Heat waves punished the West all summer and, even at the brink of fall, another sweltering blast had moved over the region. The heat was bringing record-setting temperatures in the Southwest and exacerbating a volatile fire situation farther north.

The heat wave, which began early this week before peaking Thursday, coincided with an announcement from National Oceanic and Atmospheric Administration that the months of June through August matched the Dust Bowl summer of 1936 as the hottest on record for the Lower 48 states. “A record 18.4% of the contiguous U.S. experienced record-warm temperatures,” NOAA wrote Thursday.

This latest bout of exceptionally high temperatures was generated by a large and unusually strong zone of high pressure or heat dome parked over the Four Corners area, a situation that occurred repeatedly in recent months. Temperatures near the core of the heat dome were generally 10 to 20 degrees above average, and its influence extended everywhere west of the Central Plains.

On Thursday, record highs in the 90s and low 100s were forecast from California to Colorado; Denver and Phoenix hit at least 95 and 111 degrees, both records for the date. Excessive heat warnings were issued for parts of the desert Southwest, including Las Vegas, where the forecast high was around 105 degrees. In Death Valley, it was predicted to hit 120 degrees.

On Tuesday, Las Vegas hit 108 degrees, tying its Sept. 7 record. Death Valley reached 122 degrees, the hottest temperature ever recorded this late in the calendar year anywhere in the world. Wednesday brought record heat to parts of Southern California; Paso Robles Airport in San Luis Obispo County soared to 106 degrees, among other records.

By Friday and especially Saturday, the heat was forecast to ease some in the West as the heat dome drifted eastward and weakened slightly. Over the weekend, the core of the heat was expected to shift into the Southern Plains.

‘Critical’ fire risk

The weather setup threatened to worsen an already exhausting fire season in the northern half of Western states.

After hot and dry weather this week has further primed the landscape to ignite and burn, dry lightning followed by strong winds could lead to new fires and rapidly spread fires already on the ground.

The potential for lightning was highest in Northern California and Oregon, but it also extended into Nevada, Idaho and Montana.

Meteorologists with the National Interagency Fire Center labeled it a “high risk” event for much of Northern California on Thursday and Friday “due to critical combination of abundant lightning and increasing winds following a prolonged period of unusually warm and dry conditions with critically flammable fuels.”

The fast-moving Caldor Fire moved into South Lake Tahoe, Calif., forcing evacuations and sending residents to evacuation centers in Reno, Nevada. (Erin Patrick O'Connor, James Cornsilk/The Washington Post)

A northward surge of monsoon and Pacific moisture fueled the lightning threat, combined with a low pressure zone moving into the West Coast triggering thunderstorm development. Although a mix of wet and dry thunderstorms were forecast, rainfall in any given area was expected to be limited due to their fast movement.

In addition to erratic winds from thunderstorms, the system was ushering in strong southwesterly winds while passing through, potentially wreaking havoc on the many ongoing fires in the region, including the Caldor and Dixie fires.

“We’re expecting a pretty big change in the weather starting tomorrow,” Zach Tolby, incident meteorologist on the Caldor Fire, said at a Wednesday evening briefing.

“We’re expecting winds to start to pick up along the ridges Thursday evening and last through the day on Friday,” he said.

Red flag warnings for high fire danger were placed in effect for Northern California, including parts of the San Francisco Bay Area and central Oregon through Friday, and were active Thursday over portions of Idaho, Montana, Wyoming and the western Plains.

Over 2 million acres have burned so far this year in California, and about 15,000 firefighters are currently battling active blazes. According to the National Interagency Fire Center, the country remains at “Preparedness Level 5” — the highest level of wildfire activity. Such activity includes “large, complex wildland fire incidents, which have the potential to exhaust national wildland firefighting resources.”

Research has established a clear link between climate change and a sharp rise in the areas burned in California in the past several decades, as increasing temperatures dry out vegetation.

It’s no coincidence that this summer’s fires have raged amid historically high temperatures. NOAA reported that California, Oregon, Idaho, Nevada and Utah all experienced their warmest June-through-August period on record.

The high temperatures have also intensified severe drought conditions. Drought currently covers 94 percent of the West, according to the Federal Drought Monitor. Nearly 60 percent of the region is in extreme to exceptional drought.