After one of California’s hottest weekends ever observed, a severe fire threat encompasses much of the state and expands over large portions of the western United States. Yet, in a shocking example of weather whiplash, parts of the Rocky Mountain states are bracing for a 60-degree temperature drop and accumulating snow within 48 hours of triple digit heat.

Over the weekend, California’s blistering temperatures, surging to an unprecedented 121 degrees in Los Angeles County, intensified existing fires and fueled new ones.

The Creek Fire in the Sierra National Forest, about 290 miles north of Los Angeles, grew from first detection Friday night to over 78,000 acres as of Monday afternoon. The explosive blaze generated towering pyrocumulonimbus clouds that triggered lightning and probably spawned fire tornadoes. At least 10 people were injured from the fire, with more than 200 rescued by the California Air National Guard after evacuation routes were cut off.

The weekend blazes pushed the area burned in California to more than 2 million acres, the most burned on record in a single wildfire season since modern records began in 1987, even before the most dangerous part of the fire season had begun, according to Daniel Swain, a climate researcher at the University of California at Los Angeles.

The Creek Fire in the Sierra National Forest, about 290 miles north of Los Angeles, grew to over 73,000 acres as of early Sept. 7. (The Washington Post)

A Cal Fire spokesman confirmed that this is the most acres burned in any year in modern record-keeping.

About 310,000 acres burn in an average fire season between Jan. 1 and Aug. 30, according to Cal Fire, the state firefighting agency. Fires in California alone have burned at least 3,300 structures to date and killed 8, according to a Cal Fire press release.

The state has already seen its second-, third- and fourth-largest fires on record in 2020, which are still burning.

Smoke from these blazes was streaming from coast to coast over the Lower 48 states.

On Monday, red flag warnings for severe fire danger, spanned from Colorado to the West Coast, including California, Oregon and Washington. Nicky Naus, a fire weather forecaster with the National Interagency Fire Center (NIFC) in Boise, Idaho, said via twitter that the fire event taking shape from the Cascades, Sierra Nevada Mountain range on southward to the coastal mountain ranges along the West Coast is “potentially historic.”

The most extreme fire danger Monday focused in northwest Oregon, where a combination of very strong winds, low humidity and a parched land surface could cause fires to erupt and rapidly spread. The National Weather Service’s Storm Prediction Center described an “extremely critical” fire situation and warned of “a volatile environment supportive of rapidly spreading fires exhibiting extreme behavior.” Winds in the high terrain could gust to 60 to 75 mph, while even Portland was expecting to see wind gusts to 35 to 50 mph Monday night.

“We are on track for an extremely dangerous period of fire weather beginning this afternoon [Monday],” the Weather Service office in Portland wrote.

Amid continuing heat and a forecast of strong winds, the U.S. Forest Service closed eight national forests due to the fire threat, beginning Monday evening. Agency officials said the closure would be evaluated daily.

The ingredients responsible for these volatile fire conditions in the Pacific Northwest are similar to what drives the infamous Santa Ana winds in Southern California, namely zones of high and low pressure in close proximity driving offshore winds through the mountains, where they rush downhill and dry out the air. The high-pressure zone sinking out of Canada toward the Rockies is abnormally strong for the time of year, while low pressure lurks along West Coast.

As this high pressure sinks south, it will increase California’s fire risk Tuesday and Wednesday. This will set up Southern California’s first Santa Ana wind event of the season. Typically, these events are more commonly seen in October and November.

“During the peak of the event Tuesday night into Wednesday morning, wind gusts of 40 to 50 mph are expected across the mountains, with gusts of 30 to 45 mph in the valleys and coasts,” wrote the Weather Service in Los Angeles. “Also of note, is the critically dry fuels in place following this record heat wave.”

On Monday, even before these winds arrive, the fire danger in California was not as high as in the Pacific Northwest as a result of lesser winds but still “elevated” and, in some places, “critical,” because of high temperatures observed since the weekend and very dry air.

Severe wildfires in the Colorado Rockies sent smoke billowing 40,000 feet into the air and prompted evacuations as ash fell from the sky. (The Washington Post)

Temperature plunge and snow forecast in the Rockies

The high-pressure zone from Canada driving the potentially fire-fanning winds is predicted to bring a sudden and historic drop in temperatures over parts of the Colorado Rockies along with an early season snowstorm.

After climbing to about 100 degrees over the weekend, Boulder and Denver are forecast to see highs near 90 on Monday afternoon amid smoke-filtered sunshine. But, upon the onset of strong winds from the north, the mercury will plummet. Temperatures are forecast to plunge into the 40s by Monday night and near freezing by Tuesday morning.

Denver and Boulder are both under winter weather advisories starting midnight Tuesday for 3 to 7 inches of snow by early Wednesday. High-elevation areas could see from 8 to 14 inches. This would be two days after it was raining ash in these same areas from the Cameron Peak wildfire.

Denver is expected to see its shortest time on record between posting a temperature of 100 degrees on Saturday and snow on Tuesday.

Weekend heat records

Numerous locations in the West experienced their hottest September day Saturday or Sunday and, in some cases, both days. A few spots saw their highest temperatures ever observed in any month.

On Sunday, 99 percent of California’s population was under an excessive-heat warning or heat advisory, according to the Weather Service office in Sacramento.

The Weather Service in Los Angeles called the heat “epic" and “unprecedented.” It wrote: “It cannot be stressed enough that is a very dangerous and life-threatening heat wave."

Some noteworthy temperature records that fell across the West include:

Temperatures in California on Monday were predicted to be high and, in some cases, record-challenging, but not quite as high as they were over the weekend. For much of the upcoming week, temperatures in the state are forecast to remain higher than normal but lower than the weekend levels.

Studies show human-caused climate change is tilting the odds in favor of more frequent, severe and longer-lasting heat waves, as well as larger wildfires throughout large parts of the West. Research published last month, for example, shows climate change is tied to more frequent occurrences of extreme-fire-risk days in parts of California during the fall. (Meteorologists define the fall as beginning Sept. 1.)

Some recent climate studies have shown that extreme heat events in parts of the world would most likely not have occurred were it not for the human-caused increase in greenhouse gases, such as carbon dioxide, from the burning of oil and gas for energy.

Andrew Freedman and Scott Wilson contributed to this article.