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The percentages in weather forecasting

If the meteorologist sees a 60 percent chance of rain over half the area, he’ll say 30 percent chance of rain.

Dear Answer Person: If a weather forecaster says there is a 10 percent chance of rain today, 20 percent tonight, and 30 percent tomorrow, what happens if it rains early today? Does it stay 20 percent chance for tonight or is that gone? If it doesn’t rain earlier today, do you add the 10 percent to the 20 percent, making it 30 percent chance of rain tonight? Or what ? — Jeffrey Michael Austin Foxmore, La Jolla

Put away the calculator and clear your mind of all extraneous thoughts. Weather predicting is not simple. Though you may be sorry you asked, ’cause you’ll know less about the likelihood of rain in your particular neighborhood after I explain this than you did before. But if you’re a faithful reader, maybe you’re used to that situation. Anyway, if the National Weather Service says there’s a 30 percent chance of rain in the San Diego area tomorrow, what they’re saying is the odds are three in ten that someplace (or places) in the area will get wet. In some large geographical areas, the probability is expressed slightly differently. If the meteorologist sees a 60 percent chance of rain over half the area, he’ll express the prediction as a 30 percent chance of rain. Doubly misleading, it seems to me. But weather predictors know the general public doesn’t understand the system anyway, so maybe it doesn’t matter.

The National Weather Service’s system of calculation is called the Precipitation Probability Program. It’s based on computer models of weather patterns, and they’ve been refining the models since the early 1970s. They now can predict weather for the next 72 hours with about 85 percent accuracy, though weather prediction is still a bit of an art as well as a science.

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Dear Answer Person: If a weather forecaster says there is a 10 percent chance of rain today, 20 percent tonight, and 30 percent tomorrow, what happens if it rains early today? Does it stay 20 percent chance for tonight or is that gone? If it doesn’t rain earlier today, do you add the 10 percent to the 20 percent, making it 30 percent chance of rain tonight? Or what ? — Jeffrey Michael Austin Foxmore, La Jolla

Put away the calculator and clear your mind of all extraneous thoughts. Weather predicting is not simple. Though you may be sorry you asked, ’cause you’ll know less about the likelihood of rain in your particular neighborhood after I explain this than you did before. But if you’re a faithful reader, maybe you’re used to that situation. Anyway, if the National Weather Service says there’s a 30 percent chance of rain in the San Diego area tomorrow, what they’re saying is the odds are three in ten that someplace (or places) in the area will get wet. In some large geographical areas, the probability is expressed slightly differently. If the meteorologist sees a 60 percent chance of rain over half the area, he’ll express the prediction as a 30 percent chance of rain. Doubly misleading, it seems to me. But weather predictors know the general public doesn’t understand the system anyway, so maybe it doesn’t matter.

The National Weather Service’s system of calculation is called the Precipitation Probability Program. It’s based on computer models of weather patterns, and they’ve been refining the models since the early 1970s. They now can predict weather for the next 72 hours with about 85 percent accuracy, though weather prediction is still a bit of an art as well as a science.

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Previous article

Make a wedding-and-wine statement from Nordstrom Rack and Schutz

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Don't forget Escondido

We're still spaying cats in San Diego
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4S Ranch Allied Gardens Alpine Baja Balboa Park Bankers Hill Barrio Logan Bay Ho Bay Park Black Mountain Ranch Blossom Valley Bonita Bonsall Borrego Springs Boulevard Campo Cardiff-by-the-Sea Carlsbad Carmel Mountain Carmel Valley Chollas View Chula Vista City College City Heights Clairemont College Area Coronado CSU San Marcos Cuyamaca College Del Cerro Del Mar Descanso Downtown San Diego Eastlake East Village El Cajon Emerald Hills Encanto Encinitas Escondido Fallbrook Fletcher Hills Golden Hill Grant Hill Grantville Grossmont College Guatay Harbor Island Hillcrest Imperial Beach Imperial Valley Jacumba Jamacha-Lomita Jamul Julian Kearny Mesa Kensington La Jolla Lakeside La Mesa Lemon Grove Leucadia Liberty Station Lincoln Acres Lincoln Park Linda Vista Little Italy Logan Heights Mesa College Midway District MiraCosta College Miramar Miramar College Mira Mesa Mission Beach Mission Hills Mission Valley Mountain View Mount Hope Mount Laguna National City Nestor Normal Heights North Park Oak Park Ocean Beach Oceanside Old Town Otay Mesa Pacific Beach Pala Palomar College Palomar Mountain Paradise Hills Pauma Valley Pine Valley Point Loma Point Loma Nazarene Potrero Poway Rainbow Ramona Rancho Bernardo Rancho Penasquitos Rancho San Diego Rancho Santa Fe Rolando San Carlos San Marcos San Onofre Santa Ysabel Santee San Ysidro Scripps Ranch SDSU Serra Mesa Shelltown Shelter Island Sherman Heights Skyline Solana Beach Sorrento Valley Southcrest South Park Southwestern College Spring Valley Stockton Talmadge Temecula Tierrasanta Tijuana UCSD University City University Heights USD Valencia Park Valley Center Vista Warner Springs
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