As expected following Monday's Fire Weather Watch, the National Weather Service has issued a Red Flag Warning covering most of the Bay Area, San Francisco included, starting Thursday night.
The Diablo winds are coming, and with them, along with extra-low humidity levels, comes high risk for wildfires. The Fire Weather Watch issued Monday is set to begin at 11 am Thursday and extend until Saturday night, and now the National Weather Service has issued a Red Flag Warning covering the same area, but starting at 11 pm Thursday. This includes almost all of the Bay Area, with the exception of the Marin Headlands, and parts of the Marin and Sonoma County coastline.
The warning extends through 5 pm Saturday, when the winds are expected to die down.
The Santa Cruz Mountains and coastal Santa Cruz County are outside the warning area, however parts of the Monterey County and Big Sur coast are inside the warning zone.
Conditions appear to be worsening in terms of fire danger, and the weather service is now predicting stronger winds and humidity levels as low as 10%.
Wind gusts, predicted at 30 to 40 miles per hour as of Monday, may go up to 45 miles per hour, forecasters are now saying, with isolated gusts of 65 miles per hour at the highest ridgelines in Napa and Sonoma counties.
"Stay vigilant!" the weather service warns.
The watch has been upgraded to a Red Flag Warning for critical fire weather conditions Thur night through Sat. Daytime RH as low as 10% and strong offshore winds 25-35 mph with gusts to 45 mph are expected. Gusts to 65 mph possible in the highest terrain. Stay vigilant!#CAwx pic.twitter.com/McklAEahwr
— NWS Bay Area 🌉 (@NWSBayArea) October 15, 2024
It is rare for San Francisco to be included in these warnings, given our unique position at the tip of a peninsula which is typically protected from the bulk of these offshore winds. But the latest warning indicates that we will be feeling the Diablo winds and their strange effects, blowing east to west, starting Thursday night. And while the city is not full of the kind of dry fuels found in less populated parts of the Bay Area, there could be the danger of a small fire rapidly pickup momentum from these winds, which could be disastrous in a densely populated area.
The weather service warns against lighting outdoor fires of all kinds, and against driving over dry grasses or vegetation with a car, which could also spark a fire.
As SFist noted yesterday, some of the Bay Area's worst wildfires in recent years were sparked in October under Diablo wind conditions. For a primer on Diablo winds — which are the NorCal equivalent of Santa Ana winds — see a primer here. Basically, a high pressure system in the Great Basin, in Nevada and Utah, will cause warm air to get pushed over the Sierra Nevada, into the dry Sacramento Valley, and up and over the ridge lines in the Bay Area, compressing and warming as it goes, and further dropping humidity levels as it rushes over the highest elevations and down toward sea level.
