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 http://www.utexas.edu/depts/grg/kimmel/nwsforecasts.html
Probability of Precipitation
Descriptive Terms Used
||Slight Chance, Isolated
("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).