Except for a few months — such as in March, when we had above average rainfall — a ridiculously persistent ridge of high pressure has driven the majority of the Gulf of Alaska storms north of the Central Coast over the last six years.
That condition has produced one of the worst droughts seen in San Luis Obispo, Santa Barbara, Kern, Fresno, King and Ventura counties, according to the National Drought Mitigation Center.
However, a trough of low pressure along the West Coast this month has resulted in above average rainfall for the Central Coast for October. San Luis Obispo County Regional Airport has recorded 1.81 inches this month — normal rainfall for October is 0.88 inches.
Rainfall also is above average to the north: Seattle will probably exceed the all-time record for rain in October of 8.96 inches that fell in 2003. Eureka has already recorded 9.73 inches this month; normal October rainfall there is 2.66 inches and the record is 13.04 inches, set in 1950. San Francisco has recorded 2.25 inches, which is more than double the average for October in that city.
For reasons that are still unclear, the atmosphere seems to be locked in a wet pattern. PG&E meteorologist Mike Voss told me that it could be a bit of an El Niño hangover from last year, but was quick to point out that nobody really knows for sure.
“We cannot conclusively point to global warming as the ‘cause’ of this month’s substantial rainfall. It could be the result of the random, chaotic nature of the atmosphere,” he wrote last week. “But I can tell you that climate models driven with increased greenhouse gases show a dramatically increased amount of early fall precipitation, particularly during October.”
As Cal Poly physics professor Robert Echols will tell you, the warmer the atmosphere the more water vapor it can hold, which can lead to more intense rainfall events.
In San Luis Obispo County, I recently received an email from John Neil of the Atascadero Mutual Water Co. The company has collected rainfall data since 1914 at a rain gauge at the confluence of the Salinas River and Atascadero Creek.
For the first time since 2010, Neil recorded more than 2 inches of rain this month at that rain gauge.
After he carefully reviewed the historical data, Neil discovered that in past years when this station received more than 2 inches of rain during October, the following months also received above-average rainfall. That meant that during those rain seasons (which run from July 1 to June 30) total rainfall was higher than normal.
That led me to review rainfall data from the Diablo Canyon Power Plant rain gauge; I noticed the same trend of a wet October leading to near-normal to above-normal precipitation for the rain season.
I must say, I’ve been forecasting weather along the Central Coast since 1992, and believe that Neil’s observations could be wonderful news indeed. Keep in mind, though, that nothing is guaranteed and only time will tell the story.
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PG&E Safety Tip: Halloween is a time for children of all ages to indulge in a little too much candy, dress up in silly costumes and enjoy the nip of autumn in the air. Please use or make costumes that are easy to see at night. Consider adding reflective tape to costumes and trick-or-treat bags. Have kids carry glow sticks or flashlights to help them see and be seen by drivers. Slow down and be especially alert in residential neighborhoods.
John Lindsey is PG&E’s Diablo Canyon Power Plant marine meteorologist and a media relations representative. Email him at firstname.lastname@example.org or follow him on Twitter @PGE_John.