At a Glance

A shift in the upper-level pattern since mid-September has brought snow to parts of the West.It is not that rare for snow to be on the ground in many of these locations for the first day of fall.

Snow is not the first thing that comes to mind when most think of the first day of fall. However, it greeted some in the northern Rockies and Sierra Nevada mountains to start autumn this year.

The snowy scenes were courtesy of a low-pressure system and unusually chilly temperatures for this time of year. The cooler-than-average conditions were due to a strong upper-level trough, or southward dip in the jet stream, that has been in place over the West since mid-September.

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The for portions of the northern Rockies was observed at the end of last week, when this chilly pattern first began to take shape. Up to 24 inches of snow was reported at the base of Great Divide ski area near Marysville, Montana, from this first round of snow.

Late this week, another low-pressure system moved through the West with a blast of colder-than-average temperatures, along with rain and snow. Snow has impacted travel in many areas, includingMontanaFriday morning.

This latest system has brought up to 23inches of snow near Moran Junction, Wyoming.

In addition, this system brought the first snow of the season to areas in the northern Sierra Nevada mountains. Generally, light snow has been reported so far, but the wintry conditions led to a in California on Interstate 80Thursday.

Several Wednesday and Thursday in the Sierra, including Sonora, Ebbetts and Tioga passes. Ski lovers, however, are enjoying this early taste of winter.

As of Friday morning, 3.5 percent of the contiguous U.S. was covered in snow, with an average depth of 0.1 inches, according to the (NOHRSC).

The image below showsmost of the snow is in the northern Rockies, but it's also seen in parts of the Cascades and Sierra Nevada mountains.

The last time snow was observed on the ground on the first day of fall was in 2013, when 0.1 percent of the contiguous U.S. had snow on the ground, specifically in the Sierra.

Areas of blue colors on the map indicate where snow is on the ground on the morning of Sept. 22, 2017.

(NOAA)

Since records began at the NOHRSC on Oct. 1, 2003,there has not been a first day of fall with a higher percentage of snow cover. Prior to this year, the highest percentage for the first day of fall was 2.8 percent in 2006.

How Common Is Snow Before the Autumnal Equinox?

While not common for snow to accumulate in most of the U.S. in September, it doeshappen. The average first measurable snow, meaning at least 0.1 inches, occurs in September in the higher elevations of the Washington Cascades and parts of the Rockies in western Montana, eastern Idaho and central Colorado – many of the spots where snow is currently observed.

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Snow in the Sierra Nevada mountains is less common than the Cascades and northern Rockies, but does occur from time to time.

The Tahoe, California, area reported snow on Thursday and the earliest snow on record for Tahoe City is Sept. 11, 1952, when an inch accumulated. The average date of first snow here, however, is not until Oct. 28.

Areas outside of the West and Alaska have also recorded snow prior to the start of fall, including Duluth, Minnesota, and Marquette, Michigan. However, the average first snow here is not until mid-October.

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Additional snow is possible in parts of the northern Rockies into this weekend, before a weather pattern change begins. The upper-level trough will slide eastward as a ridge of high pressure, or northward bulge of the jet stream, builds over the West next week.

This will put a hold on the snowy scenes like the one below seen in Montana on Thursday.

Additionally, the warmer temperatures that are expected by late next week will allow much of the snow currently on the ground to melt.

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