Weather Watch

Persistent atmospheric river creating ‘breathtaking’ rain totals

San Luis Creek flows over San Luis Bay Drive on Feb. 6.
San Luis Creek flows over San Luis Bay Drive on Feb. 6. jjohnston@thetribunenews.com

The National Oceanic and Atmospheric Administration has used sea surface temperatures (SST) in a central equatorial region of the Pacific Ocean called Niño 3.4 as the standard for classifying El Niño (warmer-than-normal SST) and La Niña (cooler-than-normal SST) events since 1950.

The fortune-telling SST cycles in Niño 3.4 are categorized by the amount they deviate from the average SST — in other words, an anomaly.

More times than not, this anomaly is a valuable tool in predicting rainfall amounts along the Central Coast, but there have certainly been many exceptions. Two very strong El Niño events (1982-83 and 1997-98) produced about 200-percent-of-average rainfall along the Central Coast. Last year’s very strong El Niño event did not.

Dawn Dunlap at the Walter Ranch near Cambria has received 57 inches of rain so far this year.

The seawater temperatures in Niño 3.4 this year have been slightly cooler than normal, hence the classification by NOAA of a weak La Niña condition. The name La Niña, meaning “the girl,” originates from Spanish; it’s analogous to El Niño, meaning “the boy” or “the Christ child.”

Since 1950, weak La Niña conditions produce dryer than average years, but there have been exceptions. The 1994 rain season saw more than 42 inches at PG&E’s Diablo Canyon nuclear power plant. The 2004 rain season produced nearly 36 inches of rain at the power plant. Normal rainfall at that location is about 20 inches per year.

This year — like the weak La Niñas of 1964, 1994 and 2004 rain seasons — has seen a strong blocking high in the Gulf of Alaska/Aleutian Islands split the jet stream into the polar to the north and the Pacific jet stream to the south like a sand bar in the middle of a river. This condition has caused the jet stream to shift farther southward and steered significant surges of subtropical moisture from the Pacific Ocean toward California.

For the first time in nearly six years, Santa Margarita Lake is full and spilling over into the Salinas River. Mark Hutchinson, deputy director of SLO County Public Works, talks on Feb. 8, 2017, about the role of the lake, also known as the Salina

This condition is referred to as an atmospheric river, pineapple express or — in the meteorological community — “turning on the hose.” So far this year, like other weak La Niña years that produced massive amounts of precipitation, atmospheric rivers have come through in a big way. Much of the moisture has been in the lower levels of the atmosphere. This low-level subtropical water vapor combined with persistent southerly winds along the coast and the lack of convective dynamics has produced heavy precipitation along the coastal mountains and especially coastal mountain slopes.

In these locations, the southerly winds have lifted the low-level subtropical moisture over the coastal mountains and cooled it 2.7 degrees Fahrenheit for each thousand feet of elevation. This process is called the saturated adiabatic lapse rate, which rings out the moisture from the heavens like squeezing a wet sponge or mop — in other words, orthographic enhancement.

This year’s atmospheric rivers have made greater-than-expected differences in precipitation totals. For example, as of Friday, the Paso Robles Municipal Airport on the east side of the Santa Lucia Mountains was at about 13 inches or 150 percent of average. On the ocean side of these mountains, both Cal Poly with 27 inches and SLOweather.com with 29 inches are at nearly 200 percent of average rainfall for this time of the year.

Atascadero Lake reached its capacity Tuesday, February 7, 2017, and is now draining into Atascadero Creek. It's the first time in seven years the lake has been full.

The two very strong El Niño events (1982-83 and 1997-98) that fashioned storms with strong atmospheric dynamics that created storm-force winds and precipitation that came from the higher levels gave more uniform rainfall percentages of average throughout the Central Coast.

Because of this year’s predominance of atmospheric rivers, rainfall amounts in the Santa Lucia Mountains have been breathtaking. Dawn Dunlap at the Walter Ranch near Cambria is at 57 inches so far this year. She has several rain gauges; one at 800 feet, one at 1,200 feet and one at 1,600 feet. Over the years, Dunlap has observed between a 20 percent and 30 percent increase in rainfall amounts for each 400 feet elevation increased. Debby Mix of the Circle 3 Ranch, at 2,150 feet of elevation in the San Simeon Watershed between Vulture Rock and Rocky Butte, has logged 78 inches.

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Doc Miller has been keeping track of the rain for 36 years at his property on Pine Mountain at 2,650 feet of elevation. So far this year, he has recorded 95 inches of rainfall! Miller recorded a little more than 49 inches this January, the wettest month on record. He has estimated that multiplying the San Luis Obispo County’s Rocky Butte rain gauge data by 1.5 times comes close to his data. By the way, Rocky Butte is at 64 inches of precipitation.

With rainfall like this, no wonder Lake Nacimiento went from 25 percent to 86 percent of capacity in just a little more than one month, despite releasing more than 4,000 cubic feet a second for the purpose of maintaining flood control space.

The atmosphere may continue to produce above-average rainfall into March, as it seems to be locked in this pattern.

As a bonus, SLO Weather’s Chris Arndt told me that this year is turning out to be the best mushroom season in years. He is seeing all manner of them pop up. The rain is also bringing the promise of a stellar wildflower year in late February, March and April, but that story is for a future column.

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I will be with spatula this Tuesday to help raise funds for water wells in developing nations. The Well Worth It campaign, launched by Cannon in 2010, will host a pancake breakfast fundraiser at its office, 1050 Southwood Drive in San Luis Obispo from 7 to 8:30 a.m. Tuesday. Access to clean water remains an issue in many parts of the world. Well Worth It has raised enough money to fund the construction of 19 wells worldwide. For a $10 donation during its fourth-annual pancake breakfast, you can enjoy unlimited pancakes, eggs, sausage, OJ and coffee (kids eat free with each paid adult) from Popolo, a family-owned restaurant. And you won’t want to miss the special treat: chocolate chip pancakes!

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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