Weather Watch

Dueling gauges at Paso airport

It’s no wonder people everywhere along the Central Coast are talking about rainfall totals these days. The Cal Poly rain gauge recorded 9.66 inches of rain during December. That’s the wettest December since 1996, which produced 10.88 inches of rain.

Many people who live along the Central Coast are passionate about weather and find the statistics of weather intriguing. And no other statistic resonates like rainfall totals.

Paso Robles Municipal Airport Manager Roger Oxborrow has been repeatedly questioned about the rainfall amounts. The airport has two rain gauges, both stationed in the open expanse of the airport between the taxiways, tarmacs and runways. One gauge is often reported and the other one rarely.

The one that is often reported in the news is the tipping-bucket type of rain gauge operated by the National Weather Service as part of the Automated Surface Observing System that typically operates at airports and other locations throughout the United States. The automated system provides such critical information as wind, temperature, pressure, cloud-base height and runway visibility.

A tipping-bucket rain gauge sends information over the Internet. Captured rain is funneled into a very small bucket, and when the equivalent of one-hundredth of an inch of rain accumulates, the bucket tips over and drains the water while at the same time activating an electronic switch.

The other rain gauge at the airport is a standard rain gauge operated by the city of Paso Robles. This type of gauge has been in use for more than a century.

It 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 is one-tenth the cross sectional area of the cylinder.

Hence, the amount of rain is exaggerated by a factor of 10, producing precise rainfall measurements. A calibrated wooden scale is put into the tube and withdrawn. The wet portion of the scale indicates the depth of the water and the amount of rain to one-hundredth of an inch. If more than 2 inches of rain has fallen, the excess water is caught in the outer shell of the cylinder and can be measured later.

Laurie Engstrom, also of the Paso Robles airport, has meticulously measured and recorded the rainfall information from its standard rain gauge over the years.

Her records indicated that the standard rain gauge measured 4.27 inches from Dec. 17 through 23. This matches well with other rain gauges in the area, such as Hog Canyon near Paso Robles, which measured 4.88 inches, and Shandon, which reported 4.69 inches during this period.

The National Weather Service tipping bucket measured only 1.64 inches, even though both gauges at the airport are separated only by a few hundred yards.

So if you thought that the rainfall at the airport was reading low, you’re probably right. Technicians at the Weather Service will be coming out to the Paso Robles airport to check the functionality of its tipping-bucket rain gauge.

This week’s forecast

A 1,005-millibar low- pressure system, 200 miles to the west of San Francisco at 4 a.m. Saturday, will continue to move southeastward parallel to the California coast.

This low-pressure system and associated cold front will produce moderate to heavy rain and fresh to strong (from 19 to 31 mph) southwesterly winds with gusts to 40 mph along the coastline this morning.

Scattered thundershowers are possible by this afternoon and evening. Showery weather is expected to continue into Monday morning.

Total rainfall amounts should range between 0.25 and 0.75 of an inch in the North County, and between 0.75 of an inch and 1.25 inches in the coastal valleys. The coastal mountains could receive up to 2 inches of rain by Monday.

A strong 1,030-millibar eastern Pacific high will take a position about 600 miles to the west of San Luis Obispo and will produce dry weather for the rest of the week.

Dense valley fog formation is likely under northeasterly (offshore) wind conditions.

Surf and sea report

The California NOAA marine buoy 357 nautical miles west of San Francisco reached 23 feet with a 16-second period on New Year’s Day.

The same west-northwesterly (290-degree deep-water) swell will arrive along the Central Coast this morning at 7 to 9 feet (with a 15- to 17-second period), building to 9 to 11 feet (with a 15- to 16-second period) this afternoon and will continue at this height and period through tonight.

Combined with this west-northwesterly swell will be 3- to 4-foot southerly (185-degree shallow-water) seas today.

A 7- to 9-foot west-northwesterly (290-degree deep-water) swell (with a 10- to 14-second period) is forecast along our coast on Monday.

A series of intense storms in the mid-Pacific with hurricane-force winds will produce long-period swell trains along the Central Coast this week.

A 6- to 8-foot west-northwesterly (290-degree deep-water) swell (with an 18- to 20-second period) will arrive along our coast Tuesday, increasing to 7 to 9 feet (with a 15- to 17-second period) Wednesday.

Another long-period west-northwesterly swell will arrive along our coast Thursday through Friday at 6 to 8 feet (with an 18- to 22-second period).

Seawater temperatures will range between 53 and 55 degrees through Friday.

Conservation tip

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John Lindsey is a media relations representative for PG&E. He is also a local weather expert and has lived along the Central Coast for more than 23 years. To subscribe to his daily weather forecast or ask him a question, e-mail