Is El Niño coming to SLO County this winter? One sign could arrive in October

The Climate Prediction Center says that El Niño conditions are being observed in the Equatorial Pacific, and El Niño is anticipated to continue through the Northern Hemisphere winter (with greater than a 95% chance through January-March 2024).

Historically, an El Niño condition, more times than not, produces above-average rainfall along the Central Coast.

Another method is to look at historical rainfall data. Sometimes, the answer to the formidable rainfall question is right under our noses. Let me explain.

Since 1914, the Atascadero Mutual Water Co. has collected rainfall information at a rain gauge at the confluence of the Salinas River and Atascadero Creek, forming a database that contains more than a century of rainfall statistics.

Civil engineer John Neil is the manager of the Atascadero Mutual Water Co. He carefully reviewed the historical data from the company’s 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.

In other words, since 1914, the months that follow a wet October throughout the rain season experience above-average rainfall. Neil calls this the 2-inches-of-rain-in-October rule.

I’ve been forecasting weather along the Central Coast since 1991, and my confidence in Neil, the oracle from Atascadero, has grown considerably. A few long-range models indicate a wetter-than-normal October for the Central Coast.

This week’s forecast for SLO County

In the meantime, moderate to fresh (13 to 24 mph) northwesterly winds (onshore) winds will allow low marine (stratus) clouds with pockets of fog and mist to develop during the overnight, clearing back to the ocean by the afternoon and will continue through Monday.

High temperatures on Sunday through Monday will reach the low 80s in the inland valleys (Paso Robles) and the mid-70s in the coastal valleys (San Luis Obispo). The beaches will remain in the 60s.

By Sunday, an early-season storm system will move across the northern Pacific and strengthen to 964 millibars off the Washington state coastline.

On Monday, the associated cold front will produce rain as far south as the Golden Gate Bridge. The main impact along the Central Coast will be strong to gale-force (25 to 38 mph) northwesterly winds along the coastline on Tuesday through Saturday.

These winds will usher in a cooler air mass, and temperatures will drop to the 70s in the inland valleys and 60s in the coastal valleys and along the beaches. Marine low clouds will continue to form overnight and quickly clear during the morning and afternoon along the coastline.

The long-range models diverge between a potentially wet weather pattern by mid-October, while few other models indicate a large area of high pressure developing over the Great Basin.

Consequently, Santa Lucia (northeasterly) winds could develop, producing dry and warm conditions.

Surf report

A 2- to 4-foot northwesterly (300-degree, deep-water) sea and swell (with a 4- to 12-second period) is expected along our coastline on Sunday through Monday morning, increasing to 3- to 5-feet Monday afternoon and night.

Strong to gale-force (25 to 38 mph) northwesterly winds along the coastline will generate an 8- to 10-foot northwesterly (300-degree, deep-water) sea and swell (with a 5- to 12-second period) on Tuesday through Friday, decreasing to 5- to 7-feet on Saturday into next Sunday.

Seawater temperatures will range between 60 and 63 degrees through Monday, decreasing to 58 to 61 degrees on Tuesday through Wednesday, further lowering to 55 to 57 degrees Thursday through next Sunday.

This week’s temperatures

LOWS AND HIGHS, PASO ROBLES

LOWS AND HIGHS, SAN LUIS OBISPO AND COASTAL VALLEYS

John Lindsey is a retired PG&E marine meteorologist. Email him at JohnLindseyLosOsos@gmail.com or follow him on Twitter @PGE_John.

This story was originally published September 24, 2023 at 5:00 AM.