Here's When Winter Weather Typically Arrives

Climatologist Dr. Brian Brettschneider of the University of Alaska-Fairbanks compiled the map below that captures the median date for when the first winter weather conditions of the season have historically arrived in the United States and Canada.

Brettschneider used an index called the Accumulated Winter Season Severity Index (AWSSI) to create the map using weather data over the past 40 years.

For any given location, the start date of winter, based on the AWSSI, is defined as when the first measurable snowfall (0.1 inches or greater) occurs, or when the first high temperature of 32 degrees or lower is recorded. The start date is Dec. 1 for any location that does not see either prior to that date.

As you would expect, the median date when northern climates historically see their first measurable snow or high temperature 32 degrees or lower is before winter officially begins.

For parts of northern Canada and Alaska, the median date is long before the official start of winter, occurring in September or even earlier.

October is when it begins to feel like winter for much of the Rockies and Northern Plains. Average October snowfall for parts of the Rockies is above six inches, while much of the Northern Plains into northern Michigan and northern New England often measure at least two inches of the fluffy, white stuff in October.

Farther east, the median date for the arrival of the first winter weather conditions of the season is in November in a swath from the Northeast into much of the Midwest.

Winter starts Dec. 1 in much of the South, based on the AWSSI index criteria, since the median date of the first measurable snow or first high temperature 32 degrees or lower is after that date. In some years, parts of the South never see measurable snow or a high temperature of 32 degrees or lower.

(MORE: Winter 2020-21 Outlook)

During the course of the winter season, the AWSSI tracks the severity of winter weather conditions in real-time for several cities across the U.S.

The index takes into account the "intensity and persistence of cold weather, the frequency and amount of snow and the amount and persistence of snow on the ground," according to the Midwest Regional Climate Center. Wind and mixed precipitation, such as freezing rain, are not a part of the index.

The index uses five categories – mild, moderate and average to severe and extreme – to rate the severity of winter weather at any given point in the season.

You can track the AWSSI during the winter ahead at this link.

Average First Snowfall and Freeze

The average first snowfall of the season can be as early as September in the highest elevations of the West, based on 30-year average data.

Most of the northern tier experience their average first snow by the end of November. Farther south, snow may not fall every season.

The first freeze of the season usually occurs in September in much of the higher elevations of the West and near the Canadian border.

Most of the Northeast and Midwest record their first 32-degree low temperature in October.

November is when areas from the mid-Atlantic coast into most of the South typically experience their first freeze of the season, but some areas near the Gulf Coast, Florida and the Southwest wait until after Nov. 30, if a freeze occurs at all.

(MORE: How Early Has Your Area Experienced Freezing Temperatures)

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