We’re on track for a La Niña winter. What does that mean for SLO County?

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Steve Wilson swilson@star-telegram.com

The latest runs of numerical weather models suggest a predominantly dry pattern will continue through mid-November, as the storm track remains directed over the Pacific Northwest.

This setup is typical of a classic La Niña pattern.

So, what exactly is the La Niña?

It occurs when cooler than normal ocean temperatures develop in the Equatorial Pacific. This typically shifts the jet stream — and the Gulf of Alaska storms it steers — toward the Pacific Northwest, leaving the Central Coast drier than usual.

However, there have been a few La Niña years that brought significantly above-average rainfall to our area.

Adding to the La Niña’s impact this season is the negative phase of the Pacific Decadal Oscillation, which tends to amplify La Niña’s effects along the Central Coast.

Another piece of the long-range weather puzzle is John Neil’s observations. Neil, the manager of the Atascadero Mutual Water Company, reviewed historical data from the company’s rain gauge and observed a pattern: when the area receives more than 2 inches of rain in October, the rest of the season (which runs through June 30 of the following year) tends to be wetter than average.

Since 1914, seasons following a wet October have generally experienced above-average rainfall — a trend Neil refers to as the “2-inches-of-rain-in-October rule.”

This October, however, was dry. So San Luis Obispo County should expect a dry winter.

Another historical insight: the last two rainy seasons brought well-above-average precipitation. However, rain records from Cal Poly’s Irrigation Training and Research Center, dating back to 1869, show no instance of three consecutive years of above-average rainfall during a negative Pacific Decadal Oscillation phase.

All these factors point toward a potentially drier-than-normal year, but ultimately, only time will tell.

On Sunday, a pattern of moderate to fresh (13 to 24 mph) and at times gusty Santa Lucia (northeasterly/offshore) winds developing during the overnight and morning will start, shifting out of the northwest and increasing in the afternoon.

The northeasterly winds may reach strong to gale-force (25 to 38 mph) levels on Tuesday afternoon, decreasing on Wednesday and Thursday.

This classic fall weather pattern will produce primarily clear skies with cool and crisp nights and mornings with mild afternoons.

The inland valleys (Paso Robles) and coastal valleys (San Luis Obispo) will hit the low to mid-70s.

The beaches will range between the mid to high-60s.

The long-range models do not indicate any significant rain through the end of November.

Surf report

A 5- to 7-foot northwesterly (295-degree deep water) sea and swell (with a 7- to 15- second period) is forecast along our coastline through Sunday.

A 956 millibar storm with hurricane force winds moved into the Gulf of Alaska last Thursday.

A 7- to 9-foot northwesterly (290-degree deep water) swell (with an 18- to 20-second period) will arrive along our coastline on Monday, becoming an 8- to 10-foot northwesterly (300-degree deep water) sea and swell (with a 7- to 17-second period) on Tuesday through Wednesday.

A 4- to 6-foot northwesterly (300-degree deep water) sea and swell (with a 5- to 15-second period) is forecast on Thursday, decreasing to 3- to 5-feet with the same period on Friday into Saturday.

Surface seawater temperatures will range between 53 and 55 degrees through Saturday.

On this date in weather history (Nov. 3)

1890: The temperature in Los Angeles reached 96 degrees, a November record for 76 years. (David Ludlum)

1927: Somerset, Vermont, was deluged with 8.77 inches of rain to establish a 24 hour record for the state. (The Weather Channel)

1961: A rare November thunderstorm produced snow in Casper, Wyoming. (The Weather Channel)

1987: Twenty one cities, mostly in the Ohio Valley, reported record-high temperatures for the date. The afternoon high of 80 degrees in Colombus, Ohio was their warmest reading of record for so late in the season. Showers and thundershowers associated with a tropical depression south of Florida produced 4.28 inches of rain in Clewiston in 24 hours. (The National Weather Summary and Storm Data)

1989: Cold weather prevailed in the central United States. Six cities in Texas, Minnesota and Michigan reported record low temperatures for the date. The low temperature of 7 degrees above zero at Marquette, Michigan, was their coldest reading of record for so early in the season. (The National Weather Summary)

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.