow than it has been in recent Decembers, that does not mean I can guarantee it.
Weather models struggle extensively with snow in the extended range. Take the image example of the European weather model, typically one of the most accurate models. Wednesday afternoon the model, which forecasts the next 10 days, was forecasting a big storm for the Mid-Atlantic region next weekend. Thursday morning, the latest run of that very same model was quiet for the same period. So again, be careful what you read on social media, especially if they are talking big snow beyond a week in the future. When we are confident we are looking at any snow falling in our region we will absolutely let you know.