Probability of Precipitation in the Short Term

Checking the weather forecast can help you predict whether or not precipitation is on the way in the next five to seven days.

The probability of precipitation forecast is one of the least understood elements of the weather forecast. The probability of precipitation is simply a statistical probability of 0.01" inch of more of precipitation at a given area in the given forecast area in the time period specified.

Using a 40% probability of rain as an example, it does not mean (1) that 40% of the area will be covered by precipitation at given time in the given forecast area or (2) that you will be seeing precipitation 40% of the time in the given forecast area for the given forecast time period.

If a forecast for a given county says that there is a 40% chance of rain this afternoon, then there is a 40% chance of rain at any point in the county from noon to 6 p.m. local time.

Reference: NWS Brochure, "Is it Going to Rain Today? Understanding the Weather Forecast" found at

Probability of Precipitation


Descriptive Terms Used

0% None
10% Slight Chance, Isolated
20%  Slight Chance
30-50% Chance, Scattered
60-70% Likely, Numerous

("Rain this afternoon")

Probability of Precipitation Beyond This Week

Beyond a 5-7 day window, meteorologists agree that a reliable forecast just isn't possible now.

Beyond five to seven days, weather or climate "outlooks" are given, rather than "forecasts." Outlooks are more generalized, and will focus on whether the average precipitation and temperature are more likely or less likely to be above the thirty-year normal (A – green), below the thirty-year normal (B – tan), or near normal (N – white).

Map of 8-14 day weather outlook valid Feb 18-Feb 24, 2010
Contents: Weather and Drought

National Weather Service Forecast

Long-term Outlook - National Weather Service Climate Prediction Center

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