Here's When Winter Weather Typically Arrives

 Chris Dolce and Linda Lam

Published: October 18, 2020

Winter officially begins on Dec. 21 this year, but depending on where you live, true winter weather conditions can begin well before this date or even after it passes.

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.

How much snow typically falls 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 categories are site-specific ... because what constitutes a severe winter, say, in Washington D.C. or Atlanta would be considered mild in Chicago or Minneapolis," said Dr. Barbara Mayes Boustead, co-creator of AWSSI and a forecaster at the National Weather Service in Omaha, Nebraska.

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.

When the average first measurable snowfall (at least 0.1 inches) occurs, based on data from 1981-2010.

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)

The average date of the first freeze, based on data from 1981-2010.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.