Why has it been so hot in San Luis Obispo County?

A heat wave is classified as a prolonged period of extremely high temperatures for a specific region or season. But what is a heat wave in one region can be called typical for another area.

If our coastal communities like Morro Bay or Lompoc reached into the 80s over a few days, it would be classified as a heat wave, while just a short distance away in the inland valleys — Paso Robles or Santa Ynez — it would be considered cool at this time of year.

Yes, the Central Coast is a land of many microclimates.

Historically in August and September, it almost seems inevitable that monsoonal moisture with higher humidity levels and a chance of thunderstorms will follow a heat wave.

In a reversal, this month’s heat wave started with monsoonal moisture from the remnants of hurricane Elida that created scattered rain showers and thunderstorms along with wild temperature swings.

On Aug. 13, Diablo Canyon went from 59 degrees at 12:30 p.m. to 83 degrees just one hour later at 1:30 p.m. Paso Robles only saw a high of 74 degrees under cloudy skies with light rain showers. In comparison, Santa Ynez reported a high of 104 degrees!

The Santa Maria Airport broke its daily high-temperature record on Aug. 13, when the thermometer reached 93 degrees, besting the previous record of 89-degrees set back in 1991. The next day, the Santa Maria Airport reported 89 degrees, which tied the previous record recorded in 1929.

Not only is the Central Coast hot, but sweltering heat is shattering records throughout the Western United States.

Last week, Phoenix broke the record for most days at 110 degrees or greater in one year, which was 33 days set back in 2011.

As of Aug. 15, Phoenix has seen 40 days at 110 degrees or more in one year. Unfortunately for Phoenix, more days with 110 plus degree temperatures are expected this week.

The weather models continue to indicate that warm to hot conditions will continue through Aug. 22.

Remember, it is not so much how hot it is on a single day, but the heat’s longevity is more important.



Most of us can take one or two days of hot weather. Studies have shown that after three days of excessive heat, people suffer substantial added effects, like irritability and heat-related illnesses.

To make matters more concerning, an upper-level low-pressure system about 750 miles southwest of the Central Coast will continue to entrain subtropical moisture and send it northward to California. These additional surges of monsoonal moisture are expected to move over the Central Coast from time to time this upcoming week, increasing the relative humidity levels, making it more difficult for your body to cool off.



So why has it so hot?

A large and robust dome of high pressure centered over the Four Corners area shifted westward toward California. It may reach nearly 600 decameters (dm) in height over Central and Southern California.

Meteorologists determine the thickness of the atmosphere by analyzing 500-millibar upper-level charts.

In other words, this chart will tell you how high the pressure is three or four miles above the Earth’s surface. The higher the 500-millibar line, the thicker the atmosphere, and the warmer it will be.

Anything above 580 dm will usually give hot temperatures, if near 600 dm, record-breaking. Paso Robles was forecast to reach 111 degrees on Monday, 113 degrees on Tuesday and 114 degrees on Wednesday. If these temperatures develop, they will break daily high records.

The all-time high in Paso Robles is 115 degrees set back on July 20, 1960, and again on Sept. 2, 2017.

As the atmosphere continues to warm due to increasing amounts of carbon dioxide and other greenhouse gases from the burning of fossil fuels, the thickness of these high-pressure domes will also increase, which will bring an ever-greater number of heat waves.

I reviewed historical temperature records from throughout California and found that the record-high temperatures occurred about twice as often as record lows. In other words, if the climate were stable, the ratio of record highs to record lows would be even.

According to Climate Central, higher temperatures from carbon pollution “cause more Americans to retreat indoors and turn on their air conditioners. Air conditioning consumption is expected to increase by 59% in U.S. homes by 2050.”

This story was originally published August 18, 2020 at 5:05 AM.