The red-flag warnings in effect for the entire North Bay, East Bay Hills, interior valleys, Santa Cruz Mountains and San Francisco coast through 5 p.m. local time Monday are rare for December, since typically this region begins to see steady rainfall from storms sweeping inland from the Pacific. However, this year, a sprawling area of high pressure has diverted storms northward into Canada and Alaska, causing flooding and contributing to blizzards in parts of Alaska, while the Golden State has been left high and dry.
The National Weather Service in San Francisco is forecasting near-record warm conditions for this time of year on Monday, with highs in the 70s, which is about 10 to 15 degrees above average for this time of year. Winds have been gusting greater than 60 mph at Mount Saint Helena and Mount Diablo, the Weather Service said, with the potential for stronger winds to make it to lower elevations on Monday.
Vegetation is at or near record dry levels, and the Weather Service cited “massive rainfall deficits” in a morning forecast discussion zeroing in on the wildfire threat.
“The combo of strong winds, record warmth and record dry fuels is just too much to ignore. Any new ignition sources could lead to rapid fire growth overnight into Monday morning. Should we get to lunch time Monday with no new starts the threat looks to greatly decrease through the afternoon as winds slowly ease,” the Weather Service wrote in an online forecast discussion.
“Mid and long range forecast remains dry so any new starts will have room to grow before mother nature is able to help out.”
Since 2006, the San Francisco office has only issued two red-flag warnings in December, one in 2017 and the other in 2013, during California’s recent historic drought. “While it’s been done in the past, it’s not normal for us to be thinking about fire weather in December,” said Brian Garcia, a meteorologist at the Weather Service’s forecast office in San Francisco.
The last significant December fire, according to the Weather Service, was the Pfeiffer Fire near Big Sur in December 2013.
Although November storms have limited the scope of fire danger in Northern California, parts of the Bay Area and Sacramento Valley have had very little rain and remain at risk for a large fire. For example, Oakland has reported only 0.17 inches of rain since Oct. 1, the start of the water year. The region has also been rapidly drying over the last two weeks due to high pressure that has brought plenty of sunshine and warmer weather, with little meaningful precipitation since Nov. 18.
In its decision to issue the red-flag warnings, the Weather Service cited the combination of high winds, record warmth and widespread record dry fuels, along with continuing extreme drought.
Growing threat for Southern California
While December red-flag warnings are rare for the Bay Area and Sacramento forecast offices, they are more common in Southern California, which receives less autumn rainfall and experiences frequent Santa Ana winds in the cooler months.
However, the extremely dry conditions now seen across the region are not normal for December. Vegetation in some areas is near record dry levels. Over the last several weeks, conditions have deteriorated considerably in Southern California, given the lack of rain and the very dry air that has been in place due to repeated Santa Ana winds, which come whipping through canyons and down mountain slopes, drying as the air compresses.
The legacy of extreme summer and autumn heat also looms large. “Fuel moisture is record low right now in critical mountain and foothill areas, as if it was October still,” said Alex Tardy, a meteorologist at the National Weather Service in San Diego. “We have really dried out rapidly since the early November rainfall, but we started off September and October with record low [dryness] due to the lack of monsoon season rainfall and the extreme hot record May to October 2020 period in the same areas.”
Last week, wildfires broke out during a Santa Ana event in the Los Angeles region. Now there’s another round of strong winds to contend with, and more are likely on tap for the rest of the month, computer models show.
This latest round of Santa Ana winds will feature moderate to strong winds spreading over the Southern California coast and mountains, as well as a portion of the Central Valley, with the core of the event expected Monday night through Tuesday afternoon.
In a somewhat unusual setup, strong easterly winds will also impact San Luis Obispo and Santa Barbara counties. Fire danger is running especially high in these areas, which have received minimal rain in the last several months. Only 0.05 inches of rain have fallen in Santa Barbara, for example.
While the mountains of Southern California could see damaging wind gusts between 60 to 70 mph, lower elevation areas won’t be spared, with sustained winds of 20 to 30 mph and gusts to 55 mph expected in inland San Diego County. “It will be a very windy day in these valley areas,” the Weather Service wrote.
Southern California Edison is warning thousands of customers it might preemptively cut off power avoid sparking a blaze that could grow rapidly in these conditions.
The renewed risk of fast-spreading wildfires — and evacuations — comes as reported coronavirus cases and hospitalizations are skyrocketing in California. Because of shrinking ICU capacity, much of the southern two-thirds of the state, along with five Bay Area counties, were placed under a stay-at-home order at 11:59 p.m. Sunday night. Such an order, and fears of contracting the virus, could complicate or delay decisions to evacuate if a wildfire were to ignite and threaten homes.
The dueling messaging of health officials’ stay-at-home orders vs. “Are you SET to evacuate if necessary,” as the Weather Service office in Los Angeles states on its website and via Twitter, is a stark and challenging reality as California closes out its extreme year.
Diana Leonard is a science writer covering natural hazards. She holds master’s and PhD degrees in geography from the University of California at Berkeley, where she studied physical geography, natural hazards and climate science, and researched severe thunderstorm and tornado climatology.