San Francisco's first significant bout of "real summer" or "September summer" looks to be headed here on Monday, with the possibility of those unsettling offshore winds that leave the city feeling hot hot hot.

You've got today's persistent marine layer to endure, and another weekend of morning drear and only slightly warmer afternoon temperatures possibly. But just in time for the workweek to begin, a heatwave is going to land in the Bay Area and last a couple of days, and this time SF won't be left out of the party.

Yes, the week of Labor Day brought us some balmy days that probably made some of your apartments uncomfortably warm. But as the Chronicle meteorology team explains, a real warm-up starting Monday could bring 80- or even 90-degree highs to San Francisco proper, as the weather pattern takes a dramatic turn.

"Weather models predict the system to align in a way that suppresses the daily sea breeze, compresses the marine layer and even flips winds offshore at low levels," the Chronicle says.

Offshore winds, for those not in the know, are the opposite of what SF experiences 350-some-odd days of the year, and they happen when the wind begins flowing out of the East Bay and out to the ocean — something that often results in SF's hottest days of the year, with our natural air conditioner, the ocean breeze, suddenly absent. (These are also akin to the Diablo or Santa Ana wind patterns that can make for hellish moments during fire season, but that's not what's being forecast here.)

Downtown SF and Oakland are expected to see a high in the 80s on Monday, but as the Chroncile notes, "If offshore flow dominates, both cities could make a run at 90 degrees Tuesday." The meteorologists note that a very similar pattern took hold on September 13, 2019, and Oakland and SF saw temps in the 90s that day.

Other forecasts indicate that the sun and warm days will extend further into the week, and when these offshore wind patterns take hold, they can do so for several days, so brace yourself. Prepare to swelter if you don't have AC. And buy a fan if you don't have one.

Photo by Mauro Lima