Weather Watch

Here’s why NOAA’s latest weather outlook is good news for SLO County rainfall

The Climate Prediction Center released the latest outlook for El Niño this winter, and it may be good news. Here’s why:

The Climate Prediction Center was advertising a neutral condition — the infamous “El Nothing” or weak El Niño developing this winter. It reported Monday that “El Niño is favored to form in the next couple of months and continue through the Northern Hemisphere winter 2018-19 with a 70 and 75 percent chance.”

More importantly, the strength of this expected El Niño event should have a bearing on the amount of rainfall the Central Coast will receive. Typically, in San Luis Obispo and Santa Barbara counties, the higher the classification of the El Niño event, the more rain it will create.

Let me explain, NOAA 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 fortunetelling SST cycles in Niño 3.4 are categorized by the amount they deviate from the average SST — in other words, an anomaly over a three-month period.

El Niño conditions like neutral conditions — or El Nada or weak El Niño — typically doesn’t produce any reliable seasonal rainfall predictions along the Central Coast, but a moderate, strong or very strong more times than not do.

Over the last month, the numerical models have increased the predicted El Niño category to a moderate strength level for this winter.

Historically, a moderate El Niño condition traditionally produces about 113 percent of average rainfall along the coast from San Francisco to San Luis Obispo County. Farther south, it generates approximately 124 percent of average precipitation along the coastline from northern Santa Barbara County toward San Diego, according to Jan Null, a former National Weather Service lead forecaster and PG&E meteorologist. Null is recognized as an expert on El Niños and La Niñas and their relationship to California’s weather. His studies can be viewed at ggweather.com.

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A few other models are indicating a little less rainfall, or about 110 percent of above-average rainfall in San Luis Obispo County and normal rainfall amounts in Santa Barbara County.

With that said, here are my rain season’s predicted rainfall totals: Cambria, 24 inches; Paso Robles, 14; San Luis Obispo, 25; Nipomo, 20; Santa Maria, 14; Lompoc, 16; and Santa Ynez, 20.

So how do these above-normal seawater temperatures in an area so far away have such a profound effect upon California’s weather? The answer is in the winds, or should I say the upper-level winds.

These warmer waters in the eastern Pacific produce a more considerable amount of evaporation. As this water vapor ascends into the atmosphere, it often condenses into thunderstorms and releases tremendous amounts of latent heat, which further decreases the atmospheric pressure. This area of low pressure, in turn, changes the path of the southern branch of the polar jet stream, pulling it farther southward toward the Central Coast.

John Lindsey’s column is special to The Tribune. He is PG&E’s Diablo Canyon marine meteorologist and a media relations representative. Email him at pgeweather@pge.com or follow him on Twitter: @PGE_John.
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