Weather Watch

Want to know how much it’s raining? Here’s how you can measure rainfall from home

San Luis Obispo Creek swells as more rain comes to SLO County

More water was flowing through San Luis Obispo Creek on Saturday, January 7, 2017, as heavy rain continued to hit San Luis Obispo County.
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More water was flowing through San Luis Obispo Creek on Saturday, January 7, 2017, as heavy rain continued to hit San Luis Obispo County.

An area of high pressure over the Aleutian Islands, combined with a stubborn ridge of high pressure over Baja California, has forced the upper-level winds to move from the East China Sea across the Pacific toward California.

That wind pattern means passenger airliners traveling from Asia to the United States may travel faster than the speed of sound, thanks to the 200 mph-plus tailwinds at about 30,000 feet.

Besides reducing your flight time from Hawaii to California, more importantly, this flow has steered a significant surge of subtropical moisture from the Pacific Ocean toward the Golden State. This condition is called an atmospheric river, Pineapple Express or, in the meteorological community, “turning on the hose.”

So far, much of the moisture has been in the lower levels of the atmosphere. This low-level subtropical water vapor combined with persistent southerly winds along the coast and the lack of convective dynamics has produced heavy precipitation along the shoreline, coastal mountains and especially coastal mountain slopes.

In these locations, the southerly winds have lifted the moist air mass along the coastal mountains and cooled it 2.7 degrees for each thousand feet of elevation. This process rings out the moisture from the heavens like squeezing a wet sponge or mop.

This condition created vast differences in precipitation totals throughout the Central Coast. For example, this season, Rocky Butte has recorded a total of 24.7 inches, well on its way to the normal annual total of 39 inches, while Paso Robles, which usually receives 14.9 inches for the rain year, has recorded just 5.73 inches so far.

These dramatic differences in rainfall totals have resulted in a flurry of emails and phone calls, with readers wondering if the rainfall reports are legitimate or how to measure it. One thing I’ve learned over the years is that Central Coast residents are passionate about their weather statistics — and for good reason, because many are directly involved in agriculture or public safety.

Arroyo Grande Creek breaches the beach, making it tricky for vehicles to cross to the camping area of Oceano Dunes State Vehicular Recreation Area on Saturday, January 7, 2017.

You can measure precipitation with a low-tech bucket or cup as well as using one of the many types of rain gauges. A standard rain gauge, a type that has been in use for more than a century, consists of a large metal cylinder (20 inches high and 8 inches in diameter) with a funnel on top. It drains rain into a narrow tube that has one-tenth the cross-sectional area of the cylinder. Because of this, the amount of rain is exaggerated by a factor of 10, producing precise rainfall measurements.

Another type of rain gauge is a tipping bucket. The advantage to this kind of gauge is that the data can be sent in real-time over the internet, unlike a standard rain gauge that’s read manually. A tipping bucket funnels rain into a small bucket, and when the equivalent of one-hundredth of an inch of rain accumulates, the bucket tips over and drains the water while activating an electronic switch that allows remote recording and reporting of rainfall.

The problem with a tipping bucket is during heavy downpours you can lose a tiny amount of precipitation during each bucket tip, thereby slightly underreporting rain. At home, I have a Davis Vantage Pro2 weather station. Data from this station and other Central Coast locations can be found at www.SLOWeather.com.

Another type of rain gauge is a weighing type. This type of measure captures the rain in a fixed bucket. The bucket sits on top of a sensitive weight scale. The scale translates the accumulated weight of the rain into readings of a hundredth of an inch. Cal Poly uses this type of instrument.

Official NWS observations are taken either manually with standard National Weather Service equipment or automatically with installations such as the Automated Surface Observing Systems at the Paso Robles and San Luis Obispo airports.

The series of storms with gale-force southerly winds today through Monday and again Tuesday night into Thursday should be more convective in nature, and rainfall differences between the coastline and inland areas should be less severe than last week’s systems.

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PG&E invites interested members of the public to attend an information fair Thursday to learn about temporary job opportunities during upcoming maintenance outage work at Diablo Canyon nuclear power plant. The fair will be held at the PG&E Energy Education Center, 6588 Ontario Road, San Luis Obispo, at the following times: 2:30 p.m., 4:30 p.m., 5:30 p.m. and 7:30 p.m. Outage job seekers need only attend one session.

John Lindsey’s column is special to The Tribune. He is PG&E’s Diablo Canyon marine meteorologist and a media relations representative. Email him at pgeweather@pge.com or follow him on Twitter: @PGE_John.

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