Weather Watch

Accurate measure of rain is as important as where it falls

It’s no wonder people along the Central Coast are interested in rainfall totals these days.

If it doesn’t rain enough, farmers and ranchers won’t be able to draw as much water as they need. If it rains enough to produce runoff, some cities, industries and large construction sites may be required to take water samples for the state Regional Water Quality Control Board. Too much rainfall can produce flooding or landslides.

Other people who live along the Central Coast are just passionate about weather and find weather statistics intriguing. Many take pride in the amount of rain they have received.

Accurate, long-term rainfall data is key to making educated decisions about climate and water resources. The home of climatology for San Luis Obispo is Cal Poly. The weather station there was established July 7, 1867, and has accumulated 144 years of National Weather Service (NWS) records. Much of the data can be seen at Southern California Climate Summaries (

For many years the Cal Poly Police Department recorded weather data, mostly because the officers were on duty around the clock. If the temperature fell below freezing in the middle of the night, they contacted the Crop Science Department, which would take steps to protect sensitive plants from frost.

Today, the keeper of the official rain gauge is Kim Busby-Porter, water quality management specialist with Cal Poly. Kim showed me many types of rain gauges stationed around the campus.

Their 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. 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A calibrated wooden scale is put into the tube and withdrawn.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 have fallen, the excess water is caught in the outer shell of the cylinder and can be measured later.

Another type of rain gauge at Cal Poly is a Davis Vantage Pro 2 weather station located at the Crop Science Unit. This station’s readings are available on the Internet. I have one at my home in Los Osos, whose readings can be found at

This weather station utilizes a tipping-bucket type of rain gauge. It funnels rain 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. This allows remote recording and reporting of rainfall. The problem with a tipping bucket is during heavy downpours you can lose a very small amount of precipitation during each bucket tip, thereby slightly underreporting total rainfall.

The official rain gauge on campus is a weighing type. This gauge 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. The precipitation totals are recorded on a 400-foot-long strip chart paper that is punched by the recording mechanism. Once a month, Kim carefully downloads the chart paper and sends it to the NWS.

Official NWS observations are taken either manually with standard National Weather Service equipment or automatically with installations such as the Automatic Surface Observing Systems at the Paso Robles and San Luis Obispo airports. There are several thousand official NWS sites throughout United States.

Their observations are officially archived and certified by the National Climate Data Center (NCDC). NCDC was designated by the Federal Records Act of 1950 as the official United States archive for climate data records.

Today’s forecast

A tightly wound 1,002- millibar low-pressure system and associated cold front will bring moderate rain and fresh to strong (19- to 31-mph) southeasterly winds this morning. Because of the instability in the atmosphere, there is a chance for a thundershower as well. Rain will turn to scattered showers this afternoon.

Rainfall amounts are expected to range between 0.33 and 1.00 inches, with greater amounts in the coastal mountains. Snow levels will range from about 3,000 feet north to 4,500 feet south.

Gusty northeasterly (offshore) winds will produce clearing tonight into Monday with cool temperatures and patchy dense ground fog by Monday morning.

Generally dry weather will occur Monday into Wednesday morning.

Another potentially wet weather system will cross from north to south Wednesday night into Thanksgiving Day. This system will be accompanied by rain and gusty southerly winds. Preliminary indications show snow levels in the 4,000- and 7,000-foot range. At this time, the brunt of the storm will move into Northern California.

Skies will clear Friday into Nov. 27 for generally fair weather.

Today’s surf report:

Today’s 6- to 8-foot northwesterly (300-degree deep-water) swell (with an 11- to 13-second period) will remain at this level through Monday.

Combined with this northwesterly swell will be 3- to 5-foot southerly (190-degree deep-water) seas today.

This northwesterly (300-degree deep-water) swell will decrease to 3 to 4 feet (with an 8- to 10-second period) Tuesday and will remain at this level through Wednesday morning.

A 6- to 8-foot west-northwesterly (280-degree deep-water) swell (with a 15- to 17-second period) will arrive along the Pecho Coast on Wednesday night, increasing to 9 to 11 feet Thanksgiving Day. This swell will decrease to 8 to 10 feet (with an 11- to 13-second period) Friday.

Note: The high tides will be increasing to nearly 7 feet later next week.

Preliminary analysis: The longer-range charts are indicating a high-energy west-northwesterly swell arriving along the Central Coast on Nov. 28.

Seawater temperatures

Seawater temperatures will range between 54 and 56 degrees through Monday, increasing to 55 and 57 degrees Tuesday through Thursday.

Call 911 to report downed power lines

If you see a downed power line, assume it is energized and keep yourself and others away. Call 911 immediately to report the location of the downed line, then call 800-743-5002, PG&E’s 24-hour emergency service line. During and after a storm, keep away from flooded areas and downed trees, as these areas could be hiding an energized power line.

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 nearly 25 years. To subscribe to his daily weather forecast or ask him a question, email