Rainfall forecasts include probabilities like 30 or 40 percent. Who makes those numbers?

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Dear Tom,
Rainfall forecasts include probabilities like 30 or 40 percent. Who makes those numbers?

Jennifer Mellis

Dear Jennifer,
The probability of measurable precipitation is calculated by each computer model used in weather forecasting, but the probability numbers sometimes vary considerably between models. The forecaster interpreting the model data can choose one model over another, make a compromise among models or strike out on his own. Modifiers corresponding to the numerical percentages are used in the actual forecast. In the central U.S., no mention of precipitation corresponds to zero and 10 percent, slight chance refers to 20 percent, chance to 30 and 40 percent, good chance to 50 percent, likely to 60 and 70 percent, and unqualified precipitation to 80, 90 and 100 percent.

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