Did the wind that prompted the PG&E power shut-offs live up to the hype? PG&E, meteorlogists weigh in

Firefighters work to contain a wildfire burning off Merrill Dr. in Moraga, Calif., on Thursday, Oct. 10, 2019. The area is without power after Pacific Gas and Electric preemptively cut service hoping to prevent wildfires during dry, windy conditions throughout Northern California. less Firefighters work to contain a wildfire burning off Merrill Dr. in Moraga, Calif., on Thursday, Oct. 10, 2019. The area is without power after Pacific Gas and Electric preemptively cut service hoping to prevent ... more Photo: Noah Berger, AP Photo: Noah Berger, AP Image 1 of / 78 Caption Close Did the wind that prompted the PG&E power shut-offs live up to the hype? 1 / 78 Back to Gallery

Pacific Gas & Electric shut off power to large swaths of the San Francisco Bay Area Wednesday in anticipation of the strongest wind event since October 2017, when gusts penetrated valleys and fanned flames in the Wine Country Fires that scorched some 245,000 acres.

Meteorologists didn't forecast blustery conditions as extreme as in 2017, but because wind is difficult to predict with accuracy (especially amid the Bay Area's complex terrain), the potential was there for winds to heighten to 2017 levels if conditions were just right.

Even in weather events of lesser proportions, power lines and equipment can be damaged, and PG&E prepared by shutting off power in portions of Central and Northern California.

Many across the region closely watched the weather, waiting for the winds. Did they ever arrive?

The consensus among forecasters is the winds definitely materialized and continued to blow Thursday morning and into the afternoon.

And while the event wasn't nearly as severe as 2017, it was likely the strongest fire season wind event in the past two years. While some notable gusts knocked mountain tops and led to reports of wind-caused damage to PG&E equipment according to PG&E spokesperson Melissa Subbotin, widespread high winds didn't penetrate the valleys.

"It had the potential to be a very strong wind event, but 2017 was a very special event," said Brian Garcia, a forecaster with the National Weather Service office in Monterey. "This is the strongest fire season wind event that we've seen since the 2017 fires and when I say fire season we're talking the fuels are all dried out, the relative humidities are low and the winds are high, and so it's the trifecta of those things coming together that creates this critical fire danger. But no, this event wasn't more severe than 2017 and we didn't forecast that."

Jan Null, a consulting meteorologist with Golden Gate Weather Services, added, "If you cherry pick the spots, it might look equivalent to 2017, but if you look at the wind speeds in the valleys, it looks like a lot less."

PG&E is standing behind its decision to shut off power and says the weather is presenting critical fire conditions.

"We have seen extreme weather in parts of our service area that were included in the public safety shutoff," Subbotin said. "Customers not impacted by the public safety shutoff may have experienced power outages due to equipment being damaged by the event. Also customers being served by transmission lines de-energized in the PSPS action may have been included although they may have not lived in the extreme weather areas."

Elevations above 1,000 feet and ridgetops in the north and east Bay Area hills and mountains saw significant winds and isolated gusts overnight Wednesday. The top wind speed recorded by a gauge on Mount St. Helena was 77 mph, and a 75 mph wind hit Mount Diablo. Sustained winds at higher elevations were in the 20 mph to 30 mph range and this is what the NWS had forecast.

"We were estimating winds of 80 mph to 90 mph in 2017," said Garcia. "What we saw last night was probably a good 10 to 20 mph less."

Nonetheless, PG&E said equipment in the areas of Mounts St. Helena and Diablo was impacted.

"The company has received many preliminary reports of vegetation-related damage to its equipment in these areas," said Subbotin.

Valleys were forecast to see overnight speeds averaging 10 to 20 mph with isolated gusts up to 40 mph, and while winds picked up in some inland areas, the gusty conditions were less than was possible.

"We're measuring these high winds on the ridge tops and you have to have dynamics set up just right so they get down into the valleys," said Jan Null, a forecaster with Golden Gate Weather Services.

A gauge in Coffee Park, a neighborhood that burned to the ground in October 2017, measured a top wind speed of 17 mph overnight Wednesday. "In 2017, we saw winds up to 60 mph in Coffee Park," said Null.

At a gauge in Calistoga, a couple miles from where the 2017 Tubbs Fire started, a 31 mph gust was recorded. Two years ago, winds were over 50 mph.

When winds make it down to the surface in the valleys, the nighttime temperature inversion (with warm air aloft and colder air toward the ground) is often weaker or non-existent. This scenario never unfolded overnight Wednesday.

"Generally, stronger temperature inversions help separate the winds aloft from the surface," said Craig Clements, director of the Fire Weather Research Laboratory at San Jose State. "So, some areas may have more of this decoupling than others.

"Nailing that [a wind event] with a weather forecast model is very difficult. I still think it's prudent to turn off the power. It's what we need to do."

Clements added the Bay Area's complex terrain makes predicting wind events difficult.

The northeasterly winds are being blown from the Central Valley toward the coast, and en route they encounter mountainous barriers that impact their trajectory.

"Wind in California is erratic and when the winds hit the terrain, they can do different things," said Clements. "Even though winds on Mount St. Helena were 70 mph last night, Calistoga wasn't windy. This is hard to forecast. I've suggested setting up a network of high resolution, wind profilers. I've proposed a statewide network. This would help in determining where winds may work their way to the surface and where they may not."

Meteorologists agree offshore wind events in fall months aren't entirely unusual, but more research needs to be done to determine their exact frequency in recent decades.

"The strength of these events is something we'll be watching in the coming years and we'll be looking at if they have increased over time," said Garcia. "Looking at these strong events, we need to look into it a little deeper to see if they're becoming more common as a result of a change in climate. Regardless people do need to be prepared for these strong events especially as we are expanding our population into the wild land urban interface."

Null says PG&E also needs to adapt their equipment and systems to be better prepared for high winds and dry weather.

"This is not a 100-year storm," said Null. "You have to ask, 'Is it the meteorology or is it an infrastructure that's not able to handle it. Is it deferred maintenance? Have they not done their job? You should not have to turn off power if you have wind that's going to occur multiple times a year."

FULL PG&E OUTAGE COVERAGE:

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Amy Graff is a Digital Editor at SFGATE. Email: agraff@sfgate.com