Fall is about to begin. When will SLO County see rain?

The arrival of the autumnal equinox — the beginning of fall — will occur at 12:20 p.m. on Wednesday, Sep. 22, when the sun will be directly over the equator.

We will lose the greatest amount of daylight of the year on that day, about three minutes at our latitude.

Not only is the autumnal equinox near the peak of the hurricane season, but the equatorial regions are also now absorbing much more of the sun’s energy as the Arctic Circle moves toward perpetual darkness of the winter solstice on Dec. 21, the longest night of the year.

Nature never likes anything out of balance, and soon massive mid-latitude cyclones, many times larger in size than hurricanes, will begin to develop in the vast expanse of the North Pacific Ocean to attempt to equalize the atmospheric temperature differentials between the equatorial belt and the northern latitudes.

As these mid-latitude Pacific storms develop, will the upper-level (jet steam) westerly winds increase and primarily travel at a direct path across the Pacific and bring rain to California, or will these winds shift further northward and carry these tempests into the Pacific Northwest?

The answer to this weather question will help to determine if it will be a dry or wet year.

Depending on the strength of these low-pressure systems, historically, a dry year will experience between one and four storms.

In contrast, a wet year will encounter between five and nine storms — or even more significant rainfall events. However, just one big atmospheric river (AR) event can alter that statement.

According to the Climate Prediction Center, most models indicate borderline or weak La Niña conditions to emerge this fall and continue into this winter.

La Niña events tend to produce less precipitation as the upper-level winds shift farther northward.

Unfortunately, the condition — combined with another large-scale ocean water temperature cycle — may produce lower-than-average winter rainfall this year.

Overall, moderate or strong La Niña conditions typically produce lower-than-average winter rainfall. Under borderline or weak La Niña conditions, the long-range rain forecast is much less certain.

In fact, there have been weak La Niña years that have produced well above average rainfall amounts. The Farmer’s Almanac 2021–2022 Extended Weather Forecast predicts “average wintery temperatures and precipitation” for the western United States, including California.

With that said, the predicted La Niña may happen within the context of a larger climate event, a resurgence of a cool phase of the basin-wide Pacific Decadal Oscillation (PDO).

The PDO is a long-term fluctuation of the Pacific Ocean that waxes and wanes between cool and warm phases approximately every five to 20 years.

During most of the 1980s and 1990s, the Pacific was locked in the oscillation’s warm phase. But conditions changed in 1999, when the cool phase, which resembles a large La Niña, has tended to dominate the Pacific Ocean.

“The reemergence of this large-scale PDO pattern tells us there is much more than an isolated La Niña occurring in the Pacific Ocean,” said Bill Patzert, a retired oceanographer and climatologist at NASA’s Jet Propulsion Laboratory in Pasadena.

“This multi-year Pacific Decadal Oscillation ‘cool’ trend can intensify La Niña or diminish El Niño impacts around the Pacific basin,” Patzert told me.

Patzert’s observation on how the cool phase of the PDO produces below-average precipitation seems to be verified by the ongoing drought raging throughout the western United States.

According to NASA’s Earth Observatory website, Lake Mead, the largest reservoir in the United States, dropped at its lowest level in late August since Franklin Delano Roosevelt was president.

Needless to say, this upcoming rain season will be vitally important for all of us. Many San Luis Obispo County residents are asking when will the first rains of the year arrive.

On Friday, Sep. 17, at 5 p.m. I will be on Dave Congalton’s “Hometown Radio” show on KVEC-FM for the Guess the First Day of Rain Contest.

Call 805-543-8830 during the show and predict the first day the Central Coast will receive at least 1 inch of rain over 24 hours at any calibrated rain gauge in the region, such as Cal Poly, Santa Maria Public Airport or San Luis Obispo County Regional Airport.

A few Central Coast residents have reported a significant emergence of tarantulas, the big, hairy weather prognosticator. According to longtime residents, you can expect rain in about three weeks.

Speaking of big and furry spiders, our first rains of the season usually arrive around Halloween into the first half of November, with a 12% chance of rain on any given day.

The chances of rain increase to 20% in December through January, peaking at a 24% chance in February. The chances of precipitation gradually decrease from March through May.

It is not unusual to have dry Octobers, but we have seen some surprisingly intense autumn rainstorms in the past.

On Oct. 13 and 14, 2009, the Central Coast experienced something unlike any other October storm since records have been kept, breaking many 24- and 48-hour rainfall records for the month.

The Santa Lucia mountains in Monterey County received more than 21 inches of rain. The San Luis Obispo County Water Resources rain gauge on the top of Rocky Butte near San Simeon recorded 13.7 inches of rain.

Nacimiento Lake went from 9% of capacity to 18% in just 24 hours.

I received emails from readers that reported between seven and 10 inches of precipitation along Highway 41 between Morro Bay and Atascadero.

Both Cal Poly and Diablo Canyon Power Plant reported about 2.5 inches of rain. Lompoc City Airport reported 0.66 of an inch of rain on Oct. 13 and 2.03 inches on Oct. 14.

Oddly, the Santa Maria airport only saw about three-quarters of an inch of precipitation.

If the current models verify, we could see lower than average amounts for rainfall in California this winter, but only time will tell the story.