Firefighters who have been making steady progress at containing the second- and third-largest wildfires in California history are about to face yet another extreme weather event, this time in the form of prolonged, potentially record-shattering heat and dangerous fire weather conditions across the state. The heat is poised to commence in much of California on Friday, peaking in many areas on Sunday, and then continue through Labor Day and possibly beyond.

Excessive heat watches and warnings are in effect across parts of Arizona, Nevada and much of California during this period, and may need to be extended.

The National Weather Service forecast office in Los Angeles is using strong language to describe the potential for conditions that could cause deadly heat illness, an increased likelihood of large fires and temperatures that don’t just break daily or even monthly records, but all-time hottest readings for any calendar day on record.

“A widespread heat wave with record high temperatures expected will bring dangerous heat and elevated fire weather conditions nearly everywhere Friday through at least Labor Day,” the Weather Service warns. “There is an exceptional risk for heat illness and power outages.”

Jennifer Murillo was living with her husband and his grandparents when the LNU Lightning Complex fire destroyed their home near Vacaville, Calif. on August 17. (The Washington Post)

The Weather Service is predicting temperatures in some areas on Sunday to be up to 22 degrees above average, describing it as a “brutal day.”

“Many records are likely to fall and in fact there is a chance that some all time record highs will recorded. These extreme max[imum] temps combined with lows in the mid 70s to lower 80s will make Sunday one of the most hazardous in recent memory,” the Weather Service said in an online forecast discussion.

Computer models are showing a potentially volatile scenario developing on Sunday into Monday, with a dip or trough in the jet stream diving south into Montana and Utah, leading to the formation of a strong high pressure area over Nevada. These weather systems may turn winds offshore in parts of California, which will serve to heat coastal areas up even more as the air descends from the mountains to the east. Widespread high temperatures of between 110 and 115 degrees are forecast for areas just inland, while coastal locations also exceed the century mark.

Days with dry offshore winds tend to be when California sees some of its fastest-spreading wildfires, particularly in Southern California. However, the state has already been dealing with devastating fires that have burned 1.5 million acres since Aug. 15. Any additional blazes will further tax already weary fire crews and California residents who have been living under a blanket of hazardous smoke for weeks.

The NWS is forecasting that this heat event will be hotter than the last one, which took place in mid-August and helped touch off the large wildfires in the San Francisco Bay region. It was during that heat wave that Death Valley, Calif., recorded a high of 130 degrees, one of the hottest temperatures ever reliably recorded on Earth. Between Friday and Monday, Death Valley is forecast to top 120 degrees and is under an excessive heat warning.

Weather Service forecasters in Los Angeles are advising people to stay indoors during this heat wave, calling it an “exceptionally dangerous event."

That heat wave also brought rolling blackouts due to heightened electricity demand, and there are concerns that this weekend could see more such disruptions.

“Temperatures this high, and this widespread, are rarely ever seen in this area. All daytime outdoor activities should limited or canceled. Those without air conditioning should make preparations now to stay cool. Extreme stress on our power infrastructure may lead to power outages.” forecasters stated.

In the San Francisco Bay area, the heat is also expected to be intense, although areas right along the coast may escape due to onshore winds. Some of the large fires that have each charred hundreds of thousands of acres could see renewed growth during this period, with the worst-case scenario involving periods of offshore winds that would combine with the hot temperatures to cause humidity levels to plunge. Some computer models, such as the European model, suggest this scenario.

Studies show that climate change, which is heightening the odds for and severity of extreme heat events, is also leading to more frequent occurrences of extreme fire risk days in parts of California during the fall (meteorologists define the fall as beginning on Sept. 1. In addition, climate change has been tied to a trend toward larger wildfires in parts of the American West. While California has dealt with its spate of fires, Colorado has also logged its largest wildfire on record.

Fire weather fears

One way to look at how volatile the fire weather conditions are in California, and how the upcoming heat wave could put parts of the state back in peril is through a metric that effectively measures the thirst of the atmosphere, known as the Evaporative Demand Drought Index. This index shows the ability of the atmosphere to dry out the landscape through high temperatures, clear skies, low humidity levels and other ways. It is a reliable indicator for fire danger — the higher the index, the greater the fire threat.

This summer, the EDDI has consistently been in its highest drought category, E4, over parts of California, a level that can be expected only 2 percent of the time based on records going back to 1980. Now, with a potentially historic heat wave descending this weekend over California and the Pacific Northwest, those drying effects will only intensify, pulling more moisture from plants and soils.

Scientists with the U.S. Forest Service and Cal Fire, which is California’s state firefighting agency, have seen evidence of high stress on vegetation in the last few months. Conditions have reached record dry levels periodically and then recovered during cooler, more humid intervals.

Shrubs and tree canopies, for example, are running drier this summer than they were in 2018, California’s worst fire season on record when multiple megafires burned nearly 2 million acres. That level of dryness in live trees allows fires to spread into canopies, burning hotter with “extreme fire behavior” that can be impossible for firefighters to control.

With the core of the fire season still ahead, 2020 could easily surpass the area burned in 2018.

Weather in the last week in California has helped firefighters gain containment on some of the largest fires burning near populated parts of the greater Bay Area. The return of a deep layer of cooler, more humid air off the Pacific, known as the marine layer, has improved conditions on coastal fires, like the CZU Lightning Complex in Santa Cruz that has burned about 85,000 acres and the Walbridge Fire in Sonoma County, which has burned nearly 55,000 acres. Calmer winds have also helped.

The upcoming intense heat wave, however, could send many areas back into record dryness, making all vegetation susceptible to quick fire spread. Daily winds that blow from the ocean to the valleys are also expected to return. In addition, offshore winds in parts of California could be quite strong, potentially requiring advisories or warnings of their own and leading to extreme fire behavior.

“Any kind of localized breeze will activate the fires,” said Brent Wachter, a predictive services meteorologist with the U.S. Forest Service in Redding.

Some computer models are showing the possibility of a widespread, strong offshore wind event Monday into Tuesday, which would take place just after the heat wave has peaked, drying out vegetation even more in the process.

The mid-August heatwave and lightning siege that wrought havoc in California also affected parts of the Pacific Northwest; more than a dozen large fires burning in central and eastern Oregon and Washington that required heavy commitment of firefighting resources. Much of Oregon is in severe to extreme drought, according to the latest U.S. Drought Monitor.