Weather Watch

Past rainfall shows La Niñas are not all alike

As widely reported last year, La Niña was supposed to produce below-normal rainfall this year. Our experience so far this winter reminds us that not all La Niñas are created equal.

La Niña, which generates cooler-than-normal sea surface temperatures in the central and eastern equatorial Pacific, usually produces below-normal rainfall. According to meteorologist Jan Null’s studies (http://ggweather.com/enso.htm), the long-term rainfall average during La Niña events is about 87 percent of normal on the Central Coast, and even less in Southern California — about 70 percent — as the storm track is shifted northward.

The Cal Poly rain gauge (the official home for climatology records in San Luis Obispo) recorded 9.66 inches of rain in December. That’s the wettest December since 1996, which produced 10.88 inches of rain.

So far this rain season, Cal Poly has recorded 13.25 inches of rain, or about 175 percent of normal. Even if it didn’t rain again until the end of the rain season in June, San Luis Obispo would finish at 57 percent of normal.

The rain gauge at Diablo Canyon nuclear power plant has recorded 17.61 inches, or about 200 percent of normal. SLOweather.com (an informative local weather site) in the Irish Hills on San Luis Obispo’s Westside has already recorded 20.43 inches of rain, or 250 percent of normal.

As you head further south, the percentages of normal rainfall increase.

The Santa Maria, Santa Barbara and Los Angeles airports had their wettest Decembers on record, and each airport was near or over 500 percent of normal rainfall at the end of the year.

The lake and reservoir levels of San Luis Obispo County have significantly increased over the last few months. As of last Friday, both Lake San Antonio and Nacimiento Lake were at 53 percent of capacity, Whale Rock Reservoir was at 60 percent, Lopez Lake reached 70 percent and the Salinas Reservoir near Santa Margarita was over 100 percent and spilling.

Careful measurements by PG&E hydrographers in the Sierra indicate the snowpack is at 200 percent of average. This snowpack acts as a storage reservoir for water. As this reservoir melts, it slowly releases water for the needs of forests, agriculture, households and endangered species. It also provides the energy for hydroelectric power used during the longer days of spring and summer.

During a normal rain and snow season, a little more than 20 percent of the electricity delivered to PG&E customers is from hydroelectric power.

These varying rainfall conditions across the Central Coast were due to a nearly stationary low-pressure system, which took a position off the Oregon coast. Along with a persistent west-southwesterly jet stream, it steered moist, subtropical air toward the Central Coast from Dec. 17 through 22.

The surface charts from that period indicated an occluded front stalled over the Central Coast on Dec. 18 and 19, which produced nearly continuous rain that entire weekend.

I love the rain, but even I become weary. The rain brought areas of flooding and mudslides to the Central Coast, especially to the region around Oceano.

As I wrote two weeks ago, the same type of pattern developed in December 1964 and produced some of the worst floods on record in Northern California.

Most of the models are indicating that La Niña will continue to strengthen through March and should produce below-normal rainfall through the rest of the rain season. However, there are no guarantees for any given year. With that being said, it looks like you may be able to put your umbrellas away for awhile as we head into a midwinter dry spell.

Upcoming forecast:

Dense tule fog in the San Joaquin Valley has stubbornly persisted over the last few days, with afternoon temperatures only reaching the high 30s to the low 40s. However, there is a chance for some dissipation by this afternoon as colder and drier air filters in from the northwest.

The San Joaquin Valley’s dense fog and drizzle will spill over into the North County but should burn off by this afternoon with temperatures reaching the mid-50s under partly cloudy skies in Paso Robles.

North to northeasterly (offshore) winds this morning will produce mostly clear and sunny weather along the beaches and in the coastal valleys, with temperatures reaching the low 60s in San Luis Obispo.

As the winds shift out of the northwest (onshore) later today, the marine layer may develop along the beaches this evening through tonight.

Gentle to moderate (8 and 18 mph) northeasterly (offshore) winds Monday and Tuesday will give mostly clear and sunny conditions along the coastline with warmer afternoon highs and cooler overnight lows.

A series of weather systems will move into Oregon and Northern California this week with periods of heavy rain. A stationary 1,024 millibar Eastern Pacific High located 400 miles to the southwest of San Luis Obispo will keep the storm track north of San Luis Obispo.

However, the trailing edge of a cold front will produce increasing mid-to-high-level clouds Thursday and the chance for rain showers Thursday night into Friday morning. If it does rain, precipitation amounts will be less than 0.50 inches.

Surf report:

Today’s 6- to 8-foot northwesterly (310-degree deep-water) sea and swell (with a 5- to 13-second period) will continue at this height and period through tonight.

A 4- to 6-foot northwesterly (300-degree deep-water) swell (with an 11- to 18-second period) is forecast along our coastline Monday, decreasing to 3 to 4 feet (with a 9- to 15-second period) Tuesday.

A 2- to 3-foot west-northwesterly (285-degree deep-water) swell (with an 11- to 14-second period) will develop along our coastline Wednesday.

A 3- to 5-foot west-northwesterly (280-degree deep-water) swell (with an 8- to 18-second period) will arrive along our coastline Thursday, increasing to 5 to 7 feet (with a 15- to 17-second period) Friday.

Seawater temperatures will range between 52 and 54 degrees through Monday, increasing to 53 and 55 degrees Tuesday through Friday.

Conservation tip:

For information about PG&E’s hydroelectric facilities, visit www.pge.com/mybusiness/edusafety/systemworks/hydro.

John Lindsey is a media relations representative for PG&E. He is also a local weather expert and has lived along the Central Coast for more than 23 years. To subscribe to his daily weather forecast or ask him a question, e-mail pgeweather@pge.com.

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