Bob Henson ·

Above: Flames rise from the La Tuna Fire on September 2, 2017 near Burbank, California. About 100 Los Angeles firefighters were expected to return soon from Texas, where they were helping those affected by Hurricane Harvey. Image credit: David McNew/Getty Images.

The steamy, fiery summer of 2017 hit a new crescendo this weekend across the U.S. West, which is getting its hottest Labor Day weekend on record in many locations—and in some spots, the hottest weather ever observed. Overall, "this is the greatest statewide heat wave ever recorded in California,” proclaimed WU weather historian Christopher Burt on Saturday night. Burt based his conclusion not only on the heat’s intensity but on its widespread nature well beyond California’s usual scorching locations. Even an escape to the cool Pacific shore was pretty much futile, as easterly downslope winds funneled scorching air from the interior into coastal sections that are normally mild and sometimes chilly even in midsummer. Readings also soared above 110°F across California’s Central Valley, although such heat is not quite so unusual for late summer in that area.

California’s Bay Area has been the focal point of the weekend’s most extraordinary heat. Temperatures soared to 106°F in downtown San Francisco on Friday and 102°F on Saturday. Friday’s reading was the hottest ever measured in downtown SF, where temperatures have been observed since 1874. Friday’s 106°F handily topped the previous record of 103°F from June 14, 2000, and Saturday was only the second high of 102°F in downtown history, matching Oct. 5, 1987. “To put this in perspective, the average high temperature for the city these two days is just 71°F,” said Chris Burt, who lives in the East Bay region. “Friday night’s temperatures failed to fall below 85°F at several hill locations near me (I dropped to 81°).” He added: “It is so hot in our home I can hardly think. No air conditioning, of course.” Heat-related illnesses overwhelmed San Francisco hospitals on Friday, according to the Bay Area NWS office. It would not be shocking to see multiple Bay Area fatalities during this heat wave, given the multi-day intensity of the heat and the Bay Area’s lack of air conditioning.

Figure 1. Haze and smoke shrouded the San Francisco skyline on Friday, Sept. 1, 2017, as seen from Treasure Island. Dozens of cooling centers opened throughout California, schools let students out early, and outdoor events were cancelled as temperatures soared from a heat wave expected to last through the Labor Day weekend. In normally cool and foggy downtown San Francisco, temperatures reached an all-time record high of 106°F degrees Friday afternoon. Image credit: AP Photo/Ben Margot.

On the Marin County coast, the Point Reyes lighthouse station hit 91°F on Saturday, breaking its all-time record of 90°F from Oct. 3, 1917, almost exactly a century ago. Remarkably, the temperature at Point Reyes at midnight Friday night was a sweltering-for-the-location 86°F—just 4°F below the previous all-time high.

San Francisco International Airport hit 104°F on both Friday and Saturday. Although this has been reported as an all-time record for that measuring site, Burt says the airport actually reached 106°F on Jun. 14, 1961, and 105°F on Jun. 14, 2000.

Other all-time highs set or tied in the Bay Area on Friday and Saturday include:

Salinas Airport, CA: 109°F on Saturday (old record 105°F on Oct. 5, 1987)

King City, CA: 115°F on Saturday (old record 113°F on Sept. 2, 1955)

Moffett Field, CA: 106°F on Friday and Saturday (tie, first set Jun. 14, 2000)

One of the most naturally air-conditioned cities in the contiguous U.S. is Eureka, on California’s far northern coast. On Saturday, Eureka matched its all-time record high of just 87°F , first set on Oct. 26, 1993. This is the lowest all-time high for any reporting station in the nation, according to Chris Burt. Eureka’s weather records extend all the way back to 1886.

Hundreds of miles to the south of Eureka, San Luis Obispo reportedly set an all-time record high of 114°F on Saturday, topping the old record of 112°F.

In Reno, NV, the Reno/Tahoe International Airport hit 100°F on Saturday, which marks the city’s 14th hundred-degree day of the summer. This breaks the previous summer-long total of thirteen 100°F days (1972) in airport records that date back to 1937.

Portland, Oregon is expecting its first-ever Labor Day weekend with temperatures of 90°F or better on all three days. The city hit 98°F on Saturday, and WU is predicting 90s in Portland through at least Tuesday.

Figure 2. MODIS visible satellite image of California on September 2, 2017, showing widespread areas of smoke from wildfires. Image credit: NASA.

Figure 3. The La Tuna Canyon fire burns in the hills above Burbank, California, early on Saturday, Sept. 2, 2017. Image credit: Robyn Beck/AFP/Getty Images.

A summer of heat culminates in an outbreak of wildland fire

It’s been an awful fire season in the Western U.S., thanks in large part to extreme heat. According to Chris Burt, at least 15 cities in the Western U.S. had their hottest summer on record in 2017:

LAS VEGAS, NV: 93.2° (previous 93.1° in 2016), period of record (POR) 1937-

DEATH VALLEY, CA: 103.5° (previous 103.3° in 2016) POR 1911-

BISHOP, CA: 77.8° (previous 77.5° in 2016) POR 1943-

NEEDLES, CA: 98.6° (previous 98.3° in 2006) POR 1893-

TONOPAH, NV 76.1° (previous 75.1° in 2016) POR 1954-

SALT LAKE CITY, UT: 80.9° (previous 80.7° in 2013) POR 1874-

RENO, NV: 77.2° (previous 76.2° in 2007) POR 1888-

ELY, NV: 68.3° (previous 67.9° in 1909) POR 1893-

PALM SPRINGS, CA: 94.7° (previous 93.6° in 2016) POR 1922-

REDDING, CA: 84.1° (previous 83.8° in 2015) POR 1893-

SACRAMENTO (DOWNTOWN), CA: 79.0° (ties 1996) POR 1877-

BLUE CANYON, CA: 70.3° (previous 69.6° in 2006) POR 1944-

BAKERSFIELD, CA: 86.6° (previous 85.3° in 1981) POR 1889-

FRESNO, CA: 85.3° (previous 83.5° in 1981) POR 1887-

PASO ROBLES, CA: 74.7° (previous 74.0° in 1961) POR 1894-

The National Interagency Fire Center said on Saturday that more than 25,000 firefighters and fire support personnel are spread out across the Western U.S., fighting 68 large uncontained wildfires—including 21 in Montana, 18 in Oregon, and 12 in California. More than 700 homes have been evacuated because of the La Tuna fire raging in the Verdugo Mountains just four miles north of downtown Burbank. Having spread to 5000 acres as of late Saturday, La Tuna is the largest wildfire in memory within the city limits of Los Angeles, according to L.A. fire chief Ralph Terrazas and mayor Eric Garcetti.

Smoke from wildfires has been causing serious health problems in many western states over the past month. On Monday, Eugene, Oregon suffered its worst air pollution since the 1980s, when levels of fine particle matter (PM2.5) soared as high as 216 micro­grams per cubic meter. This corresponds to an Air Quality Index (AQI) in the purple range, “Very Unhealthy.”

Figure 4. The Air Quality Index (AQI) for the combined effects of ozone and PM2.5 pollution for September 1, 2017. Widespread areas of pollution in the red zone, “Unhealthy”, were recorded in California and Oregon, due to PM2.5 from wildfire smoke and the presence of high levels ozone from the extreme heat. Image credit: U.S. EPA.

On Friday, wildfire smoke caused PM2.5 pollution levels in San Francisco to spike up to 62 micro­grams per cubic meter—a code red AQI in the “Unhealthy” range. The other deadly air pollutant we are most concerned about, ground-level ozone, forms via chemical reactions that are more efficient at high temperatures. The extreme heat of the past few days has caused over 75% of California to violate the federal standard for ozone pollution. High levels of both ozone and PM2.5 were recorded again on Saturday across much of California and Oregon, with pollution levels in the "orange" and "red" AQI range for most of the citizens in those states.

The hottest weather in coastal California’s Mediterranean climate often occurs in September and October, so residents will need to be vigilant about wildfire risk, especially during any episodes of fierce Santa Ana downslope winds. Across the parched Northwest U.S., strong winds ahead of the season’s first Pacific cool fronts may also lead to periods of heightened fire risk over the next few weeks.

Jeff Masters co-wrote this post.



