Just last week, a similar bout of snow forced the closure of Interstate 80 near Laramie, Wyo.
A juiced-up summer storm meets cold air
The offseason visit by Old Man Winter comes as a robust shot of frigid air swings through at the upper levels, bringing stormy weather and cold-enough temperatures to flip rain over to snow.
The system had plenty of moisture to work with, dropping upward of two inches of rain across much of the region between the Cascades and Rockies, from the Northern Tier to the northern Rockies; 3.3 inches of rain fell from Monday through Wednesday morning in Big Sky, Mont., with 3.6 inches coming down in Clancy. Even Bozeman topped three inches of rain, greater than its average for the entire month of June.
But in Bear River State Park in Wyoming, that moisture came down as snow — at times heavy. The average high temperature for this time of year? 71 degrees.
A few mountain passes were expected to see between six and eight inches of snow, with elevations as low as 5,500 or 6,000 feet. While the snow won’t stick around, it’s certainly not every day you see a winter wonderland in mid-June.
“It’s a pretty rare event,” said Bob Nester, a senior meteorologist at the National Weather Service office in Missoula, Mont. “I’ve been here 20 years, and it’s only happened a handful of times.”
How unusual is it?
Butte, Mont., which didn’t quite see snow this time around, has only recorded June snow 11 times between 1893 and 2001, according to Nester. The city sits at roughly 5,000 feet. Nester explained that, in extreme northern Idaho and adjacent parts of western Montana, snow from the ongoing storm primarily fell above the 6,000-foot elevation.
“The focus of all the snow was above 6,300 or 6,400 feet,” explained Nester. “May and June are the wettest months of the year, especially in southwest Montana, so this is welcome.”
Nester recalled one June storm that dropped six inches on Missoula in early June 2001, prompting the city’s mayor to declare a disaster.
“That one was memorable,” recounted Nestor. “I was on the overnight shift, and we kept getting calls from people saying it sounded like firecrackers, with large [tree] limbs cracking.”
In Idaho, pockets of snow developed with the low-pressure system’s “comma head” of wraparound moisture, where showers overlapped a jet of cold air streaming into the region. Snow levels there were able to drop to 5,500 feet overnight Tuesday into Wednesday morning.
“We had reports of up to 3.5 inches of snow that fell overnight, mostly in the central part of Idaho where the highest mountains are,” said Jack Messick, a meteorologist at the National Weather Service office in Pocatello, Idaho. “If affects travel, obviously. And at some of the lower elevations [that see snow], if it’s not coniferous trees, if they’re deciduous, usually those [trees] are not used to having snow when their branches have leaves on them.”
Messick warned that could affect high-altitude vegetation in areas where leafing has already occurred.
“It’s all pretty rare, and as far as the amounts go, maybe once every five years or so we have a snowfall here in mid-June that gets this low and substantial,” Messick said. “It’s basically from a low [pressure system] that came out of the Gulf of Alaska and interior Canada, and sagged to the southeast.”
The low has brought a mixed bag of weather, ranging from summertime storms to a burst of wintry mischief.
“Usually the air aloft is very cold, and that makes the air very unstable,” favoring rising air and storms, Messick said. “We get thunderstorm activity the first day, then you get heavy rainfall amounts, and that can contribute to snow” if the rain falls quickly and heavily enough.
The good news for those eager for summer?
“Clouds will recede, and we’ll be warming into the 70s up to 4,500 feet,” said Messick, whose office is forecasting a dramatic improvement in the weather Thursday and Friday.
But in a year as wild as 2020, don’t get too comfortable.
“We’ve had snow on the Fourth of July in Pocatello at 4,500 feet back in the 1970s,” Messick said, laughing. “That was bad.”
Offseason snow has been a recent theme across the Northern Plains and Rockies. Parts of Montana saw two to four feet of snow to close out September.