Corridor
This is the corridor where the haze is not always smoke, and saying so is most of the job.
The fires that drive it are in the northern Sierra, the Coast Range, the Sierra foothills, and northern California, with Nevada’s Great Basin fires adding to it on southerly and easterly setups. Where that smoke ends up is decided by three basins and one long-range ride.
Sacramento sits at the bottom of a valley with mountains on three sides and collects smoke from all of them; fires in the northern Sierra, the Coast Range, and the foothills all drain into the Central Valley, and the valley holds smoke the way a bowl holds water. Fresno, further down the same valley, starts from a worse baseline. The San Joaquin vents poorly in every direction, so agricultural dust, diesel, and industry are already in the air before a single fire starts. Reno has the worst address of all in a bad California year, directly downwind of the northern Sierra with smoke crossing the crest on westerly flow and dropping into the Truckee Meadows, where the surrounding ridges keep it. Salt Lake City takes the same westerly and southwesterly flow into a bowl between two ranges, and the mountains do the rest.
Denver is the long-range end. Colorado’s own western slope and the Utah and Nevada Great Basin feed it, but the increasingly common modern pattern is high smoke from California or the Pacific Northwest carried a thousand miles east on upper-level flow, which is why the Front Range disappears on days when nothing in Colorado is burning.
The named events are 2018, 2020, and 2021. The Camp Fire in November 2018 gave Sacramento the worst air quality in the world for days from ninety miles away and closed schools across the region. The 2020 season repeated it and put the Creek Fire in the Sierra directly east of Fresno. The 2021 Dixie Fire ran Reno through weeks of degraded air and repeated cancellations, and 2020 and 2021 both put Salt Lake among the worst air quality in the world on multiple days.
What makes this corridor different from the other two is that four of its five cities have a native haze that looks exactly like smoke and is not: Denver’s summer ozone and brown cloud, Salt Lake’s winter inversion, Central Valley agricultural dust and fog, and the San Joaquin’s year-round baseline. Every page below that has one says so plainly and gives you the tell, because a page that lets you mistake ozone for smoke has not answered the question you came with.
Listed in the order the smoke reaches them, not alphabetically. Each level here is read when this page loads and carries the time it was read. Open a city for its clear time and the rest of the forecast. Each page also carries that city's own distances and its own named sightlines, so you can check a level against what you can actually see from there.
Smokeshow is for informational and educational purposes only. It is not health, medical, or safety advice. Forecasts are model estimates and can be wrong, sometimes by a lot. Descriptions of what you might smell, see, or feel are generalizations, not predictions about your body. For decisions about your health, outdoor activity, or air quality safety, rely on official sources like AirNow.gov, the National Weather Service, and your local health authorities, and talk to a medical professional about your own situation.