Forget the snow, Boise is going to get how cold? Single-digit temperatures are on the way

The full attention of Boise residents heading into Saturday afternoon is the impending snowstorm that’s set to drop at least 8 inches on the city. But once the storm blows through, another impending weather event is on the horizon:

Bitterly cold temperatures that will drop as low as 5 degrees in Boise.

An Arctic blast is plunging deep into the United States this weekend, forcing temperatures in some parts of the country to dip to as low as -13 degrees. Farther north, in Edmonton, Canada, temperatures will dip to as low as -51 degrees — frostbite on exposed skin will occur in mere minutes.

Things won’t get that cold in Boise, with the Boise Mountains shielding the Treasure Valley from the worst of the cold, but it still won’t get above freezing in Boise until Thursday.

Why is it so cold?

The cold temperatures started moving into Boise on Friday as a weakened polar vortex allowed bitterly cold temperatures to dip southward. The polar vortex is a band of strong westerly winds — similar to jet streams — about 10 to 30 miles high that contains all of the cold air that sits at the North Pole.

But once every couple of years, the polar vortex weakens, resulting in dangerously low temperatures. Scientists aren’t certain why the polar vortex weakens, but the most likely reason is climate change.

Sea ice loss and warmer sea surface temperatures than usual can affect the strength of the polar vortex, according to the National Oceanic and Atmospheric Administration, meaning even though there’s an overall warming trend, individual winter events can be much more severe.

When will it be coldest in Boise?

It may seem cold on Saturday afternoon, but it will get even colder.

The low temperature in Boise in the early hours of Sunday morning will be 14 degrees before rising to 24 during the day, according to the National Weather Service in Boise.

It’ll then drop overnight again before bottoming out at 5 degrees in the early hours of Monday. But it could get even colder than that.

“We’ll definitely see the potential for single-digit temperatures in Boise, maybe even close to zero, or below zero, so keep an eye on that,” Weather Service meteorologist Jaret Rogers told the Idaho Statesman on Saturday.

“But after about Tuesday morning, it starts to warm up a bit, a little bit closer to normal,” Rogers continued. “It’ll be below normal in the middle of the week and maybe closer to normal by the end of the week.”

Perhaps surprisingly, Boise still won’t be nearing record lows. The record low for Jan. 15 — when Boise will hit its lowest temperature of 5 degrees this week — is -26 degrees, set in 1888.

And even that isn’t the record low for Boise. The coldest-ever recorded temperature in the City of Trees is -28 on Jan. 16, 1888.

Cold temperatures affecting snow depth

The frigid temperatures plunging into Boise affect not only how cold we feel but also the snow depth.

Snow that forms in colder temperatures is typically fluffier and less heavy than snow in temperatures closer to freezing. Rogers said that such cold temperatures leave little moisture in the atmosphere, resulting in snowflakes with much less water content.

“There’s a whole relationship between how much moisture is in the atmosphere and how the individual snowflakes form; it gets into something we call microphysics,” Rogers said.

“The formation of an individual snowflake, and then multiply that, of course, by millions of snowflakes,” he continued, “it’s just the way the relationship between the temperature and how cold the air is and how much moisture is in the air and how that affects how the snow forms.”

The Weather Service remains confident that about 8 to 9 inches of snow will fall in Boise by Sunday morning, with the heaviest snow falling between 5 and 10 p.m. on Saturday. There’s also a 20% chance that the city will get more than 10 inches, which would place this storm as one of the top 10 heaviest in Boise history.