It hasn’t rained since late April along the Central Coast. On April 24 and 25, a weak low-pressure system moved eastward over San Luis Obispo County and produced about a third of an inch of rain at the Diablo Canyon nuclear power plant’s gauge. Since then, there has been no measurable precipitation. As of today, that’s 150 straight days without rain.
This dry spell sparked my curiosity: What was the longest period of consecutive days without rain in coastal San Luis Obispo County? After I reviewed 21 years of rain data and weather forecasts from the power plant, it was obvious that this year was one of the longer phases of dry weather on record.
So far, the longest dry spell occurred in 2004 when it didn’t rain for 206 days (March 27 through Oct. 18).
I decided to filter out periods of coastal drizzle that can produce up to a few hundredths of an inch of precipitation. This gave a truer representation of actual rainfall across a greater portion of the county, because the inland areas received considerably less coastal drizzle.
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Some years, like 2003, saw heavy rain in May. Last year, it rained 1.25 inches from June 4 through 6, reducing the length of the year’s dry spell. Also, some years had significant monsoon rains. Back in 1996, nearly a third of an inch of rain was recorded on July 27 from a subtropical system.
One group paying particularly close attention to rainfall totals are farmers and ranchers. Believe it or not, most of these people are perfectly happy with these dry summers.
Our climate is classified as Mediterranean, meaning the annual rainfall pattern includes a dry summer and wet winter. The predominant weather feature is the Eastern Pacific high — an area of high pressure over the northeastern Pacific Ocean. Usually by June, the Eastern Pacific high has shifted northward, keeping the storm track far to the north. As we head toward winter, the probability of rain gradually increases. Todd Morris of the National Weather Service in Oxnard told me that the probability of precipitation peaks Feb. 22 in San Luis Obispo, with rain occurring 35 percent of the time.
After that, the probability of rain decreases through summer, reaching a low around Aug. 20, with rain only occurring in 6 percent of the days.
John Salisbury, a sixth- generation farmer in Avila Valley, told me, “The dry spell this year produced an ideal grape-growing season, although we could have had some later spring rains and a little less heat in the North County.”
He went on to say, “It is OK with us grape growers to hold off on rain until the second week of November; after that, let it pour!”
The same sentiment was echoed by rancher and Cambria historian Dawn Dunlap. She said her father often said it’s preferable if it doesn’t rain until November. If it rains before then, the grass seeds can germinate prematurely. Without the sustained rains of winter, most of the grass won’t reach maturity, leaving less feed for their cattle.
This is what occurred in 2009. One of the strongest October storms on record slammed into the Central Coast on the 12th and 13th. It produced 7.55 inches of rainfall at SLOweather.com in western San Luis Obispo at Chris and Katie’s home. Not to be outdone, Paul’s home along Highway 41 at Toro Creek recorded 7.80 inches of rain. All this rain caused the grass seeds to germinate. However, the rest of October and November were nearly dry, and the grass withered away.
Joy Fitzhugh, a Cambria cattle rancher and legislative analyst for the San Luis Obispo County Farm Bureau, said, “Summer rains can actually leach out the nutrients in the dry grass. If it rains more than a half an inch during summer, the dry grass will often deteriorate, leaving behind barren earth.”
Not only can summer rain damage golden grasses, but it also can produce fires. In years past, hay barrels have become wet while still in the field during a summer rain. Later, an unsuspecting rancher would store the hay bales in a barn. Like putting wet grass clippings in a compost pit, these wet bales of hay would compost and eventually catch fire, burning down the barn.
The main weather feature today will be an upper-level low-pressure system over Northern California that will bring decreasing northwesterly winds, a deeper marine layer and cooler conditions today. However, pleasant fall weather will continue through the week with night and morning coastal low clouds and fog.
Today’s high temperatures will range from the low to mid-60s along the beaches. The North County will reach the mid-80s, while the coastal valleys will reach the high 70s.
Fair weather with slightly warmer temperatures is likely by the end of this week and into next weekend, with a chance for subtropical moisture streaming north into the Central Coast by Saturday.
Today’s surf report
Today’s 3- to 5-foot northwesterly (300-degree deep-water) swell (with an 8- to 10-second period) will continue at this height and period through tonight.
A 975-millibar storm developed in the Gulf of Alaska on Friday night with 50- plus knot northwesterly winds. A 5- to 7-foot west-northwesterly (290-degree deep-water) swell (with an 18- to 20-second period) from this early season storm will arrive along the coast Monday morning, peaking Monday afternoon through Tuesday at 6 to 8 feet (with a 16- to 18-second period).
A 4- to 6-foot northwesterly (300-degree deep-water) swell (with an 8- to 13-second period) will develop Wednesday and will remain at this height and period through Thursday. A 3- to 5-foot northwesterly (300-degree deep-water) swell (with an 8- to 10-second period) is expected Friday through Saturday.
Arriving from the Southern Hemisphere: A 1-foot Southern Hemisphere (200-degree deep-water) swell (with a 15- to 17-second period) will arrive today into Monday.
A 1-foot Southern Hemisphere (220-degree deep-water) swell (with an 18- to 20-second period) will arrive along the Pecho Coast on Wednesday, peaking at 1 to 2 feet (with a 15- to 17-second period) Thursday.
Seawater temperatures will range between 54 and 56 degrees through today. Seawater temperatures will increase to 55 and 57 degrees Monday and remain at this level through Friday.
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John Lindsey’s column is special to The Tribune. He is a media relations representative for PG&E and longtime local meteorologist. If you have a question, send him an email at firstname.lastname@example.org.