A large area of high pressure, or a heat dome, is projected to build across the West at the same time as a large dip, or trough, in the jet stream delivers cold air to the Midwest and East beginning this weekend and continuing into early October.
The bifurcated weather pattern, with a warm West and a cool East, will favor unusually hot conditions as well as little to no opportunities for badly needed rain in the hardest-hit areas such as Butte County, Calif., where 15 were killed in the North Complex Fire. That fire had burned 304,492 acres and was 78 percent contained as of Wednesday.
In California alone, since Aug. 15, wildfires have burned a seasonal record of more than 3.6 million acres, destroyed over 6,500 structures and killed 26. The toll has been significant in Oregon as well, where fast-moving blazes swept through communities such as Talent and Phoenix, claiming lives.
In California, another heat wave is the last thing that’s needed now. The extreme wildfire conditions originally yielded massive blazes during August’s record-breaking heat wave, which was followed by a dry lightning event. Then another record-breaking heat wave hit in early September, which was immediately followed by an unusual and widespread land-to-sea, or offshore, wind event along the entire West Coast. Now another combination of heat and dry winds is expected to settle into Northern California this weekend, lasting potentially till early October in other parts of the state.
Late September through October is the season for dry winds blowing from land to sea in California, but not for this kind of heat. The heat will intensify the fire threat, with many large fires still burning throughout the state.
Scott Rowe, a meteorologist with the National Weather Service in Sacramento, expects temperatures this weekend to rise to around 100 degrees, which is about 15 degrees above normal for the Sacramento area.
“While the calendar says it’s autumn, it definitely won’t feel like that in Northern California this weekend,” he said.
The heat will not be as significant as in the past two heat waves, in August and early September, he said. Sacramento hit 112 degrees during the August heat wave. Numerous high-temperature records were set across California in the September heat event as well.
However, the upcoming warmth will coincide with offshore winds that are expected to bring increased fire weather concerns to Northern California.
“We are expecting there to be elevated and even possibly critical fire weather conditions this weekend,” Rowe said.
Fire weather watches are in effect in parts of the San Francisco Bay area, including the North Bay mountains, for Saturday through Monday, though the warm, dry weather conducive to fires is likely to last longer. Watches are also up for other parts of Northern California, including higher elevations in and around Sacramento, where fires are still burning. Some if not all of these watches are expected to be upgraded to Red Flag warnings for Saturday through at least Monday.
“We’re very concerned about fire weather at the highest altitudes of the coastal range,” said Brayden Murdock, a meteorologist with the Bay Area National Weather Service, where the North Bay mountains and East Bay hills could see gusts of up to 50 mph.
According to Daniel Swain, a climate scientist at the University of California at Los Angeles, the fire weather watches are likely to be expanded to other areas in California, but there’s uncertainty about how strong and widespread the dry winds will be.
“It does look like we may see a return to some record heat in the next 5-7 days along with moderately strong offshore winds,” Swain said in an email, noting that this is “not an ideal confluence given the exceptional/record dryness of vegetation and the preexisting wildfire crisis.”
Fire meteorologists with the Forest Service in Redding are forecasting a rapid return to extreme fuel dryness this weekend, which could make the winds more impactful, particularly given the number of large wildfires still burning, such as the North Complex fire in Butte and Plumas counties.
Rain would help tame the fire threat, but it isn’t on the horizon yet for most of the state. On average, Sacramento receives its first 10th of an inch of rain on Oct. 1. Last year that rain didn’t arrive until Nov. 26.
In contrast, parts of Oregon may see some rain before the warm and dry weather sets in again, but the moisture may not be enough to lower fire risks significantly.
In Southern California, where firefighters are still wrangling the 109,271-acre Bobcat Fire in Los Angeles County, the upcoming heat will complicate their efforts.
The National Weather Service (NWS) office in Los Angeles is anticipating possible record warmth from the weekend into early next week, along with offshore winds. High temperatures exceeding 100 degrees are forecast for inland areas and may be achieved yet again in downtown L.A., especially early next week, the NWS stated on its website.
“Very hot and dry conditions will bring an elevated threat for new large fires this weekend, with near record heat expected to build across the area from late this weekend through much of next week,” said the U.S. Forest Service’s outlook for Southern California.
Looking ahead, Swain says the fall outlook is “not especially reassuring” as the weather pattern for California, at least, seems locked into continued warmer- and drier-than-average conditions.