We are a couple of weeks into the Fall season, and much of the western United States, including southern California, is grappling with Extreme Heat Warnings. My hometown Atlanta Braves lost to the Los Angeles Dodgers in the Major League Playoffs, but both teams battled scorching hot temperatures. According to Associated Press writer Beth Harris , the temperature at the start of the game was 100 degrees. That’s just 3 degrees below the hottest temperature ever recorded at Dodger Stadium. Unfortunately, Sunday could be even hotter in some parts of southern California.

How Hot Will It Be Sunday?

While many high temperature records were broken or tied in California on Saturday, I detected an ominous statement while reading the National Weather Service - San Diego office discussion for Sunday. “Today will likely be the hottest day of the week as high pressure aloft and gusty Santa Ana winds combine to produce daytime high temperatures ranging from the mid 90s to low triple digits in the coastal areas and inland valleys,” NWS Forecasters wrote. Such temperatures are 15-25 degrees above normal, and will likely last through Thursday.

While the daytime high temperatures often get the headlines, I am equally or even more concerned about the elevated nighttime low temperatures. Much of southern California is under an Extreme Heat Warning. “Dangerously hot conditions with high temperatures in the upper 90s to 103 and warm nights with low temperatures in the low to mid 70s expected,” continued the National Weather Service in San Diego.

High Pressure aloft is a primary culprit. Experts like Daniel Swain saw it coming. “Global model ensembles are in strong agreement that a prolonged and anomalous nearly West-wide autumn heatwave will develop this week and persist for 10+ days,” Swain wrote on his Linkedin page last week. “While some daytime records may be set, this event will be more notable for its persistence and breadth,” He continued. Swain also made note of the overnight minimum temperatures and elevated humidity along the southern California coast.

The Santa Ana winds are also contributing to the autumn heatwave too. The surface weather map gives a clue with the position of high pressure systems in the West. “Santa Ana Winds occur when air from a region of high pressure over the dry, desert region of the southwestern U.S. flows westward towards low pressure located off the California coast,” explained the NWS website. “This creates dry winds that flow east to west through the mountain passages in Southern California,” It continued. The Santa Ana winds are common from September to May, but this year the timing couldn’t be worse given the magnitude of this ridge of high pressure.

Santa Ana winds are often very warm or hot because of physics. As the air moves down the westward slopes of the mountains, something called adiabatic compression happens. That compression warms the air. These winds often appear from September through May and often increase wildfire risk too.

I consider Daniel Swain to be one of the top experts in the world on western U.S. weather. Something else caught my eye in his post about what he called “teeth gnashing” online that “El Niño is a bust.” Such hot takes are sadly part of the “I need clicks and shares" landscape that we face in social media these days.

“Not only could that not be further from truth, but autumn precipitation in California tells us almost nothing about the winter to come,” Swain wrote. He went on to say, "The effects of even the strongest El Niño events on the North Pacific storm track, and subsequently California precipitation, are not expected to emerge until November *at the earliest,* and possibly not until mid-late December! What concerns me as an atmospheric scientist is that this El Niño is already breaking records, and it is not even close to its peak yet, so I caution people to back off with the false narratives.

I often lament in this space that people underestimate heat even though it has killed more people in the U.S. over the past three decades than tornadoes, hurricanes, lightning or floods, according to NWS statistics. Just this week, Arizona State University announced the establishment of the nation’s first Center for Heat Preparedness.

That’s a step in the right direction towards elevating public perception of heat and prompting actions that we see when tornadic storms or hurricanes approach. “This August was the hottest month ever reported globally, and extreme heat is the leading cause of weather-related deaths in Phoenix, across Arizona and much of the Southwest,” noted a press release issued by Arizona State University.

There are still several days of this western heatwave to go. Whether you are attending an MLB or NFL game, working, or doing other activities, please treat the Extreme Heat Warnings with the same level of caution that you would for a more telegenic or tangible weather threat like a tornado or atmospheric river.