Understanding Rain Chances

The probability of precipitation is a statistical probability of 0.01″ or more of precipitation at a given area in the given forecast area in a specific time period. If there is a 30% chance of rain for example. This does NOT mean that 30% of the area will be covered by precipitation in the forecasted area, or that you will be seeing precipitation 40% of the time in the given area for the forecast period.

Here are a few examples that explain where the percentages come from.

If the forecaster is 80% certain that rain would develop but only expected to cover 50% of the forecasted area, then this would read a 40% chance of rain at ANY given location. Another example: if the forecaster expects a widespread area of precipitation with 100% coverage possible, but only 40% certain it would reach the forecasted area, this would as well, read a 40% chance of rain at any given location in the forecasted area.

The point probability of precipitation is used by the following equation:

Forecaster certainty that precipitation will form or move into the area multiplied by the Areal coverage of precipitation expected. Using the 100% certainty with only 40% of the forecasted area, the equation would look like this:

100 X 40 = 4000. Move decimal over two spaces to the left, and you get 40%

-Meteorologist Brittany Beggs

(National Weather Service Brochure, “Is it Going to Rain Today? Understanding the Weather Forecast”)