Fall arrives in Sonoma County as summer storm passes

Thursday marks the beginning of fall after a summer that included a record-setting heat wave in Sonoma County.

The season is set to kick off with a warmup on the heels of a storm that brought welcome rain to the Bay Area.

In Santa Rosa, a high of 80 degrees is forecast for Thursday, followed by highs in the mid- to upper 80s through the weekend, according to the National Weather Service.

The latest storm over the weekend came as a relief, most Sonoma County residents would agree, following a grueling heat wave that ushered in the hottest day in 120 years of Santa Rosa record keeping. The mercury reached 115 degrees in the city on Sept. 6, breaking the previous record of 113 set July 11, 1913.

This summer in Santa Rosa was the city’s third-hottest since records began in 1902, according to National Weather Service data. The average temperature was 70.9 degrees.

The only years with a higher average temperature between June 21 and Sept. 21 were 2017, with an average of 73.1 degrees, and 2019, with an average of 71.

The summer of 2020 was the city’s fourth-hottest, with an average of 70.5 degrees.

While the four hottest summers have all come in the past decade, the years with the highest averages following those “bounce around a good deal,” said weather service meteorologist Sean Miller. He said it’s difficult for meteorologists to predict whether the pattern seen in recent years will continue for Santa Rosa.

With summer in the rearview mirror, Bay Area forecasters are now keeping an eye out for common fall conditions that can promote the spread of wildfires, including low humidity and offshore winds.

While forecasters aren’t anticipating such conditions in the next week or so, “this is the peak time of year when we have the most concern for that type of thing,” Miller said.

There have been several large Northern California wildfires this summer, including the Mosquito Fire currently burning in Placer and El Dorado counties, but no major blazes in the Bay Area.

“We’ve been lucky,” Miller said.

Meteorologists and fire officials said the recent end-of-summer rain in the Bay Area wasn’t enough to eliminate the possibility of fall wildfires.

Weather service meteorologist David King said the moisture only served to “dampen fire concerns for the next week or so.”

Rainfall totals from the storm include 2.67 inches in Cazadero, 1.46 inches in Healdsburg, 0.59 inches in Santa Rosa and 0.32 inches in Petaluma.

In just over a week, meteorologists will mark the start of a new water year, a calendar the weather service uses in recording annual precipitation totals. Water years run from Oct. 1 through the end of September.

Santa Rosa has seen 27.04 inches of rain during the 2021-22 water year. That amounts to 80% of the city’s average, which is 33.7 inches, according to the weather service.

Much of that rain came near the beginning of the water year, including during a historic October storm that triggered flooding throughout Sonoma County.

On Oct. 24, at the height of that storm, Santa Rosa was pounded with 7.83 inches of rain. It was the highest one-day rainfall total since records began in 1902, shattering the previous record of 5.66 inches set Feb. 26, 2019, according to weather service data.

Aside from that fall storm, rainfall has been rather scarce.

Since Jan. 1, Santa Rosa has seen 5.63 inches of rain, which is just 26% of the normal 21.71 inches that would normally fall by this time in the calendar year.

You can reach Staff Writer Matt Pera at matthew.pera@pressdemocrat.com. On Twitter @Matt__Pera.