Weather Watch

SLO County rain totals are still below average after the storm, but more is on the way

Rain beats against the glass at the Point San Luis Lighthouse during last week’s rainstorm that hit the Central Coast.
Rain beats against the glass at the Point San Luis Lighthouse during last week’s rainstorm that hit the Central Coast. Special to The Tribune

Rainfall amounts so far this season have been about 45 percent of average for mid-January throughout the Central Coast.

Typically, both the Paso Robles and Santa Maria airports would have recorded about 5.5 inches of precipitation by this time. This rain season, which runs from July 1, 2017, through June 30, 2018, however, the weather station at the Paso Robles Municipal Airport has recorded 2.19 inches, while the Santa Maria Public Airport has seen 2.42 inches. The San Luis Obispo County Municipal Airport has reported 3.95 inches of rain, or 45 percent of average. SLOWeather.com in western San Luis Obispo has also recorded 45 percent of normal rainfall.

Nacimiento Lake, which sits entirely in San Luis Obispo County, is currently at 42 percent of capacity. Lake San Antonio, which is just north of the SLO County line, is at 31 percent. Large watersheds feed both of these lakes, but Nacimiento, as a rule, will fill up about three times faster than San Antonio due to the larger size and proximity of its watershed to the ocean. 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 County lake and reservoir capacity percentages: Lopez Lake is at 52 percent, Santa Margarita Lake at 82 percent and Whale Rock Reservoir near Cayucos at 74 percent. In Santa Barbara County, Cachuma Reservoir is currently at 39 percent of capacity.

Below average rainfall amounts and lake and reservoir levels should increase by the end of the month, and here’s why.

Several weak cold fronts will produce between a few hundreds to a tenth of an inch of rain Tuesday followed by another cold front later Thursday into Friday, which will yield between a tenth and a half inch of rain throughout the Central Coast.

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.

The two numerical models most trusted by weather forecasters — the European Center for Medium-Range Weather Forecasts and NOAA’s Global Forecast System — are advertising a series of significant storms moving through Central Coast starting Jan. 22 to Jan. 30. These low-pressure systems are forecast to produce gale-force southerly winds and periods of moderate to heavy rain.

Photographer Joe Johnston takes drive down the Cuesta Grade to the SLO Tribune office in San Luis Obispo on a rainy day in January 2018.

If these models verify, the rainfall averages could reach near normal levels for San Luis Obispo and northern Santa Barbara counties by the end of January. Unfortunately, and I’m worried sick about this, the predicted storm systems later this month could also produce moderate rain in the burn areas of southern Santa Barbara and Ventura counties, which have subsequently been hit with devastating flooding and mudslides.

Since this prediction is more than 200 hours in the future, there’s less confidence that these models 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.

However, there’s much higher confidence that the storms will produce a series of high-energy wave events along the California Coastline. The four most important factors in the formation of waves that eventually arrive along the Central Coast are wind speed, wind duration, length of wind fetch and air and water temperature differences. All these factors are coming together to produce the largest waves of the year.

A 5- to 7-foot west-northwesterly (290-degree, deep-water) swell (with a 22- to 25-second period) will arrive along our coastline Sunday, increasing to 9 to 11 feet (with an 18- to 20-second period) Monday.

This west-northwesterly (285-degree, deep-water) swell will further build to 11 to 13 feet (with a 16-to 18-second period) Tuesday. This swell will be followed by a 14- to 16-foot northwesterly (295-degree, deep-water) swell (with an 18- to 20-second period) Thursday.

Many of us visit the beaches to enjoy the waves along the Central Coast. But ask anyone who has spent significant time at sea, and I’m sure they will have their own stories of vexing experiences with waves. Waves are powerful and full of energy, so regardless of whether they appear small or large, never turn your back to them and always be respectful of the ocean.

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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