Last weekend, residents of Los Angeles and other areas of coastal California could be forgiven for thinking they had overslept like Rip van Winkle and woken up in the heat of September or, over the last few days, that they had suddenly been transported to humid Miami.

The disorienting weather has been the result of a combination of an unusually early, record-setting heat wave, monsoon conditions over the Southwest and the mountainous local topography. The heat has taxed the electrical grid, leaving thousands without power; endangered human health; and kicked off a flurry of wildfire activity. “It’s been quite the week,” says Daniel Swain, an atmospheric scientist at the University of California, Los Angeles.

Whereas most of the contiguous U.S. sees its hottest summer days in July and early August, the coastal areas of southern California typically see the mercury spike in September. The median date for the hottest temperatures of the year for Los Angeles is September 24, for example—the latest date of any major U.S. city, according to Brian Brettschneider, a climate scientist at the University of Alaska Fairbanks’s International Arctic Research Center. Several of the all-time record high temperatures set in southern California during this heat wave bested ones set in previous Septembers: Record highs of 114 degrees Fahrenheit in Burbank and 111 degrees F at U.C.L.A. in western Los Angeles on July 6 unseated records set in September 1971 and 1939, respectively.

The typical late timing of peak temperatures in coastal southern California, compared with the rest of the country, is the result of the region’s peculiar climatology and topography. Like the rest of the Northern Hemisphere, temperatures rise there during the spring. But in May and June clouds known as a marine layer develop along the coast because of the temperature difference between the warmer land and cooler sea. Those clouds block sunlight and keep temperatures relatively cool. The marine layer starts to break up in July, allowing temperatures to rise again. But it is not until the end of summer, when high pressure develops over the desert and prevailing winds shift to flow from land to sea instead of the other way around, that things really heat up. Those winds push air downslope along the region’s mountain ranges, compressing and heating it. (This also causes the famed Santa Ana winds that have fanned the flames of so many devastating wildfires.)

The timing of the hottest day of the year across the contiguous U.S. The coast of southern California sees temperatures peak much later than the rest of the Lower 48. Credit: U.S. National Oceanic and Atmospheric Administration

The recent heat wave was the result of a high-pressure system parked over North America that gradually bled westward. This caused those downslope winds to uncharacteristically develop in July, sending temperatures skyrocketing. The early timing of this heat wave means temperatures got an additional boost, thanks to longer summer days and the season’s more intense sunlight. On July 6 the temperature in Riverside topped out at 118 degrees F; Escondido hit 112; Long Beach Airport peaked at 109, and downtown Los Angeles reached 108.

But searing daytime highs were not the only feature of the unusual heat wave. As temperatures stayed sultry throughout the night, they posed a threat to human health because the body is less able to recover from the heat of the day. Overnight low temperatures have generally been in the mid-70s to the low 80s F—more typical of daytime high temperatures for this time of year. Swain recalls one night at the peak of the heat wave when the temperature in western Los Angeles was still 100 at midnight, “which is really kind of wild.” The temperature overnight in Riverside on July 7 bottomed out at a toasty 82, beating out the previous record high overnight low of 80 set on September 2, 2007. Burbank had the same record that day, besting its previous record of 81 set in September last year. The array of all-time record highs as well as record high minimum temperatures shows “this has been an extraordinary heat wave,” he says. And as global temperatures warm, overnight lows are rising faster than daytime highs.

Although the mercury has been gradually dropping, residents have been plagued by another oddity for the area: humidity. Walking to work on Tuesday, Swain says he was sweating through his clothes despite the early hour and relatively cloudy conditions and notes the humidity was “on par with Miami.” The moisture is flowing in from the active annual Southwest monsoon centered over Arizona. “You’d never get this in the autumn,” he says, because the monsoon isn’t around then. The humidity also adds to the potential health burden from the heat wave because it makes temperatures feel hotter in a region where many lack air-conditioning. “Things just aren’t designed for these kinds of heat waves,” he adds.

But these extreme bouts of heat will likely become more common—and temperatures ever higher—as the world warms. And as they do, the records set this week will eventually themselves be toppled. “It’s the future,” Swain says. “At some point we will exceed [this heat] again in the next 10 to 20 years.”