Weather Watch

The rain season is almost over in SLO. But a big storm — even a tornado — could still hit

Take a rainy day trip from San Luis Obispo to Morro Bay

San Luis Obispo County received steady rain on Saturday, March 10, 2018. Take a drive from SLO to Montana de Oro and Morro Bay in the wet weather.
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San Luis Obispo County received steady rain on Saturday, March 10, 2018. Take a drive from SLO to Montana de Oro and Morro Bay in the wet weather.

The long-range models and charts are no longer indicating any significant storms on the horizon through the end of April, which is not uncommon as the rain season that runs from July 1 through June 30 starts to wind down.

Typically, the month of April receives roughly a third of the characteristic rainfall amounts of March, while the month of May only collects about one-tenth of March’s averages. It’s rare to receive rain in June, but it does happen from time to time. However, with that said, I’m always leery about predicting the end of the wet season as the Central Coast has experienced some of its most severe storms during the months of April and May, and here’s why.

Almost like clockwork, the Eastern Pacific high off the California coast strengthens in spring. This condition frequently weakens cold fronts as they head down the California coastline, which diminishes the strength of the pre-frontal (southerly) winds but tends to increase the post-frontal (northwesterly) winds. The northwesterly winds of spring are further enhanced by the higher amount of sunlight as the days grow longer.

Let me explain: As the days grow longer, the air in the inland valleys naturally warms. As the valley’s surface air heats, it expands and rises like a hot air balloon. This condition in turn produces lower pressure at the earth’s surface. Meteorologists refer to this as a thermal low. Nature never likes anything out of balance, and consequently, the higher air pressure out over the ocean forces air inland to fill the void left by the thermal low.

As if on cue, moderate gale-force to fresh gale-force (32 to 46 with gust to 55 mph) northwesterly winds are forecast along the coastline this Tuesday, Wednesday and Thursday. Partially counteracting the forces that weaken cold fronts in spring is the growing amounts of sunlight that heats the earth’s surface, which in turn warms the surface air and causes it to rise into the atmosphere.

This convection circulation can severely destabilize the atmosphere as the relatively warm air slams into the cold air above, which can produce a tempest. Here are some examples of late-season storms: On April 2, 2014, in the middle of the night, the Doppler weather radar at Vandenberg Air Force Base indicated a thick red line of severe weather over Cayucos moving southeast through the Chorro Valley toward San Luis Obispo. Later that morning, I received photographs and emails from numerous residents in Cayucos documenting 1-inch in diameter hailstones. This band of thunderstorms may have produced a weak tornado at Camp San Luis.

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Hailstones in Cayucos. Courtesy of Fallon and Brance Molnar

On April 7, 1926, a storm from the Pacific moved through the Los Osos Valley and produced lightning. The lightning struck large oil tanks along Tank Farm Road in San Luis Obispo. Altogether, more than 5 million gallons of oil burned over five days. Burning oil made it to Avila Beach by way of San Luis Obispo Creek. Intense heat from these fires produced hundreds of fire whirls — many of them showed characteristics of actual tornadoes.

On May 5, 1998, I was living on the corner of Kentucky and Fredericks streets in the neighborhood near Cal Poly, where it touched down. At first, I thought it was a fast-moving train along California Boulevard. However, my anemometer — a device to measure wind force — was fluctuating from 60 to 70 mph.

Tree branches were breaking, and then I saw debris rotating in a counterclockwise direction, clear evidence that a tornado was occurring. Later that day, the National Weather Service came out to the site and confirmed that a low-level tornado had indeed occurred.

On May 21, 2006, a cold front stalled over our area and produced nearly 3 inches of rain at the Diablo Canyon Ocean Lab, the highest amount of precipitation for a signal day in May.

Now, the good news: If we don’t receive any more precipitation this season, most locations will finish the rain season with above-normal averages, finishing between 110 and 130 percent of typical. Rocky Butte receives around 40 inches each year, but as of Saturday has recorded 49 inches.

Cal Poly (home of climatology for San Luis Obispo) is over 28 inches; the annual average for that station is 22 inches. The Paso Robles Airport is at 15 inches but usually gets 12.5 inches. Much like the Paso Robles Airport, the Santa Maria Airport receives about 13 inches each rain season but so far has seen 14.5 inches of rain.

All this rain has risen lake and reservoir totals throughout the Central Coast. Lake Nacimiento went from 17 percent of capacity as of January to 87 percent of capacity as of Friday. Lake San Antonio is at 40 percent. According to data from SLOCountyWater.org, here are the other San Luis Obispo County lake and reservoir percent of capacity figures: Lopez Lake is at 59 percent, Salinas Reservoir near Santa Margarita is at 100 percent, and Whale Rock Reservoir near Cayucos is 89 percent.

In Santa Barbara County, Gibraltar Reservoir is releasing water into the Santa Ynez River, which flows into Lake Cachuma. In January, Lake Cachuma was at 31 percent of capacity but had risen to 79 percent as of Saturday. Even Twitchell Reservoir, which was mostly dry at the beginning of February, is now reporting 26 percent of capacity.

Celebrate Earth Day

Please join us, PG&E employees, Saturday, April 13, to celebrate Earth Day at Montaña de Oro State Park. The event is one of a number of service projects sponsored by PG&E and the California State Parks Foundation. If you plan to join us, register at the California State Parks website, www.calparks.org/help/earth-day/. Rangers will provide tools and supervision at the event.

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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