Dense fog to plague Treasure Valley this week, could create commute hazard in Boise area
Dense fog is set to envelop the Treasure Valley for the rest of the week, as an inversion keeps cool air underneath warm air and traps low-level fog near the surface. The National Weather Service in Boise expects the fog to stay in the forecast until Sunday.
This comes after weather officials in Boise extended an air stagnation advisory that was in effect from Nov. 29 till Friday, Dec. 6. High temperatures are expected to stay below 45 degrees this week, and lows will stay well below freezing.
Here is what’s on tap, according to the NWS.
How long will the fog last?
An inversion is when warm air traps colder air at the surface level, and instead of the temperature getting colder as you travel to higher elevation, it becomes warmer. Inversions trap stagnant air and low-level fog near the surface, causing breathing problems for those particularly sensitive to the pollutants.
Senior forecaster Les Colin told the Idaho Statesman that the colder air is stuck in lower elevations, at around 240 feet. At about 5,000 feet, you’re above the inversion.
The forecast shows that morning fog should clear by the afternoon, before rolling back in during the evening.
“(The fog) might move out a little bit on Thursday, compared to what it has been today and yesterday,” Colin said. “But the change is minimal at best from we can see, through at least Friday. On about Sunday, it looks like much more promising possibility, to completely scour it all out and blow all the fog out.”
Colin said a new weather system is expected to enter the Treasure Valley by Sunday, and that could raise the temperature and push out the inversion.
Is the fog related to poor air quality?
Inversion air can create issues for people who are more sensitive to pollutants, but Colin said residents should not connect fog to bad air.
“The fog is not associated with pollution, that’s an air quality issue of its own,” he said. “If air pollution is so bad that you can’t even see through it, that’s a genuine health hazard. But it’s not due to pollution, it’s due to moisture in cold air, a regular water cloud near the ground.”
According to Colin, the air stagnation advisory is due to limited to no ventilation or wind, as the air is not able to move. Any pollutants that are introduced will degrade the air quality.
At this time, the Department of Environmental Quality has not issued an air quality advisory. It did issue a yellow/moderate air quality forecast for Tuesday, Dec. 3, due to “increased particulate matter levels” and inversion conditions. DEQ expects the air quality to be 79.
However, fog can create dangerous driving conditions for commuters, reducing visibility greatly, with it reduced to about a quarter-mile along Interstate 84.
“Well, the dense fog is a hazard, it’s possible to to rear-end another car if you don’t keep enough distance, or you don’t have your headlights on,” Colin said.
How will the temperature be this week?
The forecast in place for the week is not expected to change much, the weather service said, with cold temperatures and calm winds into Thursday.
The forecast for Tuesday is a high temp of only 42 and a low of 24, with light wind from the west during the evening. On Wednesday there should be lighter fog in the morning and a sunny afternoon, according to the NWS, with a high of 41 and low of 26. And on Thursday partly sunny skies are expected with a high of 44 and a low of 28.
This story was originally published December 2, 2024 at 4:13 PM.
