When frigid air streamed over the relatively warm waters of Lake Erie this week, a “lake effect” snow event for the ages unfolded in the Buffalo area, with up to 88 inches of snow.
This historic event has raised the question whether the Washington, D.C. and Baltimore region could get snow from cold air passing over the Chesapeake Bay. The answer is no, but “Bay effect” snow can and does sometimes occur in southeast Virginia. The reason is simple geography.
In order for snow to form over a body of water, a cold wind must pass over the water for a sufficiently long distance to allow clouds to form and snow-making (i.e. condensation) processes to initiate. This distance over which the wind blows on the water is known as fetch. In other words, you need a lengthy fetch for lake effect or Bay effect snow to get going.
Lake Erie, Lake Ontario and Lake Superior – oriented west to east – provide an extended fetch when winds blow from the west. The fetch sets up places like Buffalo, Watertown, and Marquette to get hammered by snow bands that develop over the lakes. Westerly winds are very common over the Great Lakes in the cold season, so locations downwind can get walloped repeatedly, at least until the lakes freeze.
But the Chesapeake Bay is oriented from north to south, often perpendicular to the prevailing westerly wind direction. For Bay effect snow, the wind has to blow parallel to the body of water’s orientation and over a long distance. Even when the wind blows from a favorable northerly direction, there is not enough fetch for snow bands to form and affect Washington, D.C. and Baltimore, which lie adjacent to the north part of the Bay. (The Bay is also very narrow in its northern reaches, providing minimal area for snow bands to develop).
But in southeast Virginia, where there’s more water for the wind to pass over, Bay effect snow can and does occur under very specific conditions.
“Surface winds must be between 340 degrees and 010 degrees,” writes the National Weather Service office in Wakefield, Va., which serves southeast Va. “360 degrees is optimal. The direction of wind is oriented along the length of the Bay to provide the maximum over water trajectory conducive to the development of “Bay effect” snow bands.”
Below is a radar and satellite animiation of Bay effect snow bands that formed last winter, on January 24:
Typically, the Bay Effect snow events only produce light amounts of snow because the Bay is narrow, providing limited real estate for bands to grow and then dump snow.
But couldn’t the D.C. and Baltimore areas get “Bay effect” snow if there was a strong wind from the south moving up the Bay?
No. When the Mid-Atlantic has strong winds from the south, air temperatures are almost always above freezing – so snow is a non-starter. Furthermore, the air temperature must be substantially colder than the water temperature for either lake or Bay effect snow to develop.
Perhaps you never knew southeast Virginia occasionally gets Bay effect snow. Some other surprising places prone to occasional bouts of lake or Bay effect snow include the Great Salt Lake in Utah, and Cape Cod Bay in Massachusetts.