A monster snowstorm is rare in any given Washington winter, but they are a major part of the region’s weather history. Big snowstorms — a hefty eight inches or more — come come on average once every three winters, or once every 15 storms. The real blockbusters, those dropping a foot or more, are much more rare. They tend to come on the order of once every 10 years, but they leave a much bigger impression on our weather timeline.
With decades of winter storms under our belts, Washingtonians have gone through enough big snow days to have learned some lessons. The first and possibly most important element of a big Mid-Atlantic winter storm is the track. This is the specific path that the low pressure center takes.
This map illustrates a number of tracks for big snowstorms in the D.C. region.
A few commonalities jump out at us.
- Clustering off the mouth of the Chesapeake: Details will vary, but the sweet spot for Washington snow is a storm that passes roughly 75 miles off the Delmarva coast. Storms that have centers cross into the Delmarva area tend to deliver more snow to Washington’s far western areas.
- Carolina coast superhighway: A significant number of the biggest snowstorms in Washington tracked parallel to the coastlines of South and North Carolina.
- 40 degrees north: Storms that turn east into the Atlantic at around 40 degrees north — or just off the central New Jersey Coast — tend to give the mid-Atlantic the most snow, while avoiding areas to the north. Storms that don’t make that turn can affect the whole Northeast. The turn is more likely in a “blocking” setup (see below).
The high pressure matters too
While critically important, it’s not just the track of the low that determines if and how much snow will fall. Sometimes we see a great low pressure track for snow and rain ends up falling, even in the heart of winter.
While it might seem counterintuitive, most of the blockbuster snowstorms are comprised of some serious high pressure as well. That high pressure feeds in the cold air needed to produce the fluffy stuff.
A surface high pressure will typically begin to drop into the United States from Canada in the days before a snowstorm. That helps set up the air mass ahead of it.
As a powerful winter storm gains strength in the Gulf of Mexico or Southeast, a dome of cold, high pressure is often sitting somewhere over the Great Lakes. Then as the storm is walloping the mid-Atlantic, that high pressure is hanging in southern Ontario or Quebec, funneling cold, Canadian air south into the storm.
High latitudes are often, but not always, key
High pressure and low pressure systems well to our north play a significant role in many of Washington’s biggest snowstorms. Two key areas we look at to determine whether East Coast winter storms are likely are the polar region and the area around Greenland.
Surface high pressure near the pole (a negative Arctic oscillation) helps push colder air our way. Mid-level high pressure near Greenland (a negative North Atlantic oscillation) helps act as a steering mechanism when storms approach and pass the area.
In the map above, we see an idealized set up for a big D.C. snowstorm. It’s a composite of five dozen events since the 1800s and it shows the most typical features among them.
Other than the low-pressure trough itself, which becomes the catalyst for a storm to develop at the surface, ridging in western parts of the country and into Canada helps the low pressure to ride up the coast. Additional ridging around Greenland, often referred to as the “Greenland block,” helps box in the storm.
One final element, a 50-50 low, or Newfoundland low, further configures the pattern to be snowy for the mid-Atlantic. The 50-50 low, named because it tends to be located at 50 degrees north, 50 degrees west off the coast of Newfoundland, is an upper level low pressure system that prevents the cold high pressure from drifting out to sea. This keeps cold air locked into place to filter into the mid-Atlantic.
There are plenty of snow events that happen without the block, or under less ideal scenarios than envisioned above. But when you see these kinds of setups, it’s certainly time to take notice.