For potential clues, I pulled past weak La Nina Winter seasons (12 analog years) and looked at temperature and precipitation anomalies that followed during November through March. The image above shows a blend of global temperature anomalies from twelve previous October’s when weak La Nina conditions were developing across the Equatorial Pacific. Notice the warmer than average temperatures in October stretching across the Great Lakes into the Mid-Atlantic and New England.
The latest Sea Surface Temperature (SST) Anomalies (departure from average) map animation below shows cooler temperatures stretching from the coast of South America westward into the Equatorial Pacific, indicative of a weak La Nina.
The latest forecast models indicate that weak La Nina conditions are likely to continue through the upcoming Winter Season:
The probability of La Nina conditions continuing through February are currently running near a 60% – 70% chance.
A typical Jet Stream pattern during La Nina Winters (image courtesy NOAA):
The image above shows that La Nina winters favor a variable polar jet stream (fluctuations in amplification and strength) from the Pacific Northwest through the Midwest, Mid-Atlantic and New England. Temperatures in our region during typical La Nina winters would fluctuate between periods of mild and cold conditions. Strong La Nina winters favor much colder temperatures and less snowfall across our region as the polar jet stream is typically stronger. Weaker La Nina conditions on average have favored a better chance of average to above average snowfall.
One of the most important patterns that govern the amplification of the jet stream (deep ridges of high pressure and deep troughs of low pressure) are the Arctic Oscillation (AO) and the North Atlantic Oscillation (NAO). When the AO and NAO are both negative, the jet stream is much more amplified allowing bitterly cold air that builds across Canada to spill southward into the continental United States. In the image above under the La Nina column, notice the dramatic temperature differences across the United States when the NAO and AO are positive (much warmer) versus negative (much cooler). Unfortunately forecasting which phase the NAO and AO will be months in advance is nearly impossible. There are other very important weather patterns that impact the behavior of the global jet stream as well: The Pacific North American (PNA) Oscillation, the Eastern Pacific Oscillation (EPO), Western Pacific Oscillation (WPO) and Madden-Julian Oscillation (MJO) all of which I will touch on during the upcoming winter at times to illustrate how they may be directly impacting our local weather patterns.
Snowfall at Washington National Airport during twelve weak La Nina analog years have produced 4 winters with above average snowfall and 8 winters with below average snowfall.
A closer look at the temperature and precipitation blended anomalies during weak La Nina conditions from November through March
November: Cooler than average temperatures and near normal precipitation were more typical across the Mid-Atlantic.
December: Near average temperatures and precipitation were more typical across the Mid-Atlantic.
January: Above average temperatures and near average precipitation were more typical across the Mid-Atlantic.
February: Above average temperatures and above average precipitation were more typical across the Mid-Atlantic.
March: Cooler than average temperatures were typical across the Mid-Atlantic.
I’m less concerned about milder temperatures during winter as some of our biggest snow storms have occurred when temperatures were not bitterly cold. Just before the Blizzard of 1996 (a weak La Nina year), temperatures in the Washington Area were well above average (in the 70s) and nobody believed that a blizzard would impact our area shortly after. This is why the behavior of the North Atlantic Oscillation (NAO) is key in producing blocking (an upper-level ridge of high pressure) over the North Atlantic and Greenland which favors the development of coastal storms in the Mid-Atlantic and New England big cities of the Northeast from DC to Boston.
- Given last years dismal seasonal snowfall accumulation at Washington National Airport (nearly 3″ of snow) I think that our region will certainly see more snow than last year.
- I expect a higher threat of ice storms this year as the jet stream configuration favors a storm track to our west with warm air aloft moving on top of cold air masses that get trapped at the surface east of the mountains.
- I have seen plenty of past La Nina winters where a Nor’easter develops too far north and east of DC which is why my map below shows a higher probability of more snow from northern Delaware into the Philadelphia area.
- Lastly, with the polar jet being more dominant and storm systems moving our way from the west (as opposed from the south) I think “Miller B” type storm systems will be the predominant feature this winter.
Here is my 2017-2018 Washington DC Area Winter Snowfall Forecast:
Average Snowfall in the Washington Metropolitan Area:
Average Snowfall across the National Capital Region: