It's expected to be beach weather once again in San Francisco today, with temperatures in the mid-80s in the city, but it's going to be in the 100s out in the East Bay and up north, where Napa and Sonoma wineries are bracing for power outages tonight that could be disastrous for harvest time.

The Oakland Fire Department is preparing for worst-case scenarios Tuesday, with a red-flag warning in effect and temperatures already soaring. As Battalion Chief Zoraida Diaz tells KRON 4, the department has already "pre-position[ed] a wildland firefighting engine" in the East Bay Hills ahead of any possible fire outbreaks. The hills that overlook Oakland and Berkeley saw a devastating fire 28 years ago, in October 1991, and no one wants to see anything like that repeated.

What that week in October had in common with the October week when the Tubbs and Nuns Fires broke out in 2017, and with the week we're having now, is the phenomenon known as the Diablo winds. Similar to the Santa Ana winds in Southern California, the winds are marked by a weather pattern in which California's typical onshore wind patterns reverse in the fall, and warm winds begin blowing from inland toward the coast. The state has typically seen some of its deadliest and most destructive wildfires when these winds start blowing, which is the reason PG&E is taking precautions this week to shut off power to regions where the winds are the strongest.

Over 20,000 utility customers in Nevada, Yuba, and Butte counties lost electricity Monday evening, and tens of thousands more may see a shutoff tonight in Napa, Sonoma, and Lake counties, in addition to spots in the East Bay and the Sierras.

In the East Bay, Diaz says, "We’re encouraging people to not start any power tools outside, [and] no barbequing because any small fire can quickly escalate into a large scale event." Additionally she says people can protect their homes by clearing brush and making sure their lawns are well maintained.

Up in Napa and Sonoma, the major worry at wineries is how an extended blackout could impact their ongoing harvests. As the Chronicle reports, suppliers in the North Bay are selling out of generators, and smaller wineries in particular are facing the possibility that harvest time is going to continue running up against these pre-emptive power shutoffs.

Modern winemaking requires a lot of electricity at harvest time, in order to operate things like de-stemmers, sorting tables, crushers, and pumps, and if the process is halted for too long, whole batches of grapes could lie in waste. Then there's the issue of temperature control in wine cellars and fermentation rooms as the temps outside heat up.

"If we learned we were getting our power shut down, we’d have to cancel our picks," says Mick Unti of Unti Vineyards in Healdsburg, speaking to the Chronicle. But he explains that once the juice is in tanks, fermentation can be extremely temperature sensitive, and an outage could be disastrous.

Other wineries expressed fears of lost business if a power outage extended through a day's traffic in the tasting room, shutting down their ability to run credit cards.

PG&E warned Monday that shutoffs today were likely to begin around 7 p.m. as the wind pattern begins taking shape. More specific announcements should be made here by Tuesday afternoon.