For coastal L.A., the hottest days of summer are yet to come

While sweltering heat envelopes the nation this weekend from the desert Southwest to the Northeast, coastal Californians can’t afford to be complacent. The hottest weather of the season in the Southland is usually still to come, with all those drought and fire conditions.

A strengthening high-pressure peak stretching from coast to coast is expected to issue a nationwide over-temperature warning for the next few days, the National Weather Service said. The jet stream at this time of year has generally moved to its furthest north of the season, up near the Canadian border or beyond. This allows the most stifling heat of the summer, on average, to occur under the ridge it forms in the south, on the contiguous United States, from mid-July to early August.

But California is a slightly different story, largely dependent on its distance from the Pacific Ocean, thanks to a persistent sea layer. This gray stratigraphic cloud layer is created by coastal waters cooled by the California Current flowing south from Alaska.

In Southern California, the inland valleys are hottest in mid-August, slightly later than in the rest of the country. Closer to the coast, the hottest weather was only partially delayed from late August to September 1, said Eric Boldt, a meteorologist with the National Weather Service in Oxnard. Some of the highest temperatures occurred in September, he said, due to the Santa Ana winds off the coast and warming from the ground.

Other areas of coastal California, particularly north of Point Conception, did not have their hottest average temperatures until much later. For example, Half Moon Bay has its warmest average maximum temperatures in the second half of September and the first half of October, with most of its record readings in the low to mid-1990s taking place during the early months of the year. that month.

But why don’t all the hottest days happen when the sun is most direct in the Northern Hemisphere? Solar radiation peaks at the summer solstice on June 21. The reason for the delay over most of the United States is that the proportion of daytime heat input from the sun exceeds nighttime cooling over the next several weeks. summer solstice, until the temperature begins to drop. in late July and early August.

Notable exceptions are Arizona and New Mexico, where the hottest weather tends to occur in June, before the North American Monsoon rain clouds usually gather. Thunderstorms typically bring cooler temperatures, and the monsoon, which peaks in July and early August, provides about half of the annual rainfall for parts of the region. But in 2020, when the monsoon was essentially nonexistent, the region endured record heat.

Another big exception is along the Pacific Coast, where the sea layer acts as a heat shield.

During the summer, the West Coast is dominated by low pressure off the Pacific Ocean and low thermal pressure over the continent, said Jan Null, a veteran Bay Area meteorologist with Golden Gate Weather Services. Air moves from high pressure towards lower pressure, creating a westerly westerly wind onshore that moves down the California Coast, promoting sea layer clouds.

As we move into fall and winter, the pattern will change, Null said. The high altitude offshore weakened and moved south, while the polar jet stream began to decrease southward and bring cooler and high-pressure air masses to the Great Basin.

This leads to sea layer clouds providing less coastal protection and temperatures starting to rise. The jet stream flowing south is a harbinger of cooler winter weather to come. But the high pressure building in the Great Basin also heralds the start of the Santa Ana season in Southern California, when cool ocean water is driven away and coastal temperatures soar.

In the fall and winter, the Great Basin, surrounded by mountains, can be a giant reservoir of high-pressure air, all of which wants to escape and flow toward lower pressure. That lower pressure can be found nearby, at sea level off the California coast. To get there, high-pressure air would flow downhill and through passes and canyons. The atmosphere is constantly trying to restore equilibrium, and winds are associated with these changes in barometric pressure over short distances. So again, the high pressure flows towards the lower pressure, only this time it flows offshoreproduces the infamous Santa Ana winds to the northeast of Southern California.

The valleys of the Great Basin range in elevation from 2,000 to 6,000 feet. As this air travels down the coast, it heats up and dries out due to compression. Submerged air increases its temperature by 5.5 degrees Fahrenheit for every 1,000 feet it descends. This is called the lapse rate. For example, air flowing from a 4,500-foot mountain into the ocean, would warm nearly 25 degrees.

In addition to this difference in pressure, or pressure gradient, winds accelerate as they are forced through canyons and through, creating the Venturi effect – like the nozzle on a garden hose.

Offshore Santa Ana winds, at their strongest, can reach the coast and even Santa Catalina Island. They can hollow out the waves, much to the delight of surfers. But even weaker offshore currents could constrain the sea layer. Now, densely populated areas near the coast – where the heat could be hidden under sea clouds earlier in the season – are recording some of the highest temperatures.

Latest US drought monitor, released July 21, 2022.

The latest US Drought Monitoring data was released on Thursday.

(Paul Duginski / Los Angeles Times)

Even before the Santa Ana winds howled, coastal Southern California was beginning to see its marine advantage dwindle. Overnight minimum temperatures peaked on average from about mid-July to mid-September, with many of LA’s warmest nighttime temperatures on record in the low to mid-80s occurring in September. This means uncomfortable sleeping conditions for those who are too poor to afford air conditioning. Those who can afford it will strain the grid by keeping their air conditioners running day and night.

Furthermore, sea surface temperatures off Southern California, on average, peak in August and September, increasing humidity, said Alex Tardy of the weather services office in San Diego. That, coupled with the potential of humid monsoon flows from the southeast, could keep the day and night wet.

While marine stratigraphic clouds can provide coastal sun protection, the temperatures are moderate when elsewhere in bloom, they often bring only drizzle, but do not severely alleviate persistent drought conditions. persistent in California. In a sense, with California’s Mediterranean climate and months without rain, they save some of California’s worst temperatures towards the end of the dry season when vegetation is driest and most prone to burning.

Each year, it’s a race to see which comes first, the beginning of the rainy or windy Santa Ana season, as climatologist Bill Patzert, formerly of NASA’s Jet Propulsion Laboratory, likes to point out.

La Niña, a global climate pattern that typically leads to dry winters in the southwestern United States, is poised to continue into a rare third consecutive year. The pattern is expected to decline during the July-September period before rising again through the fall and winter, the National Weather Service’s Climate Prediction Center said earlier this month. So there’s a high chance that dry weather conditions and increased fires are likely to last – and get worse – into 2023.

https://www.latimes.com/california/story/2022-07-23/for-coastal-l-a-the-hottest-days-of-summer-are-yet-to-come For coastal L.A., the hottest days of summer are yet to come

Edmund DeMarche

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