The Coldest Time of Year is Skewing Later in the East, NOAA Analysis Finds

NOAA's National Centers for Environmental Information (NCEI) produced updated maps illustrating when the coldest day of the year, on average, occurs across the United States, based on the latest version of 30-year climate normals ending in 2020.

From the Northern Plains to the Midwest, Southeast and Northeast, the last two weeks of January are typically the coldest time of year. Only some parts of northern Michigan typically are coldest after February 1.

These maps are not a forecast, but rather show the average timeframe the coldest air of the season arrives. In any given year, the actual coldest day for your area may occur before or after the time shown in the maps.

They are derived from NCEI's U.S. climate normals, a dataset that consists of the latest 30-year average of weather data, including daily high and low temperatures, for various cities throughout the U.S.

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But NOAA's analysis found something interesting when comparing the latest version of data from 1991 through 2020 with the previous 30-year period (1981 through 2010).

They found a "small, but noticeable shift" of the average coldest day 3 to 6 days later or more in many locations east of the Rockies compared to the 1981-2010 period.

The 1991-2020 data showed more warming early in winter compared to later in winter east of the Rockies, according to NOAA.

This is consistent with a previous study by Climate Central which found winter is the fastest warming season east of the Rockies and in Alaska.

NOAA found the average coldest day had also shifted later in the northwest U.S., but the rest of the West failed to show such a pattern.

From the Front Range of the Rockies westward, this coldest time of year is usually much earlier in December or early January.

The reason for this discrepancy from east to west is the frequent plunges of frigid Arctic air chilled over Alaska and Canada into the Midwest and Northeast in January. West of the Rockies, these mid-winter Arctic plunges are less frequent. Instead, Pacific storms can usher in less cold air into the West in even in the heart of winter.

For most places in North America, the day of the year when the coldest temperature occurs is typically a week or more later than the coldest average-low temperature, according to Brian Brettschneider, a climatologist with the University of Alaska Fairbanks.

The coldest temperatures of the year usually occur when bitterly cold arctic air in Canada is pulled southward into the U.S. This Arctic outbreaks usually happen around the average coldest time of the year.

For example, the late January 2019 cold outbreak was easily the coldest air of the season in much of the Midwest, parts of the Northeast urban corridor and the Deep South.

But that's not always the case.

Another bitterly cold outbreak in early March 2019 was the coldest of the season in Great Falls, Montana (minus 32 degrees); Dallas (21 degrees); and Wichita, Kansas (2 degrees).

When All-Time Records Were Set

A number of locations set their all-time record lows in December, January or February.

Boston, New York City's Central Park and Philadelphia all set their records in a Feb. 9, 1934, cold outbreak. Washington, D.C., set its record on Feb. 11, 1899.

Many Midwestern cities set their all-time records in January, including Chicago (Jan. 20, 1985), Cleveland (Jan. 19, 1994) and Minneapolis-St. Paul (Jan. 21, 1888).

In the South, the January 1985 cold outbreak set all-time records that still stand in Charlotte and Raleigh, North Carolina; Charleston, South Carolina; and Nashville, Tennessee. However, the historic February 1899 cold outbreak, arguably the greatest in modern history, still is the all-time record holder in Atlanta; Dallas; Little Rock, Arkansas; and Tallahassee, Florida, among other cities.

Western all-time record cold varied from as early as Dec. 11, 1932, in downtown San Francisco to Feb. 15, 1936, in Great Falls, Montana. Anchorage (minus 34 degrees) and Fairbanks (minus 66 degrees) each set their all-time cold records in early to mid-January.

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