So … it’s about that time of year, and for some of you, it’s already happened. Winter’s first snow. For those who are still waiting, here’s a handy historical map that ballparks the first day of snow for an average year. It’s based on the "normals" dataset constructed here at the National Centers for Environmental Information—the same dataset from which the familiar “Today’s normal high is ____” bit of your weathercast is drawn.
Before we dive into interpretation, a few caveats: this map isn’t the average date of the first observed snowfall. Technically, it’s the date by which there’s a 50% chance at least 0.1” of snow will have accumulated. It’s based on historical patterns from 1981-2010, with some “smoothing” to account for statistical noise in the data.
Colored dots indicate the date by which there’s a 50% chance at least 0.1” of snow will have accumulated, based on each location’s snowfall history from 1981-2010. Image Courtesy: NOAA
By taking a closer look at the map, you can see Indiana favors October 1 - 31 while Kentucky leans more toward November 1 - 30. Image Courtesy: NOAA
A quick look at the map reveals the obvious: the main factors that govern the average day of your first snow are the same main factors that govern your other major climate characteristics: latitude and altitude. In general, the farther north you are, and the farther up you are, the earlier the threat of first snow. In fact, the highest elevation stations along the spines of the Rockies have a year-round threat of snow. On the other end of the spectrum, the Deep South, Gulf Coast, Desert Southwest and Hawaii have many stations that get snow so infrequently, there’s no date listed (empty circles). In these places, there just aren’t enough events to make robust statistics.
There are also a few interesting, but more subtle, regional features. Across the Plains and Midwest, instead of a straight-up east-west pattern, there’s a slight northeastward tilt, where earlier dates plunge farther southward in the plains, relative to the more Midwestern states. It’s especially noticeable along a line stretching roughly from the Texas panhandle to Chicago. This has a little bit to do with elevation, but more to do with the fact that the coldest air associated with many winter storms—including early winter storms—often barrel down the high plains, corralled by the Rockies to the west.
You may also notice subtle plumes of earlier dates peeking south from the Great Lakes. If you grew up near their southern shores, you know the drill: the first very cold, northerly winds of autumn can pick up a lot of moisture from the still-relatively-warm lakes. This often results in a rather rude, and rather early, annual introduction to winter precipitation. It’s already happened this year in parts of Michigan and upstate New York.
As you might expect, the first date of snowfall isn’t a constant thing. It varies naturally, like many other climate variables. That’s why we call them “variables,” right?
For example, take a look at Dodge City, Kansas. The dots on this graphic show the first date with one-tenth* of an inch or more snow for each cold season from 1950/51 (top) through 2014/15 (bottom).
The dots span from mid-September at the earliest to early January at the latest. That’s a three-and-a-half-month spread! Don't forget, this is solely based on climate data and is by no means a FORECAST. Hang in there snow lovers!