What explains this week's Southern California scorcher? In part, a phenomenon called 'seasonal lag'

A late-summer heat wave is scorching Los Angeles and the broader West Coast this week. Downtown Los Angeles is flirting with triple-digit temperatures, maybe even for consecutive days. Hotspot towns such as Claremont and Woodland Hills hit 100 and 106, respectively, on Wednesday, with even higher numbers forecast for Thursday. The last time it was this hot for this long, people were still disinfecting their groceries in year one of the pandemic.

As AccuWeather meteorologists reported, a strong area of high pressure set up over California midweek and will linger into the weekend. The center of that high will park right over Southern California, sending temperatures soaring to levels not seen since September 2020.

"September is traditionally one of the hotter months of the year in Los Angeles," AccuWeather Senior Meteorologist Heather Zehr said.

Southern California cools down from the peak of summer heat slower than areas farther north, Zehr explained. Moreover, the characteristically cooler Pacific Ocean water temperatures finally start to warm up by the end of summer, which reduces the difference between the land and sea temperatures. Warmer water temperatures can also reduce the gloomy marine layer that can ruin an otherwise sunny California day.

Add in some offshore flow—where air flows from the hot interior instead of the cooler Pacific— and it's going to be a hot one all across Tinseltown, with temperatures peaking Thursday though Sunday.

Even places right at the coast, like Santa Monica, could hit the 90-degree mark this week. Interestingly, the coastal town posts its warmest three-month average highs from August through October (70.3 F). San Diego also has warmer average highs from August through October compared to June and July. San Francisco, notoriously springlike and foggy in much of the summer, reaches an average high of 74.8 F by September, its warmest month. And while Downtown Los Angeles records slightly warmer average highs in August (84 F) than in September (83 F), its warmest average September day (mean maximum) is 99.4 F, the highest of the calendar year.

In other words, while it's going to be warm in California this week, this heat wave is right on cue.

But what explains why certain cities on the West coast have their warmest days when the Super Soakers are already put away and the sun is setting noticeably earlier?

Seasonal lag

The phenomenon at play is called seasonal lag, defined as a mismatch between when regions are most tilted directly towards the sun and the annual peak in temperature. In the Northern Hemisphere, the summer solstice in late June marks this event, leading to most major cities across the country, from New York City and Chicago to Atlanta and even Phoenix, having their warmest months from late June through August. Meanwhile, coastal cities like Los Angeles, San Diego and San Francisco really don't start feeling the warmth until the second half of the summer, some not even until summer's waning days.

Seasonal lag feels counterintuitive, kind of like the difference between astronomical and meteorological seasons. For the explanation here, let's forget about the sun for a second and focus instead on water, which makes up most of the Earth. Water, especially large oceans, tends to limit extreme temperatures, explaining in part why it can be a comfortable 75 when landing at LAX and a sweltering 95 when your family checks in at Disneyland 34 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 a lot more to warm it up than air or land. On the flip side, water also retains heat longer, leading to a much more gradual drop off, hence creating a late-summer peak in temperatures before a steady decline.

"The reason behind the lag, especially toward the West Coast, is because during the summer the land of the western U.S. heats up quicker than the Pacific which lowers pressure over land with relatively higher pressure offshore," Jason Nicholls, AccuWeather senior meteorologist, explained. "This pressure gradient favors a predominantly onshore flow off the cool Pacific into coastal areas and coastal valleys during summer. Hence, the term June Gloom was coined for the L.A. basin."

Meanwhile, heading into autumn, that discrepancy between land and sea becomes a little less pronounced which leads to a reduction in the coastal clouds and fog pattern that dominates the early part of the summer, Nicholls adds. By fall, the L.A. basin is also impacted by Santa Ana offshore wind events, which can make it feel like summer during the day even in January.

The phenomenon of seasonal lag comes into play in many other parts of the country, not just coastal California. Around 2,000 miles away in the Midwest, infamous lake-effect snow plagues cities like Cleveland and Buffalo, peaking in late fall and early winter between late November and mid-January. Because it takes much longer for the lakes to cool down than the land, this results in huge temperature differences, producing snowstorms.

As for the Southern California heat wave, where even overnight temperatures are forecast to be in the 70s with a smattering of 80s, limiting outdoor activities to the mornings and evenings and staying hydrated will be key.

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