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Bay Area’s version of Santa Ana winds triggered PG&E decision that could cut power to millions in NorCal but only handful in SJ County
The 1991 Oakland Hills Firestorm that was fueled by the Diablo Wind. - photo by Photo Courtesy Of Caltrans

The Devil’s Wind is blowing and it is raising Cain in Northern California.

The wind — referred to as the Diablo Wind after Mt. Diablo at 3,848 feet — is the tipping factor in PG&E’s decision to cut power to as many as 800,000 customer accounts for homes, businesses, and schools affecting millions of Northern Californians starting today. The planned outages are to avoid creating a catastrophic wildfire that the utility concedes their equipment probably started 11 months ago. The Butte County fire ended up killing 85 people, burned 14,000 homes, and destroyed 5,000 other structures.

For all but a few hundred people in San Joaquin County that are served by 51 PG&E accounts 10 miles southwest of Manteca across the San Joaquin River near Vernalis, the Diablo Winds expected to kick up in the wee morning hours at worst could play havoc with the Bay Area commute.

Most of us will simply suffer through dry winds today and Thursday. The humidity — that topped off at 54 percent Tuesday, is expected to plunge to 28 percent today and then 19 percent on Thursday before climbing back up on Friday. Sustained wind speed is expected to average 19 mph today and 12 mph Thursday on the valley floor.  It will be punctuated by gusts approaching 40 mph.

The impacts of the high and dry winds in concert with low humidity can be seen in the stress it will put vegetation under.

The Diablo Wind is always hot and dry. It comes from the northeast and pop up in spring and fall. It is generate by the same conditions that mimic the more famous Santa Ana Wind that is the scrooge of the Los Angeles Basin when it comes to wildfires.

Both the Santa Ana and Diablo winds are local monikers for a “foehn” wind that references down-slope wind that is dry and warm that happens on the downwards side of a mountain range such as the Diablo Range and the Altamont Hills. The winds reach the highest speed as they clear the ridge and spill down into the Northern San Joaquin Valley. They also pick up speed in canyons but not quite as strong as when wind clears mountain tops.

The winds are created when there is strong inland high pressure near the ground that abruptly pulls air aloft at the same time a lower pressure has formed off the coast. As the wind drops on the downside of ranges, it heats up from the dynamics of compression forcing the air’s humidity to drop.

Those winds race across the Central Valley then are pushed up by the Sierra created more powerful gusts and drying out vegetation.

Trying to avoid

snarling comute

PG&E and Caltrans crews were working against the clock to prevent this morning’s Bay Area commute from becoming worse than normal due to the utility’s decision to start cutting power.

The planned power outage that PG&E is moving forward with takes out the power needed to keep lights and ventilation fans going in three Caldecott Tunnels that carries commuters from Walnut Creek to Oakland via Highway 24.

PG&E as of 11 p.m. Tuesday said their efforts to install four backup generators to supply electricity to the tunnels will be completed in time to avoid a massive snarl in the Bay Area commute. Caltrans had advised commuters Tuesday they were preparing to shut down the tunnels and that they could have remained closed for several days if  PG&E wasn’t successful in getting the generators installed in time.

Only a handful of San Joaquin County residents — several hundred that are the 51 PG&E customers 10 miles to the southwest of Manteca on the west side of the San Joaquin River near Vernalis — have been advise their power may be cut.

Manteca & almost all

 of San Joaquin County

is not impacted

While Manteca and the rest of San Joaquin County are not impacted at this point during the so-called “red flag warning” event that the National Weather Service has forecasted for today and Thursday that will provide all the ingredients to create an “extreme wildfire risk”,  this is just the start of the typical Diablo Wind fall season.

It is the same dry winds that combined with dry vegetation on Oct. 19-20 to trigger the 1991 Oakland Hills firestorm. That fire, started by a grass fire that wasn’t completely extinguished, killed 25 people, injured 150, burned 1,520 acres, and destroyed 2,843 single-family homes as well as 437 apartment and condo units. The fire as it raged topped the Diablo Range visible from Manteca-Lathrop with a large orange-ish hue as it burned through the night.

Similar dry winds that move inland from the ocean but farther north are what contributed to the Butte County fire that started on Nov. 8, 2018 and killed 85 people. It is the disaster what led up to PG&E’s current policy of pre-emptive power outages to reduce the risk of wildfires during high and dry winds, low humidity, high temperatures, and heavy “fuel”, a reference to dry vegetation.

To contact Dennis Wyatt, email