Weather Watch

Here's what sparked record heat and wildfires along the Central Coast

Many Southern California locations broke their all-time high-temperature records Friday. Incredibly, the Van Nuys Airport hit 117 degrees, while both the Burbank Airport and Santa Ana reached 114 degrees. It’s not like the Van Nuys Airport sits in the middle of Death Valley; it’s the Fernando Valley.

Along the Central Coast, Cal Poly (home of climatology for San Luis Obispo) and the San Luis Obispo County Airport both hit 102 degrees around noon on Friday, breaking the daily high-temperature record of 99 set back in 1989.

In fact, most communities in the coastal regions, like Santa Maria and Lompoc reached into the high 80s to the mid-90s. Pismo Beach hit 100 degrees. In the inland valleys, like Paso Robles and Santa Ynez, the temperature soared well into the triple digits.

So, what is causing this heat wave and red-flag fire conditions? It's strong high pressure in the upper levels of the atmosphere combined with downslope winds, and here’s why.

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Throughout the world, downslope winds have different names. Along the Rocky Mountains, these downslope winds are known as Chinook — literally meaning “snow-eater” — winds. In Japan, they are called the Oroshi.

In Southern California, these downslope winds are called the Santa Anas. These famous winds may have gotten their name from the Santa Ana Mountains. However, some Southern Californians believe that the traditional name is Santanas. In Northern California, these downslope winds are called diablo winds.

Even relatively small geographic areas will have their own unique name for these winds. For example, in Santa Barbara, they’re referred to as “sundowners.”

Along the Central Coast, they are called the Santa Lucia winds. Like a semi- truck rolling down the Cuesta Grade, air from the higher elevations of the Santa Lucia Mountains flows downward along the mountain slopes toward the Pacific Ocean, pulled by the never-ending force of gravity.

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These downslope winds are technically called katabatic wind, from the Greek word katabatikos, which means “going downhill.”

As the air mass descends the side of the mountain range, it warms at the rate of about 5.5 degrees Fahrenheit per 1,000 feet of descent. Meteorologists refer to this rate of warming as the dry adiabatic lapse rate. If the air is warm at the top of the mountain range, it can be sizzling hot and bone dry by the time it reaches the valleys below. That’s what occurred in the San Fernando Valley on Friday when downslope (Santa Ana) winds helped to break all-time temperature records.

You see, as the air molecules descend into the higher atmospheric pressure close to Earth’s surface, they gain kinetic energy as they compress inward. If you’ve ever filled up a bicycle tire or a scuba tank, you’ve probably noticed them getting warmer as the pressure increased.

Locally, the Santa Lucia (northeasterly/offshore) winds flow from the land out to the Pacific and push the low coastal clouds out to sea. These winds can also create bone-dry relative humidity levels and counterintuitive time of peak temperatures.

Hot weather Pismo Beach115161
Crowds flocked to Pismo Beach on Friday as hot weather swept the region. David Middlecamp dmiddlecamp@thetribunenews.com

On Friday, the Santa Barbara Airport reached a high of 91 degrees at 1 p.m. then cooled to 79 by 3 p.m. Then the sundowner winds developed later in the evening, and temperatures soared to triple-digit levels, and relative humidity levels dropped 12 percent by 8 p.m., continuing through midnight. As Cal Fire will tell you, these atmospheric conditions combined with a seemingly never-ending drought significantly contributed to the Holiday Fire in Goleta.

I am often asked, “About what time of day do we usually have our highest temperatures in different parts of the Central Coast?”

As the sun rises, the solar radiation increases until the sun is at its highest point in the sky at noon Pacific Standard Time or 1 p.m. daylight saving time. This is called solar noon. Even though the sun’s radiation is at its strongest then, that doesn’t necessarily mean that the maximum temperature will occur at that time.

Many inland areas of San Luis Obispo County will continue to climb so long as the earth is receiving more incoming heat than what is radiating back to space. On average, the summertime temperature will peak at 3:30 p.m. in Paso Robles. Farther inland, it doesn’t peak until 5 p.m. during the summer months.

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However, it’s a different story along with our rugged coastline. We live in an area of many microclimates often caused by the clash of the Santa Lucia and northwesterly winds. This back-and-forth battle can produce hot conditions in the late morning and then cool conditions in the afternoon.

The most significant temperature gradient along the Central Coast I know of occurred on Sunday, Sept. 2, 2007. It was Labor Day weekend, when at the Pops by the Sea concert in Avila Beach there were reports of thermometers reaching 110 degrees.

A little distance away, a fishing boat at Port San Luis reported a temperature of 105 degrees near the Harford Pier. As the vessel moved past the breakwater toward the wind shift line, the temperature dropped 37 degrees in less than a quarter mile. About a half mile farther out to sea, the temperature reached 60 degrees off the Point San Luis Lighthouse.

In the interest of public safety, and following the wildfires in 2017, PG&E is implementing additional precautionary measures intended to reduce the risk of fires. To help meet the climate-driven challenge of increasing wildfires and extreme weather events, PG&E announced a comprehensive Community Wildfire Safety Program. To learn more, visit pgecurrents.com.

John Lindsey’s column is special to The Tribune. He is PG&E’s Diablo Canyon marine meteorologist and a media relations representative. Email him at pgeweather@pge.com or follow him on Twitter: @PGE_John.
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