Cool air will prevail for most of this week, as the marine layer off the San Francisco coast gathers some strength after remaining pretty weak for two days.

Saturday and Sunday saw some declining air quality thanks to wildfire smoke blowing down from the north and settling closer to ground level. But that is changing by Monday afternoon, as the National Weather Service forecasts, with the marine breeze set to push the current smoke back further inland. By late afternoon, most of the North Bay and East Bay should see the haze clear out as well as the smoke gets "advected," as the meteorologists say.

(And, P.S., this is the 21st day in September, made holy by the Earth, Wind & Fire song "September.")

Later in the week, though, temperatures are going to start to go up again, with that typical-for-late-September pattern that often brings a heatwave to San Francisco the same weekend as Folsom Street Fair — which this is, but the fair is of course a virtual event this year.

A warming pattern looks to be beginning on Friday and Saturday, as Weather Underground tells us, with Sunday likely being the peak for high temperatures around the Bay Area.

The National Weather Service also forecasts a late-week heatwave, with temperatures potentially to hit 80-ish in San Francisco on Sunday, and 98 in places like Santa Rosa and Livermore.