Published: May 9,2015
A major late-season snowstorm is underway for parts of the northern and central Rockies and the adjacent High Plains.
Winter storm warnings have been posted for parts of western South Dakota, southwest North Dakota, northwest Nebraska, central/eastern Wyoming and the mountains of northern Colorado. A winter storm watch is in effect for parts of north-central Colorado, including the Denver metro area.
Current Radar
(INTERACTIVE MAP: Winter Alerts)
The snow is being produced by a strong upper-atmospheric area of low pressure that is sliding east into the Plains from the interior West. As of late Saturday morning, snow was falling in parts of Colorado, Wyoming and western South Dakota, including the Rapid City area.
The Sierra Nevada in California has also seen some beneficial snowfall, with reports of over a foot of new snow in the southern Sierra. Light accumulating snow also fell in the mountains of Southern California on Friday.
(MORE: Record Low Snowpack in the Sierra)
But this will not just be a mountain event. Sufficient cold air nosing southward down the High Plains and powerful lift in the upper atmosphere will allow precipitation to fall as snow even in lower elevations of the High Plains. That snow will likely be heavy, as well.
When, Where and How Much
Current Winter Weather Alerts
Saturday Night's Forecast
Sunday's Forecast
Sunday Night's Forecast
Snowfall Forecast
Through Saturday Night: Snow continues from the mountains of Wyoming and northern Colorado to western South Dakota. Rain over the lower elevations of the High Plains will changeover to snow, and become heavy for some in Wyoming, north Colorado and parts of the Nebraska panhandle. This may include the Denver area depending on how much cold air is available.
North to northeast winds will also intensify over these areas, with gusts over 40 mph likely in some locations by Saturday night. Parts of western South Dakota may see wind gusts up to 60 mph.
Travel may quickly become hazardous along parts of Interstate 90 in far western South Dakota and northeast Wyoming, as well as Interstates 25 and 80 in Wyoming, and Interstate 70 in the Colorado high country. Hazardous travel may also develop on I-25 in northern Colorado.
(FORECAST: Aspen, Colorado | Casper, Wyoming | Rapid City, South Dakota)
Sunday: The snowstorm continues in the High Plains and mountains of eastern Wyoming, western South Dakota, far southern Montana, southwest North Dakota, the western Nebraska panhandle and northern Colorado. Heavy snow will likely be accompanied by strong winds, producing whiteout conditions, at times, particularly near the Black Hills of South Dakota.
Blizzard conditions are possible, though the wet nature of the snow may limit blowing and drifting. That could keep visibilities from meeting the blizzard threshold of less than 1/4 mile for three consecutive hours.
Dangerous travel conditions are likely on Interstate 90 in far western South Dakota and northeast Wyoming, Interstate 25 in Wyoming and Interstate 80 in southern Wyoming. Parts of these interstates may have to shut down for a time. Travel may also remain difficult along Interstate 70 in the Colorado high country, as well.
(FORECAST: Rapid City, South Dakota | Casper, Wyoming | Cheyenne, Wyoming)
Heavy wet snow and high winds may lead to downed tree limbs and power outages in these areas.
(FLASHBACK: Winter Storm Atlas October 2013)
Some wet snow will also fall in parts of the Denver metro area Saturday night into early Sunday morning, particularly in the foothills. In fact, Denver has the potential to see 5 or more inches of wet snow.
Snow will slowly wind down Sunday night in Wyoming and Montana and by early Monday morning in the western Dakotas. Snowy travel will likely linger into Monday in these areas.
Total snow accumulations over one foot seem likely over a swath of western South Dakota, as well as higher mountain elevations of Wyoming, northern Colorado and Utah's Uinta Mountains. The northern Black Hills of western South Dakota may see accumulations up to 3 feet in spots, and even adjacent foothill locations such as Rapid City may pick up over a foot of total snowfall.
Some lower elevations of eastern Wyoming, southeast Montana and the western Dakotas may see at least 6 inches of wet snow. Amounts less than 6 inches are expected in the northern Colorado Front Range urban corridor, including Denver, Boulder and Ft. Collins.
(MAPS: Weekly Planner)
Plan ahead for possible road closures if you have plans to drive in any of these areas over the Mother's Day holiday weekend. Heed all National Weather Service winter storm watches and warnings. Staying informed ahead of the storm will keep you from being stranded in open country.
Are May Snow Storms Unusual?
(MORE: Winter Storm Zephyr Recap)
Rapid City, South Dakota averages 1.1 inches of snow in May and has received snow in June, with the latest date of measurable snowfall being June 13, 1969.
Interestingly, according to the NWS-Rapid City NOWDATA database, the record May monthly snow in Rapid City is only 11.6 inches in 1950. So, this event has the potential to set a May record for the city.
An early May 2008 snowstorm deposited 11 inches of snow in Rapid City on May 2, 2008. That same snowstorm dumped an incredible 54.5 inches of snow in Lead, located in the northern Black Hills.
The average date for the last measurable snowfall (0.1 inches of snow or greater) in Cheyenne, Wyoming, is May 9, so this is not that unusual for the area. Casper, Wyoming, sees almost 3 inches of snow on average during the month of May.
In the Denver area, the average date for last measurable snowfall is April 27, so this is a little later than average. However, Denver has seen snow as late as June.
When digging into the record books, even one-foot snowfalls in a single May day are not as rare as you may think in this area.
Locations with at least 5 years of data that have reported their largest post-May 5 one-day snowfall of over a foot. Anchorage, Alaska-based meteorologist Brian Brettschneider found dozens of locations not just in the mountain West, but also in lower elevations from the High Plains of Montana and the Dakotas to even northwest Kansas and the Texas panhandle that have picked up over a foot of snow on any calendar day after May 5.
Brettschneider found a station near Lusk, Wyoming once measured 36 inches of snow in a single day on May 10, 1922. Three feet...on May 10!
So, while parts of the nation are dealing with severe thunderstorms, snowthrowers and snow plows will be busy in the Rockies and High Plains, on Mother's Day weekend.
Meteorologist Jonathan Erdman contributed to this report.
MORE: Best Winter Photos 2014-2015
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