Saturday marked the end of the 2011-12 California rain season, which runs from July 1 through June 30, and what a difference a year makes.
At Cal Poly — home of climatology for San Luis Obispo — 14.6 inches of rain fell this rain season, or about 65 percent of normal. The previous year Cal Poly recorded 31.5 inches, or 140 percent of normal rainfall. From 1893 through 2012, San Luis Obispo has averaged 22.4 inches of rainfall each season. (The Tribune’s rainfall total on its weather page comes from the SLOweather.com gauge on the west side of town, where more rain falls than at Cal Poly. It showed just more than 20 inches for this year).
In the North County, Paso Robles received 8.7 inches of rain, or about 68 percent of normal. Last year Paso Robles reported 17.6 inches, or about 137 percent of normal. Along the Pecho Coast, the rain gauge at Diablo Canyon nuclear power plant recorded 11.5 inches, or about 53 percent of average. Last year 28.1 inches of rain fell at Diablo Canyon.
Further southward, Santa Maria recorded 9.7 inches, or about 70 percent of normal, and Santa Barbara had 11.7 inches of rain, or about 66 percent of normal.
Elsewhere across the state, Southern California averaged only 57 percent of normal and Northern California saw 65 percent of normal.
So what caused last season’s low rainfall amounts? The La Niña cycle was probably to blame. La Niña cycles produce stronger-than-normal trade winds across the equatorial Pacific. This condition triggers a greater amount of upwelling along the North and South American Pacific coastlines, which gives rise to cooler surface seawater temperatures.
These cooler waters tend to keep a persistent ridge of high pressure over California that often forces the storm track toward the Pacific Northwest, leaving the Central Coast with less rain. The months of December, January and February, historically our wettest months of the year, only produced a combined total of 3.5 inches of rain last season. Normally, the rainfall total for that period should be about 13.9 inches at Cal Poly.
Thankfully, the ridge of high pressure moved southward and a trough of low pressure parked off the California coast during March and April, allowing a series of storms to produce much-needed rain. These rains produced decent amounts of rye grasses, which helped local cattle ranchers tremendously. However, May and June were dry.
July is the driest month of the year, averaging only about 0.03 inches of rain in San Luis Obispo. According to old rainfall records that I found, dating back to 1869, only 23 of the past 140 months of July have seen any amount of rain. Occasionally, subtropical moisture from the south will migrate across our area during late summer, producing rain and thunderstorms, but overall it’s mostly dry.
So what does today’s newly minted rain season hold in store for us? Historically, after back-to-back La Niña cycles, we frequently go into an El Niño phase, which means we could have near-normal to above-normal rainfall this year. The National Oceanic and Atmospheric Administration’s Climate Prediction Center in Camp Springs, Md., has issued an El Niño watch (warmer-than-normal ocean water in the equatorial and eastern Pacific) for late summer into the fall and winter.
However, another large-scale ocean water temperature cycle continues to lurk in the North Pacific, and it could produce lower-than-average winter rainfall.
This other large-scale cycle is the Pacific Decadal Oscillation, or PDO, which can increase or decrease the El Niño effect. The PDO is found primarily in the North Pacific. The phases of the PDO are called warm phases or cool phases. Unlike El Niño and La Niña, the PDO stays in one phase for an extended period — between 10 and 40 years.
The El Niño and La Niña phases usually last for about a year. It appears that we’re still in the cool phase of the PDO, which means it could cancel out the El Niño effect. These are long-range forecasts and should be taken with a grain of salt. Only time will tell the story.
Today’s weather report
A 1,031-millibar Eastern Pacific High about 1,000 miles to the west of San Luis Obispo County will remain nearly stationary while a thermal trough will remain over the Great Central Valley of California.
This condition will continue to produce an onshore flow that will maintain the coastal temperature inversion layer. This cool and moist layer of marine air will allow coastal low clouds and areas of fog and drizzle to form along the beaches, in the coastal valleys and even into parts of the North County this morning.
This coastal stratus will clear back to the beaches later this morning. The westerly and southwesterly facing beaches (Pismo Beach, Grover Beach Cayucos, Avila and Shell beaches) will clear by this afternoon. However, the northwesterly facing beaches (Morro Bay, Los Osos, Montaña De Oro and the Nipomo Mesa) will remain mostly overcast with some afternoon clearing.
Overall, below normal temperatures will continue with a relatively deep marine layer during the night and morning hours and fresh to strong (19- to 31-mph) northwesterly (onshore) winds each afternoon in the usual locations through Tuesday.
Temperatures are expected to cool over the Fourth of July holiday as another weak but dry weather system moves through the north. This system will produce a stubbornly persistent coastal overcast through the Fourth of July along most of the beaches. However, the cloud ceiling should remain above 1,000 feet allowing for good viewing of the firework displays at Pismo Beach, Morro Bay and Cayucos.
A warming trend will begin across the North County on Friday through next weekend as high pressure now bringing blistering heat to the Midwest moves farther west and begins to influence inland locations. Closer to the coast, slight warming is expected next weekend with continued coastal low clouds and fog and seasonal to slightly below normal temperatures.
Today’s 3- to 5-foot northwesterly (315-degree deep-water) sea and swell (with a 5- to 9-second period) will continue at this height and period through tonight. A 4- to 6-foot northwesterly (305-degree deep-water) sea and swell (with a 5- to 11-second period) will arrive along the Pecho Coast on Monday and will continue at this height and period through Tuesday.
A 3- to 5-foot northwesterly (300-degree deep-water) swell (with an 8- to 11-second period) will develop along the Pecho Coast on Wednesday and will remain at this level through Friday.
Arriving from the Southern Hemisphere: An energetic storm has developed about 500 miles to the east of New Zealand last week. A 1-foot Southern Hemisphere (225-degree deep-water) swell (with an 18- to 20-second period) from this storm is expected to arrive along our coastline late Wednesday. This swell will gradually build to 1 to 2 feet on Thursday and peak Friday and Saturday at 1 to 3 feet (with a 16- to 18-second period).
Seawater temperatures will range between 52 to 55 degrees through Friday.
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John Lindsey is a media relations representative for PG&E. He is a local weather expert and has lived along the Central Coast for more than 25 years. If you would like to subscribe to his daily weather forecast or ask him a question, send him an email at email@example.com.
Detailed rainfall totals
Adelaida, near Franklin and Las Tablas Creeks at Cal’s 21.40” Arroyo Grande, A.G. Creek, 12.73" Arroyo Grande, Rancho Grande Park at Ken's 11.19" Arroyo Grande, at Jim's Home 10.74" Arroyo Grande, Moore Lane at Lori & Carlene’s Home 9.52” Arroyo Grande, Printz Rd at Richard’s Home 12.98” Atascadero, Heilmann Regional Park 8.58" Atascadero, ramage ave at Larry’s Home 17.40 Atascadero, Northwest side at Dan’s Home 13.09” Atascadero, South at Mickey's Home 13.87" Atascadero, west side at Mike’s Home 16.57” Avila Beach, Indian Hill at Jeanette’s Home 14.06” Avila Beach, Gary’s Home 14.01” Avila Valley, PG&E Energy Education Center 14.05” Baywood Park, at John's Home 10.17" Cambria, Santa Rosa at Main 13.23" Cambria, Ellis and Richards at Runo’s Home 12.75” Cambria, Park Hill at Arthur’s Home 12.21” Cambria, Berwick Drive at Bob’s Home 15.95” Camp San Luis 12.68" Cayucos, at Mike's Home 19.33" Cayucos, Cottontail Creek at Bill’s Home 21.40” Cayucos, Marsh Road at Doug’s Home 20.51” Cayucos, San Geronimo Ranch Jennifer & Chuck 18.00” Condor Lookout Los Padres National Forest 13.31" Creston, Rich's Home 7.96" Creston, Rick’s Home on Lamplighter Way 6.34” Creston, Windrose Farm at Bill’s Home 7.19” Diablo Canyon 11.46" Grover Beach, at Pete’s Home 12.91” Hog Canyon road, west of Paso Robles 8.07” Hollister Ranch at Michael’s Home 16.38” Hollister Ranch at Kit’s Home 12.65” Islay Hill, San Luis Obispo, CA 13.14" Los Osos at Jean’s Home 10.12" Los Osos, Landfill Precipitation 11.06” Los Osos, Cuesta by Sea 8.61” Los Osos, George’s Home 10.59” Lopez Lake Dam 14.17” Morro Bay, at Dawn's Home 9.17" Morro Bay, Luzon St at Bill’s Home 13.30” Morro Bay, roof top on Rich’s Home 7.87” Nipomo, South at Dave’s Home 11.02" Nipomo, East 12.63" Oceano at Tom’s Home 9.79” Oceano, Hwy 1 overcrossing Union Pacific Railroad 9.65” Oak Shores at Paul and Karen’s Home 9.96” Paso Robles 8.68" Paso Robles Airport 8.79” Pismo Beach 9.98" Paul's House (41 West & Toro Creek) 21.40” Rocky Butte 24.65" Santa Barbara 11.67" Santa Barbara, Mission Canyon at Jaffurs Wine Cellars 13.20” Santa Margarita Fire Department 15.83" Santa Maria Public Airport 9.70" San Luis Obispo, Cal Poly 14.60” San Luis Obispo, Mission Prep High School 17.27 San Luis Obispo, San Carlos Drive at Barbara’s Home 14.46” San Luis Obispo, Toro St. & Pismo St at Bruce’s Home 12.50” SLOWeather.com 20.20" San Simeon 12.87" See Canyon, Creek Side Farms 13.20" Shandon 7.01” Templeton 11.38" Templeton, at Karl’s Home on Almond Drive 9.79”
SOURES: PG&E Weather Forecast and Tribune Weather Watch readers, SLOweather.com, NWS, SLOCountyWater.org and Golden Gate Weather Services.