What does a dry October and November mean for SLO County?

Civil engineer John Neil is the manager of the Atascadero Mutual Water Co.

Neil loves the science of hydrology and meteorology and their potential impact on people’s lives. The company has collected rainfall data since 1914 at a rain gauge at the confluence of the Salinas River and Atascadero Creek.

After he reviewed the historical data from this rain gauge, Neil discovered that when the area received more than 2 inches of precipitation during October, the rest of the rain season — which runs through June 30 of the following year — was higher than average. Neil calls this the “2 inches of rain in October rule.”

Unfortunately, the company’s rain gauge did not reach 2 inches of rain this October. In fact, it’s looking like it will not reach 2 inches by the end of November.

Typically, Santa Maria records about 2 inches of rain in October and November, while Paso Robles sees about 1.5 inches, and San Luis Obispo gets about 2.5 inches.

So far, the Central Coast has not seen any rain in these locations this October and November.

These conditions have driven most of California into the D0 (abnormally dry) category, according to the United States Drought Monitor. The next level is D1 (moderate drought), and that’s the level that much of the state will reach if it doesn’t start to rain this winter.

The last time it’s been this parched over this two-month time frame was 1980. That year only saw 0.01 of an inch of precipitation at Cal Poly, home of climatology for San Luis Obispo.

According to the National Weather Service, you need at least 0.02 of an inch to be classified as measurable rain.

After a review of the historical data from Cal Poly, when rainfall was less than 1 inch at the Irrigation Training & Research Center during the October through November period, rainfall amounts were less than average for the rain season (July 1 through June 30).

Over the past 100 years, when rainfall amounts were less than 1 inch on average, about 20 inches of rain was recorded for the entire rain season — or about 4 inches less than typical.

The only years that had above-average rainfall were 1977, which saw 48 inches, and 1937, which reported 31 inches.

From 1880 through 1920, when rainfall was less than 1 inch during October and November, San Luis Obispo recorded four years of above 24 inches. 1883 had the most at 42 inches.

So what will the rest of the rain season bring?

According to the federal Climate Prediction Center, the infamous neutral condition — known as “El Nothing” or “El Nada” — will continue into spring 2020.

The National Oceanic and Atmospheric Administration uses Niño 3.4, an area of sea-surface temperatures (SST) in a central equatorial region of the Pacific Ocean, as the standard for classifying El Niño (warmer-than-normal SST) and La Niña (cooler-than-normal SST) events.

The classification of the El Niño-Southern Oscillation (ENSO), or El Niño, La Niña or neutral conditions, historically has had a bearing on the amount of rainfall California will receive.

Typically, the higher the classification of the El Niño event, the more rain it will create.

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

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

The other large-scale ocean water temperature cycle is the Pacific Decadal Oscillation (PDO), which can increase or decrease the ENSO effect. The PDO is found primarily in the North Pacific.

The phases of the PDO are called warm phases or cool phases. Unlike ENSO, which focuses on the sea-surface temperature in the central equatorial region of the Pacific, the PDO is classified by seawater temperatures throughout the northern Pacific Ocean.

The models are indicating that it will continue to shift to the cool phase, which can mean a higher chance of below-normal rainfall.

At this time, the pool of warmer than usual seawater is also present off the Pacific Coast, where a cold patch existed in 2018. That warm “blob” is not expected to have a meaningful impact on the atmospheric circulation this upcoming winter.

According to PG&E senior meteorologist Ted Schlaepfer, the October polar sea ice extent was the second smallest on record behind 2007.

Current sea surface temperature anomalies in that area are well above average as the lack of ice will cause more of the sun’s energy to get absorbed by the ocean.

Climate model forecasts for that area are showing above-normal temperatures from December through February.

The warmer air mass this winter over the arctic is favored to produce blocking high pressure at times and disruption to the typical jet stream circulation.

Overall, slightly warmer or warmer than average temperatures are forecast for much of California, and seasonal precipitation should be below average.

Naturally, the snowpack should also be below average across the Sierra Nevada.

While most of the months of winter are expected to see below-normal rainfall, January is favored to receive more than 110 percent of average precipitation. With that said, the long-range predictions can be wrong; only time will tell the story.

Diablo Canyon tour

Back by popular demand is the Weather Watchers tour of Diablo Canyon power plant and lands. If you and your family would like to participate in this free tour on Tuesday, Nov. 26, please visit tourdcpp.pge.com to register.

It will start at 9 a.m. at the PG&E Energy Education Center, 6588 Ontario Road in San Luis Obispo, and will finish by noon.

This story was originally published November 17, 2019 at 4:50 AM.