What caused this past winter's abnormal dryness in SLO County?

Special to The TribuneJune 22, 2013 

Next Sunday will mark the end of the 2012-13 California rain season, which runs from July 1 through June 30. It should come as no surprise that it’s way below average.

This rain season, we had above-normal rainfall during November and December, followed by one of the driest January-through-June periods in San Luis Obispo County’s history. In fact, January through June in San Luis Obispo will go down as the third driest such period on record at Cal Poly (home of climatology for the city) since 1870, when weather observations started. Since January, Cal Poly has recorded only 3.5 inches of rain. Normally, it should be about 15 inches.

This has become a major issue for our brave firefighters. The early-season rains produced hefty amounts of vegetation. But the late season’s paltry rainfall combined with stronger-than-average winds — last week, northwesterly wind gusts exceeded 50 mph almost every day at Diablo Canyon — have lowered the moisture levels of potential fuels to near-record lows.

“All these conditions are coming together to make this year an especially difficult one for our firefighters throughout the state,” said Robert Lewin, Cal Fire chief for San Luis Obispo County. To help prepare for this fire season, Cal Fire has an informative website
that can be viewed at http://calfire.ca.gov/fire_prevention/fire

This rain season, Cal Poly recorded 14.3 inches, or about 64 percent of normal. Last rain season, 14.6 inches of rain fell. Normally, San Luis Obispo averages 22.4 inches of rainfall. Chris Arndt of SLOweather.com in western San Luis Obispo reported his season’s rain totals at 59 percent of normal. Along the Pecho Coast, the rain gauge at Diablo Canyon Power Plant recorded 9.1 inches, or about 40 percent of normal. Last year, 11.5 inches of rain fell at Diablo Canyon. In the North County, Paso Robles saw only 4.7 inches of rain, or just 37 percent of normal. Last year, Paso Robles reported 8.7 inches of rain. Paso Robles averages 12.8 inches of rainfall each season.

Further south, Santa Maria recorded 6.2 inches of rain, or 47 percent of normal. Last year, it recorded 9.7 inches. Santa Barbara had 9.4 inches of rain. Last year, it was 11.7 inches. Santa Barbara averages 17.7 inches of rainfall. Fresno reported just 5.7 inches of rain, or 50 percent of normal.

So what caused this season’s low rainfall amounts?

A large-scale, high-pressure ridge dominated our weather at a longitude that has never fluctuated far from the western edge of North America. This condition didn’t allow the normal wintertime storm systems to progress southward into California. However, another area of high pressure about 3,700 miles away as the crow flies may be the real culprit. According to PG&E senior meteorologist Ted Schlaepfer, this year’s drought pattern across California is related to a strong and persistent area of high pressure in the upper atmosphere near Greenland. The “Greenland Block” is a blocking high-pressure ridge that has forced the polar jet stream south over the Midwest and East Coast.

So what does the newly minted rain season hold in store for us?

The U.S. Climate Prediction Center is predicting that the current neutral conditions — the infamous El Nothing — will continue into fall. Neutral conditions generally don’t produce any reliable seasonal rainfall predictions along the Central Coast.

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. The PDO is found primarily in the North Pacific. The phases of the PDO are called warm phases or cool phases. Unfortunately, we’re still in the cool phase of the PDO, meaning we could continue to see below-average rainfall through next year. These are long-range forecasts and should be taken with a grain of salt. Only time will tell the story.

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Did you know? PG&E delivers some of the nation’s cleanest power. Nearly 60 percent of the electricity the company provides to customers comes from sources that are renewable and/or emit no greenhouse gases.

John Lindsey’s column is special to The Tribune. He is a media relations representative for PG&E and a longtime local meteorologist. If you have a question,  email him at pgeweather@pge.com.

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