Last time it rained more than an inch in a day was Dec. 22. The last time we had more than a tenth of an inch was May 7 at Cal Poly (home of climatology for San Luis Obispo). Yet other Central Coast locations haven’t seen this much rain since March 30.
Some of our rolling hills are turning from hues of golden brown to shades of gray as soils are exposed.
To make matters more concerning, Robert Lewin, Cal Fire chief for San Luis Obispo County, said vegetation moisture level surveys in San Luis Obispo County and throughout the state are indicating extremely low moisture content.
So far, 2013 is the third driest on record. The driest January-to-August on record at Cal Poly occurred in 1972, when only 2.8 inches of rain fell. The next driest was 1984, when 3 inches of precipitation were recorded. So far this year, only a meager 3.5 inches of rain have fallen from the sky. Both of these years had bone dry Septembers, followed by wet Octobers.
From a historical standpoint, this could be important. You see, John Neil of the Atascadero Mutual Water Co. told me that his company has collected rainfall data over the past 100 years at a rain gauge at the confluence of the Salinas River and Atascadero Creek. After he carefully reviewed the historical data from this rain gauge, Neil discovered that when this station received more than 2 inches of rain during October, the rest of the rain season was wetter than normal. Let us hope for a wet October.
So what does the 2014 rain season (July 1, 2013, through June 30, 2014) hold in store for us?
One of the key influences on our weather is El Niño and its sister, La Niña, which are triggered by changing conditions in the Pacific Ocean that produce warmer- or cooler-than-normal ocean temperatures. El Niños produce warmer seawater and can bring average or above-normal rainfall, while La Niña periods yield below-average seawater temperatures that usually produce below-normal levels.
This year, we have neither. The U.S. Climate Prediction Center is predicting that the neutral conditions — the infamous El Nothing — will continue through fall and maybe into winter. Neutral conditions generally don’t produce any reliable seasonal rainfall predictions along the Central Coast.
However, there is another ocean water temperature cycle that seems to be playing a larger role. It’s called the Pacific Decadal Oscillation, or PDO, and it probably has the same effect on our rainfall as El Niño and La Niña in San Luis Obispo County.
The phases of the PDO are called warm or cool phases. Unlike El Niño and La Niña events, which last only about a year or so, the PDO stays in one phase for a longer period.
There’s a growing amount of evidence that the warm phase of the PDO produces above-normal levels of rainfall, while the cool phase produces below-normal levels. According to Josh Willis of the NASA Jet Propulsion Laboratory in Pasadena, “There is some research to suggest that during negative phases of the PDO, La Niñas become more frequent and El Niños become less so.”
Unfortunately, we’re still in the cool phase of the PDO, meaning we could continue to see below-average rainfall. Nevertheless, the current seawater trends seem to indicate that we could move into the warm phase of the PDO by next year. So my prediction for this rain season: between 15 and 20 inches as measured at Cal Poly.
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