What explains the heat storm in Southern California this week? In part, a phenomenon called “seasonal lag”

Mountains of Los Angeles with palm trees at sunset. (Image credit: Getty Images)

A late-summer heat wave is scorching Los Angeles and the entire West Coast this week. Downtown Los Angeles is flirting with triple-digit temperatures, possibly even for several days in a row. Hotspot cities like Claremont and Woodland Hills reached 100 and 106 degrees respectively on Wednesday, with even higher temperatures forecast for Thursday. The last time it was this hot for this long, in the first year of the pandemic, people were still disinfecting their groceries.

As AccuWeather meteorologists reported, a strong high pressure system formed over California midweek and will last through the weekend. The center of this high will be directly over Southern California, sending temperatures soaring to levels not seen since September 2020.

“September is traditionally one of the hottest months of the year in Los Angeles,” said Heather Zehr, senior meteorologist at AccuWeather.

Southern California cools more slowly than areas farther north after the peak of summer heat, Zehr explained. In addition, the Pacific’s characteristically cooler water temperatures finally begin to rise toward the end of summer, narrowing the difference between land and ocean temperatures. Warmer water temperatures can also reduce the turbid ocean layer that can ruin an otherwise sunny California day.

Santa Monica Beach, Los Angeles, California. (Image credit: Getty Images)

Add to that the offshore currents, which bring air from the hot interior rather than the cooler Pacific, and it will be hot all over Hollywood, with highs reaching Thursday through Sunday.

Even places right on the coast like Santa Monica could hit the 90-degree mark this week. Interestingly, the coastal city sees its warmest three-month average temperatures from August to October (70.3°F). San Diego also has higher average temperatures from August to October than June and July. San Francisco, known for being springlike and foggy for most of the summer, averages 74.8°F in September, its warmest month. And while Downtown Los Angeles sees slightly higher average temperatures in August (84°F) than September (83°F), the warmest average September day (mean maximum) is 99.4°F, the highest temperature of the calendar year.

In other words, even though it’s getting warm in California this week, the heat wave is coming at just the right time.

But what explains why in some cities on the west coast the warmest days occur when the water pumps have already been removed and the sun sets noticeably earlier?

The resulting phenomenon is called seasonal lag and is defined as a discrepancy between the time at which regions are most tilted toward the sun and the annual maximum temperature. In the Northern Hemisphere, this event is marked by the summer solstice in late June. In most major cities across the country, from New York City and Chicago to Atlanta and even Phoenix, the warmest months are from late June to August. Coastal cities like Los Angeles, San Diego and San Francisco, on the other hand, don’t feel the heat until the second half of summer, some even as late as the last days of summer.

The seasonal lag seems counterintuitive, much like the difference between astronomical and meteorological seasons. To explain this, let’s put the sun aside for a moment and focus instead on water, which is what most of the Earth is made of. Water, especially large oceans, tends to limit extreme temperatures, which partly explains why it can be a balmy 75 degrees when you land at LAX airport and a sweltering 95 degrees when you check your family in at Disneyland, 35 miles inland. There are similar microclimates in the Bay Area and throughout the state.

Water has a very high specific heat capacity, meaning it takes much more to heat it than air or land. On the other hand, water also retains heat for longer, resulting in a much slower drop, so a temperature peak in late summer before a steady decline.

The warmest day of the year according to the historical average from 1991 to 2020. (NOAA)

“The reason for the delay, particularly toward the West Coast, is that the land in the western U.S. warms faster than the Pacific during the summer, which lowers the pressure over the land with relatively higher pressure offshore,” explained Jason Nicholls, senior meteorologist at AccuWeather. “This pressure gradient favors a predominantly inland flow from the cool Pacific into coastal areas and coastal valleys during the summer. Hence the term June Gloom was coined for the Los Angeles Basin.”

As fall begins, the discrepancy between land and sea becomes a little less pronounced, resulting in a reduction in the coastal cloud and fog patterns that dominate the early part of summer, Nicholls adds. In fall, the Los Angeles Basin is also affected by the Santa Ana offshore wind events, which can make it feel like summer during the day even in January.

The seasonal lag phenomenon is also at play in many other parts of the country, not just coastal California. About 2,000 miles away in the Midwest, cities like Cleveland and Buffalo are hit by the infamous lake-effect snow, which peaks in late fall and early winter between late November and mid-January. Because the lakes take much longer to cool than the land, this leads to huge temperature differences and thus snowstorms.

Given the heat wave in Southern California, where temperatures of 21 to 25 degrees Celsius are forecast even at night, it is important to limit outdoor activities to the morning and evening hours and to drink plenty of fluids.