After several inches of snow, Boise must prepare for Arctic air. Here’s what to expect
The Treasure Valley has suddenly been plunged into the steely grip of winter, and there’s no apparent end in sight.
After high temperatures reached the mid-40s as recently as Sunday afternoon, over 3 inches of snow fell in Boise by Monday morning, and temperatures will drop into the single digits by the end of the week.
National Weather Service meteorologist Jay Breidenbach characterized the incoming drop in temperature as “Arctic air.”
“(Monday) afternoon is going to be the last time we’ll be above freezing for at least the next week,” Breidenbach told the Idaho Statesman on Monday. “Maybe even longer than that, but certainly, for the next five to seven days, we’re not going to see temperatures go above freezing.”
The upcoming temperature dip is a result of the storm that passed through Boise on Monday morning, which The Weather Channel calls Winter Storm Diaz.
An over-performing snowstorm
From late Sunday night until about midday Monday, about 3.4 inches fell in Boise, according to measurements from the Weather Service.
About an inch fell before midnight before the remainder fell Monday in two separate waves — about 1.5 inches in the early hours of Monday morning and 0.9 inches later in the day.
School districts across the Treasure Valley canceled classes Monday, including the Boise, West Ada, Nampa, Caldwell, Kuna and Middleton districts.
Breidenbach said the storm over-performed, with only about 10% of weather models showing Boise to be getting over 3 inches — most models were showing about an inch of snow for the upper Treasure Valley.
The increased snowfall was because of the direction of snow bands, which lined up perfectly over Boise and dropped a continual stream of snow over the city, Breidenbach said.
“The whole area of low pressure was centered to our southeast,” Breidenbach said. “If you think about bands of precipitation spiraling around that, sometimes they’ll form bands of precipitation like you see with a hurricane. Of course, this was not a tropical cyclone storm, but we did see heavier bands of precipitation that lined up and just kind of sat right on the Boise area.”
Breidenbach said that areas closer to the Boise Foothills got around 4 inches while one measurement location near Mountain Home picked up 5 inches.
The storm brings Boise’s season total up to 9.3 inches — before Sunday night’s storm, it was at 6.9 inches, which includes the 1.6 inches of snow that fell late last week.
There’s no more snow in the forecast for Boise for at least the next week, but the effects of Winter Storm Diaz are long from over.
Cold Arctic air
By Saturday night and into Sunday morning, the Weather Service forecasts temperatures in Boise to plummet to as low as 6 degrees.
As Winter Storm Diaz moves farther east across the upper Midwest, it’ll draw a heavy dose of Arctic air into the Northwest behind it.
“If you can imagine cyclonic circulation around the storm, as it moves away from us, we’ll be in the northerly flow around that storm,” Breidenbach said. “So that’s going to drive some very, very cold air down across the entire region.”
The snow on the ground will also contribute to colder air. When there are clear skies and snow on the ground, a process called “radiative cooling” occurs. According to Breidenbach, the Earth’s surface will radiate heat into the clear sky, causing cooler temperatures near the surface and helping to amplify the cold air mass.
By Tuesday night, the temperature will be 16 degrees and progressively cooler each night.
Freezing fog is also possible in the Treasure Valley on Tuesday night into Wednesday morning.
Freezing fog occurs when temperatures are below 32 degrees, and miniature water droplets in the fog can instantly freeze on surfaces that are cold enough, including bridges, overpasses and cars.
“Here in the Treasure Valley, freezing fog is certainly possible and that can make roads a little bit slippery,” Breidenbach said. “Typically though, the roads are not going to be as impacted as much as automobile windshields, which will freeze up a little bit.”
This story was originally published December 12, 2022 at 3:41 PM.
