Here’s your Thanksgiving week forecast, Boise. Beware weekend snow, return of inversions

It will be a pleasant Thanksgiving in Boise, but for those traveling in or out of the Gem State for the holiday, the journey home may be less enjoyable.

The National Weather Service in Boise forecasts snow to move into the Treasure Valley area Sunday afternoon and persist into the early hours of Monday morning.

Weather Service meteorologist Josh Smith told the Statesman on Tuesday that although not much snow accumulation is expected on roads in the Boise area, visibility will be an issue on Sunday evening.

Those driving farther afield than the Treasure Valley will also encounter inclement weather, Smith said, specifically in North Idaho and west of the state in Oregon and Washington.

“I would say that in the northern part of the state, we’ll probably see more precipitation than we will down here,” Smith said. “I would also say the further west you get, the worse the weather gets; along the coast from Seattle, Portland, down in Medford, you could see some heavier precipitation there.

“And the mountains, for sure, we’ll get some snowfall for that area,” Smith continued. “So if you’re going to be traveling through the Cascades at all on Sunday into Monday, you will encounter some really good snowfall in that area.”

But before worrying too much about the journey home, the days around Thanksgiving will be closer to average temperatures and sunny for the Treasure Valley.

A high-pressure ridge that has sat over the Pacific Northwest for the past couple of months has been filtering cold air from the north into Idaho, resulting in below-average temperatures.

A small weather system Tuesday night helped clear the ridge and brought in some warmer air from the Pacific. The Weather Service forecasts a high of 41 degrees on Thanksgiving, which remains 4 degrees below the historical average for Nov. 24.

The return of inversions

Although the skies will be blue and clear over Thanksgiving, don’t expect that to last much longer.

For anyone familiar with a Boise winter, inversions define the city’s long and dark winter days. Inversions are when the air temperature is cold at the surface and warms up the higher you go — the opposite of normal conditions — and occur most commonly between Thanksgiving and Valentine’s Day, Smith said.

During inversion conditions, a low haze will hang over the Treasure Valley at about 1,000 feet above the surface, Weather Service meteorologist Sofia Adams told the Statesman. Adams said clouds usually sit between 1,000 and 3,000 feet above the surface during precipitation, while the cloud ceiling can be as high as 25,000 feet on calmer days.

Boise already experienced one inversion last week and could be set for another this weekend before Sunday’s snow clears out the conditions, Smith said.

Inversions can cause issues for those with sensitive lungs due to the low cloud ceiling trapping pollutants close to the surface.

“We’ll see pollution from cars and other exhaust-type things,” Adams said. “Especially during the winter months, we’ll see more particulate matter.

“So those are fine particles from generally wood-burning fireplaces and wood stoves. So those will kind of collect at the surface and won’t get mixed up into the upper atmosphere as readily as they would if we were in more of an active weather pattern.”