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

It’s going to be a pleasant Thanksgiving in Boise, but for those traveling in or out of the Gem State for vacation, the journey home may not be as pleasant.

The National Weather Service in Boise is forecasting snow moving into the Treasure Valley area on Sunday afternoon and continuing into the early hours of Monday.

Weather Service forecaster Josh Smith told the Statesman Tuesday that while not much snow is expected on roads in the Boise area, visibility will be an issue Sunday night.

Those who head further afield than Treasure Valley will also encounter inclement weather, Smith said, particularly in northern Idaho and west of the state in Oregon and Washington.

“I would say we’re probably going to see more precipitation in the northern part of the state than down here,” Smith said. “I would also say that the further west you go the worse the weather gets; Somewhat heavier rainfall could be seen along the coast from Seattle, Portland, down in Medford.

“And in the mountains we’re definitely going to get some snowfall for this area,” Smith continued. “So if you’re traveling through the Cascades at all Sunday through Monday, you’re going to find some really good snowfall in that area.”

But before you worry 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 been over the Pacific Northwest for the past several months has filtered 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 is forecasting a high of 41 degrees for Thanksgiving, remaining 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 it to stay that way for long.

For anyone familiar with a Boise winter, inversions define the city’s long and dark winter days. Inversions occur when surface air temperature is cold and warms 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 light haze will hang over Treasure Valley at about 1,000 feet above the surface, Weather Service meteorologist Sofia Adams told the Statesman. Adams said clouds typically sit between 1,000 and 3,000 feet above the surface during precipitation, while cloud cover can be as high as 25,000 feet on calmer days.

Boise already saw an inversion last week and could be set for another this weekend before snow clears conditions on Sunday, Smith said.

Inversions can cause problems for those with sensitive lungs because the low cloud cover traps pollutants close to the surface.

“We’re going to see pollution from cars and other exhaust-type things,” Adams said. “Especially in the winter months we will see more particulate matter.

“So these are fine particles from fireplaces and stoves, which are usually wood-fired. So these will somehow accumulate at the surface and not mix into the upper atmosphere as quickly as they would if we were in a more active weather pattern.”

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