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Dear Tom,
My grandson says that when you forecast a 25 percent chance of rain, 25 percent of the area will get rain. I believe it means a 25 percent chance of rain and a 75 percent chance of no rain. Please set him straight.
Dennita, Chicago
Dear Dennita,
You are correct; your grandson is incorrect. Probability forecasts of precipitation refer to the CHANCE that measurable precipitation will occur during a specific time period. Probability forecasts do NOT mean precipitation must fall on a specified percentage of the area. Here are some relevant definitions:
- Measurable precipitation: precipitation of at least one hundredth of an inch (water content; snow, melted) during the forecast period.
- Chance of precipitation: The probability of measurable precipitation occurring from 20 percent through 50 percent.
- Likely precipitation: The probability of measurable precipitation occurring from 60 through 70 percent.
- Unqualified precipitation occurrence: The probability of measurable precipitation occurring from 80 through 100 percent.
- Slight chance: When the weather forecaster believes the probability of precipitation is less than 20 percent but that there is still a chance that it might occur. This term is usually used when the forecast indicates the possibility of an isolated thunderstorm covering only a small portion of the forecast area or the expected occurrence of very light precipitation (snow flurries, for example).
The times of occurrence of precipitation:
- Today: 6am to 6pm
- The remainder of the day: Noon to 6pm
- Tonight: 6pm to 6am of the following morning
- The remainder of the night: Midnight to 6am
These are precise definitions and they apply to all weather forecasts issued by the National Weather Service.
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