The timing of the first snow across North America is highly variable, depending almost entirely on latitude, elevation, and the annual pattern of weather systems. While some areas experience snow in late summer, others must wait until the new calendar year for even a trace of white. To understand the start of the snow season, it is helpful to look at typical averages rather than the rare extremes.
Defining the Start of Winter Precipitation
Meteorological agencies distinguish between the first sighting of snow and the first event of measurable snow. The “first observable snow” may consist of a few flakes that melt before touching the ground, or a trace amount too small to record. This visual date can occur weeks earlier than the official start of the snow season.
“First measurable snow,” the standard used in long-term climate records, is defined as an accumulation of 0.1 inches or more. This threshold ensures the recorded event had a genuine impact on the ground surface and provides a meaningful benchmark for the start of the winter precipitation season.
Geographic Variability in North America
The first measurable snowfall can occur as early as September in the highest elevations of the continent. Peaks in the Rocky Mountains and the Cascades, for example, commonly see accumulating snow in September or early October due to high altitude, resulting in thinner atmosphere and lower average temperatures above the tree line.
Moving into the northern tier, including the Upper Midwest, the Great Lakes region, and mountainous New England, the first measurable snow typically arrives in November. Areas like the I-95 corridor, the Ohio Valley, and the central plains generally see their first snow in late November or early December. These regions lie at a lower latitude where temperatures remain marginally warm well into autumn.
Further south, the start of the snow season is delayed significantly, often until the new year. Cities in the Mid-Atlantic and the Deep South usually experience their first measurable snowfall in January or February. In these southern locations, the window for snow is short, and it is common for some years to pass without any measurable accumulation.
Key Meteorological Factors Influencing Timing
The timing of the first snowfall is governed by two primary atmospheric conditions: the temperature profile and moisture availability. Snow forms when temperatures are at or below the freezing point of \(32^\circ\)F (\(0^\circ\)C) through the entire column of air from the cloud to the ground. Even if the ground temperature is slightly above freezing, evaporative cooling as the flakes fall can sometimes allow snow to reach the surface before melting.
Macro-level climate patterns, such as the El Niño-Southern Oscillation (ENSO), influence when and where cold air and moisture align. During an El Niño winter, the Polar Jet Stream typically shifts southward, often resulting in increased moisture and snowfall across the southern United States. Conversely, a La Niña pattern tends to push the jet stream and colder air farther north, often reducing snowfall in the northern Rockies and Great Lakes region.
The Polar Jet Stream, a fast-moving ribbon of air high in the atmosphere, dictates the path of cold air masses and storm systems. When the jet stream dips into the mid-latitudes, it draws frigid air from the Arctic south, dramatically lowering temperatures and increasing the chance of early snow. A wavy jet stream pattern can bring early bursts of cold air deep into the continent, causing snowfall ahead of the average date.
The Difference Between First Snow and Sustained Snow Cover
The first snowfall is often a transient event that melts quickly upon contact with the relatively warm ground. The earth retains heat absorbed over the summer, and until this heat dissipates, the ground temperature remains above freezing. This initial warmth causes the snow to melt almost immediately, preventing accumulation.
Sustained snow cover, defined as snow remaining on the ground for an extended period, usually begins several weeks after the first flakes appear. For accumulation to last, the ground must freeze, requiring a period of consistently sub-freezing air temperatures. Depending on the latitude and depth, this sustained cover can begin four to eight weeks after the first trace of snow.
The onset of sustained cover marks the true start of the winter season for activities like winter sports and travel preparation. Once a layer of snow establishes itself, it acts as an insulator, slowing the loss of heat from the soil and helping to preserve the snowpack. This insulating effect makes subsequent snowfall much more likely to accumulate and persist.