Weather Watch

There’s a probability that forecast will be accurate

Special to The TribuneMay 19, 2013 

Three types of clouds can be seen in this photo taken at Port San Luis. Stratus, cumulus and cirrus.


There is a 100 percent probability of a 50 percent chance that it may or may not rain. Weather forecasting has made great strides over the years with advances in computer modeling. However, it is not great enough to always be able to pinpoint the exact location and time when the rain will fall. Due to this uncertainty, forecasts issued by the National Weather Service routinely include a PoP (probability of precipitation) statement. It usually refers to at least one-hundredth of an inch (0.01 inches) of rain over a 12-hour period for a specific area.

San Luis Obispo County’s PoP zones are divided into three general zones. First is the “Coast” section, which includes: Cambria, Cayucos, Morro Bay, Los Osos, Avila Beach, Pismo Beach, Arroyo Grande and Nipomo. Second is the “Interior” section, which includes Paso Robles, Templeton, Lake Nacimiento, Atascadero and California Valley. Third is the “Mountains” — much of it in Los Padres National Forest and includes Hi and Black mountains.

Understandably, there is agreat amount of confusion when a “chance of rain” statement is issued by the National Weather Service. For instance, if there’s a 50 percent chance of rain, a lot of folks believe it’s defined as precipitation that will fall over half of the forecast area. Some think that it will rain half of the time over half of the area during a particular period.

It actually means that, in 5 out of 10 cases, there will be a measurable amount of precipitation somewhere in the zone over a 12-hour period. Remember, probabilities are always given for a point in space over a standard period.

The best weather prognosticator I ever knew didn’t use “chance of rain.” Meteorologist Rea Strange of Santa Barbara — whose name was pronounced Ray — forecasted weather along the California coast for more than 50 years. He was also my mentor throughout my weather forecasting career.

He told me his customers didn’t pay him to put $20 on red No. 7 and spin a roulette wheel. They wanted to actually know whether it was going to rain or not. Unlike the dice thrown from a gambler’s hands, he had the knowledge, vast experience and that intrinsic intuition to be right nearly all the time.

It was recently announced that next year the National Weather Service will be acquiring drastically more powerful computers that run the numerical weather models. Hopefully, this will remove more of the uncertainty in weather forecasting somewhere down the road. Needless to say, I don’t think they’ll ever produce a computer that could match Rea.

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John Lindsey’s column is special to The Tribune. He is a media relations representative for PG&E and a longtime local meteorologist. If you have a question, send him an email at

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