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Lindsey: What exactly does 'percent chance of rain' mean anyways?

Lindsey: What exactly does 'percent chance of rain' mean anyways?

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Last week, a pair of TikTok videos went viral on social media, reaching millions of viewers. They both claimed that most of us incorrectly interpreted or misunderstood "percent chance of rain" on smartphone weather apps.

In these thought-provoking videos, they explain that if there is a 50 percent chance of rain, one-half of the forecast area would see precipitation and not that there is a 50 percent chance that one given location would get wet.

These videos shined a bright light on weather forecasting, so much so that Adam Montiel, on his radio show, "Up and Adam in the Morning" on Coast 104.5 FM, asked me if they were correct? Well, it depends, and here is why. 

Understandably, there is a tremendous amount of confusion when a "chance of rain" statement is issued by the National Weather Service. For instance, if there is a 50 percent chance of rain, many 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.

A few weather forecasting companies use the convention that the TikTok videos speaks about, but others do not. 

According to the National Weather Service (NWS), it actually means that, in 5 out of 10 cases, there will be a measurable amount of precipitation somewhere in the zone over 12 hours. Remember, probabilities are always given for a point in space over a standard period.

Weather forecasting has made great strides over the years with advances in computer modeling. However, it is not good enough to always pinpoint the exact location and time when the rain will fall. Due to this uncertainty, NWS's forecasts 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 12 hours for a specific spot.

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. The third is the "Mountains" — much of it in Los Padres National Forest and includes Hi and Black mountains.

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 did not pay him to put $20 on red No. 7 and spin a roulette wheel. They wanted to 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.

Over the next few years, the National Weather Service will be acquiring more powerful computers that run 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.

John Lindsey is Pacific Gas and Electric Co.’s Diablo Canyon Power Plant marine meteorologist and a media relations representative. Email him at or follow him on Twitter @PGE_John.


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