Weather apps like the iPhone's, and even the National Weather Service, under-predicted this past week's heatwave in San Francisco — and said it was only going to be a three-day warming spell, which clearly it was not. Why did this happen?

Mid-week last week we had a hint from the Chronicle's meteorology department that things had changed for the forecast.

"The strong ridge of high pressure responsible for the scorching temperatures is not behaving as forecast and has not weakened as quickly as predicted," the paper said last Wednesday. Still, even that weather team was only predicting scorching heat for one more day, on Thursday, and they did not foresee the record temperatures coming to the Bay Area by the weekend.

New records were set in Downtown San Francisco (97 degrees) and SFO (98 degrees) on Sunday, and meteorologists are now saying that this was SF's third hottest week on record — in 150 years.

Santa Rosa hit 102 on Sunday, tying a record from 1930. And San Rafael hit 107, shattering a record of 95 degrees for October 6th, which was set last year.

And the reprieve, which can't arrive soon enough, is going to come sort of slowly between now and Wednesday — with nighttime temperatures, at least, dipping to the low 60s and high 50s by Tuesday.

In most Bay Area heatwaves, Apple's Weather app and the National Weather Service alike tend to try to estimate average temperatures for the city, which typically do not accurately reflect how hot it is in the Mission, for instance, on any given day. And during a heatwave caused by a strong ridge of high pressure, SF's typical onshore breeze — the natural air conditioning from the Pacific that typically keeps our Peninsula cooler than the rest of the Bay Area — can die down completely for days at a time, leading to the sweltering conditions all those of us without air conditioning have been experiencing.

A similar wave of mis-forecast temperatures occurred during the major heatwave we experienced over Labor Day Weekend in 2017 — which was also caused by a stubborn ridge of high pressure that did not behave as expected.

And by way of example, when it comes averaging temperatures across the city in an automated, app forecast, the National Weather Service showed how the temp at noon today was 90 degrees in the Mission, but 69 out by the ocean — so would an average of 80 really be accurate for the whole city?

"It is tricky when it comes to impactful weather," says Nicole Sarment, a NWS meteorologist speaking to the Chronicle. "Obviously, across the city temperatures can range as much as 30 degrees. We’re here to provide context as best as we can across the microclimates."

Sarment added that the weather service does not amend its automated forecasts when they come out wrong.

All that said, the weather service was pretty spot on in the last few heatwaves to hit SF, typically saying that the city would remain fairly temperate while the rest of the Bay Area baked in extreme heat.

It just goes to show, though, that these fall heatwaves are sometimes a whole lot hotter in SF than the weather professionals say they're going to be.

And hey — anybody ready to go back to the natural air conditioning yet?

Previously: End to Heatwave In Sight

Top image: Photo by Mario von Rotz