The unseasonably sunny, dry weather across Northern California in recent weeks has provided a great opportunity for people to get outside, exercise and work in their yards.
But meteorologists are warning that it may be increasing the risk of wildfire in the coming days, even after many Bay Area residents had assumed that fire season was over.
On Wednesday, the National Weather Service said computer models are showing the potential for gusty offshore winds starting Sunday and extending into Monday across the Bay Area hills, an event the agency’s forecast discussion said “would likely result in critical fire weather conditions.”
Forecasters said a trough of low pressure developing in the Great Basin area of Nevada could generate strong winds blowing dry, warm air from the land toward the ocean. Such conditions are known as Diablo winds in Northern California and Santa Ana winds in Southern California, and significantly raise the risk of wildfires.
“We’re still in such a rainfall deficit,” said Brayden Murdock, a meteorologist with the National Weather Service in Monterey. “We’re well below last year. It’s something we’re keeping an eye on in the next few weeks.”
More will be known in the next few days, he said. But the dangerous conditions have already arrived in Southern California.
There, the National Weather Service issued a red flag warning from Wednesday night through Saturday from Santa Barbara to San Diego, with low humidity and gusty winds increasing the chances of fires like the Thomas Fire, which started two years ago on Dec. 4 in similarly dry conditions and destroyed more than 1,000 homes in Ventura and Santa Barbara counties. Southern California Edison warned Wednesday it was considering power shutoffs for up to 270,000 customers, mostly in Riverside, San Bernardino, Ventura and Los Angeles counties.
A #RedFlagWarning is in effect for portions of Southern CA due to gusty winds and low humidity from Wednesday evening through Saturday evening. This is critical fire weather. Exercise caution when outdoors. https://t.co/upBwccxXFO pic.twitter.com/AgwQJmYGQd
— CAL FIRE (@CAL_FIRE) December 2, 2020
The lack of rain so far this season has been highly unusual.
Two weeks ago, on Nov. 17, Northern California received a decent storm, which delivered 2 to 3 inches in the mountains from Mount Tamalpais in Marin County through the Santa Cruz Mountains to Big Sur, dramatically curbing fire risk. It also brought 2 to 3 feet of snow in the Sierra. Many ski resorts have reopened in the Lake Tahoe area, albeit with help from machine-made snow and limits from state COVID regulations that require skiers to make reservations to keep crowd sizes down.
But that storm system didn’t bring much rain to Bay Area cities, the East Bay Hills or the Diablo Range. On Wednesday, San Francisco was at only 11% of normal rainfall for this date, and San Jose and Oakland were at 5%.
In fact, San Francisco received only .54 inches of rain between July and November. That ranks as the 8th driest July-to-November period in San Francisco since 1849, when modern weather records began, according to Golden Gate Weather Services in Half Moon Bay.
Worse, no chance of a good rain appears in the forecast for the Bay Area over the next 10 days
“It’s very significant that we’re seeing the potential for red flag fire conditions in Northern California this late in the season,” said Daniel Swain, a climate scientist at UCLA. “It’s unusual but not unheard of in Southern California. But up in the Bay Area, and moister parts of Northern California, it’s pretty extraordinary.”
The reason for the dry weather? Persistent ridges of high pressure air along the West Coast have been blocking incoming storms off the Pacific, including several larger atmospheric river events, and pushing them north into British Columbia and Southern Alaska, Swain said.
He noted that shifts in weather patterns due to climate change are generating warmer, dry conditions later into the year, extending fire season.
“The winter has not been getting drier over the long run in California, though the autumn has,” Swain said. “Unfortunately what that means is that fire season lasts later into the autumn and increasingly coincides with strong offshore winds.”
Fire commanders at Cal Fire have been noting in recent years that fire season is lasting longer as the climate warms. This year, La Niña conditions — where Pacific waters near the equator are cooler than normal due to wind patterns — is also complicating the picture. Historically, La Niña winters have been more likely to be dry in Southern California, although there has been little clear pattern further north.
So far, the dry end to 2020 hasn’t done major harm to the state’s water supply. After a relatively dry winter last year, water officials are hoping for a wet winter this year. They still have until late March or April for big storms to break through, and statewide, reservoirs are at 86% of their historic average, filled by a wet winter two years ago.
“It’s a slow start to the year, a dry start,” said Chris Orrock, a spokesman for the state Department of Water Resources. “But we do have some good carryover storage from 2019. Our reservoirs are still doing OK. It’s still early. It could change. Hopefully it will.”
