What does ‘chance of rain’ really mean? PG&E meteorologist offers an explanation

A pair of TikTok videos recently went viral on social media, reaching millions of viewers. They both claimed that most of us incorrectly interpreted or misunderstood the “percent of chance of rain” on smartphones weather apps.

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

These videos shined a bright light on weather forecasting. Adam Montiel, who hosts the radio show “Up and Adam in the Morning” on Coast 104.5 FM, asked me if the videos 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 (NWS).

For instance, if there is a 50% 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.

According to the NWS, it actually means that, in five 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 forecasts routinely include a probability of precipitation (PoP) statement. It usually refers to at least one-hundredth of an inch (0.01 inches) of rain over 12 hours for a specific spot.

“So, a 40% chance of rain at my location means there is a 40% probability for at least 0.01 inches of acculumation,” explained Eric Boldt, warning coordination meteorologist with the National Weather Service’s Los Angeles/Oxnard forecast office.

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 section, much of it in Los Padres National Forest. It 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.

He had the knowledge, vast experience and intrinsic intuition to be right nearly all the time.

Over the next few years, the NWS 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 the weather service will ever produce a computer that could match Rea.

How to tour Diablo Canyon Power Plant virtually

Due to the COVID 19 pandemic, our team has had to suspend tours of Diablo Canyon Power Plant and the thousands of acres of pristine lands surrounding this magnificent facility.

While the tours are on hold, we made a wonderful virtual tour page of the power plant that can be viewed at https://bit.ly/3uGMK6Z.

This story was originally published March 2, 2021 at 5:00 AM.