Weather Watch

Last week’s storm put SLO County’s rain totals above average. But will it stay that way?

After a series of storms that marched across the Pacific and tore into the Central Coast with fearsome southerly winds, the deep rumble of thunder, high waves and much-needed rain, it was joyful to see that big bright yellow ball in the sky contrasted with the emerald-green hills on Saturday.

The four-day storm total at Rocky Butte, near Hearst Castle, was 6.5 inches of rain on Monday into Thursday. A vigorous cold front moved southward through the Central Coast on Wednesday night into Thursday morning and produced 3.6 inches of rain at Calvin French’s Home in Adelaida and 3.4 inches on the San Marcos Pass over a six-hour period.

So far this rain season, which runs from July 1, 2018, through June 30, 2019, rain gauges throughout the Central Coast have recorded roughly 130 percent of average rainfall for late mid-January. Last year on this date, most San Luis Obispo and northern Santa Barbara county locations were at only 45 percent of normal.

Typically, by this time, both the Paso Robles and Santa Maria airports would have recorded over 5 inches of precipitation. However, this season, the weather stations at the Paso Robles Municipal Airport and the Santa Maria Public Airport have both recorded 6.4 inches or 128 percent of average for this time of the year. Cal Poly (official home of climatology for San Luis Obispo) has seen over 11 inches of rain, while SLOWeather.com in western San Luis Obispo reported 131 percent of normal rainfall.

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Surfers surveying the high waves on a stormy Thursday morning at Montaña de Oro State Park. John Lindsey Special to The Tribune

Nacimiento Lake, which sits entirely in San Luis Obispo County went from 17 percent of capacity as of Monday to 29 percent of capacity as of Friday — a nearly a 20-foot rise in lake level. Lake San Antonio, which is just north of the SLO County line, is at 16 percent. Large watersheds feed both lakes, but Nacimiento Lake, as a rule, will fill up about three times faster than Lake San Antonio, due to the larger size and proximity of its watershed to the Pacific.

However, different amounts of precipitation that fall in our notoriously complex Central Coast microclimates can play havoc with this rule.

According to data from SLOCountyWater.org, here are the other San Luis Obispo and Santa Barbara counties lake and reservoir percent of capacity figures: Lopez Lake is at 41 percent, Salinas Reservoir near Santa Margarita is at 85 percent and Whale Rock Reservoir near Cayucos is over 74 percent.

In Santa Barbara County, Gibraltar Reservoir filled, and its dam spilled into Santa Ynez River, which flows into Lake Cachuma. On Wednesday, Lake Cachuma was at 31 percent of capacity but rapidly rose to 36 percent by Saturday.

A relatively weak cold front will produce increasing clouds and light rain Sunday night in Monday morning, but then a dry spell, which is typical for winter, will start Monday afternoon and continue through the end of January. However, average rainfall amounts and lake and reservoir levels may increase in February and here’s why.

An atmospheric model initiated at the U.S. Navy Fleet Numerical Meteorology and Oceanography Center in Monterey is advertising the potential for the southern branch of the polar jet stream to shift southward toward Hawaii and pick up momentum across the mid-latitudes of the Pacific, which will allow it to carry storms and surges of subtropical moisture to California.

If this model verifies, the rainfall averages may remain above normal levels through the rain season. However, since this prediction is over 200 hours in the future, there’s less confidence that this model will verify. Model predictions more than 72 hours are suspect; beyond 72 hours, you can either shake your fist at the sky in frustration or take the most reliable strategy — “WAS,” or wait and see.

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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