by Ed Piotrowski
Wed, June 26th 2019 at 2:27 PM
Updated Wed, June 26th 2019 at 2:35 PM
Karen Day looks out from an umbrella in downtown Wilmington, N.C., Thursday, Sept. 13, 2018. Day lives along Front Sreet and had just lost power. Florence's outer bands of wind and rain began lashing North Carolina on Thursday. (Matt Born/The Star-News via AP)
WPDE — It's one of the most misunderstood and confusing concepts in weather forecasting. Here's how the National Weather Service defines the probability of precipitation (PoP):
PoP = C x A where "C" = the confidence that precipitation will occur somewhere in the forecast area, and where "A" = the percent of the area that will receive measurable precipitation, if it occurs at all. So... in the case of the forecast above, if the forecaster knows precipitation is sure to occur (confidence is 100%), he/she is expressing how much of the area will receive measurable rain. (PoP = "C" x "A" or "1" times ".4" which equals .4 or 40%). But, most of the time, the forecaster is expressing a combination of degree of confidence and areal coverage. If the forecaster is only 50% sure that precipitation will occur, and expects that, if it does occur, it will produce measurable rain over about 80 percent of the area, the PoP (chance of rain) is 40%. ( PoP = .5 x .8 which equals .4 or 40%. )
As clear as muddy water, right?
My job is to try to make it easy for you to understand what it means when you see a chance of rain in our forecast. In the summertime, most of our rain is convective. In other words, the heating of the day causes air to rise and eventually develop into showers and storms. When I forecast a certain percentage of rain, I am simply expressing how much of the area I think will likely be covered with showers and/or storms during a given time period. This time of the year, showers and storms are most likely in the afternoon and evening. Here's a break down of the numbers we use and what they mean.
20% - ISOLATED - 1 or 2 storms in the entire area meaning almost everyone stays dry.
30% - WIDELY SCATTERED - About a 1/3 of the area experiences a shower or storm. Still, most people stay dry.
40%-50% - SCATTERED - Roughly half of the area will encounter a shower or storm.
60%-70% - NUMEROUS - Much of the area is covered so it's likely you will get wet.
80%-100% - WIDESPREAD - The entire area is covered with showers and storms so everyone gets rain! Outside of tropical systems, this rarely occurs in the summer.
By providing context to these numbers and producing a basic graphic of what the radar may look like with given percentages, hopefully, you understand what they mean when you see them in our forecasts. Keep in mind that in most apps on your phone, the chance of rain is a straight percentage from a particular model with no human interpretation. These are often wrong and too high.
What about the rest of the year? The fall, winter, and spring seasons are more complicated because of fronts and storms that produce rain that is not convective and at various times of the day. I'll address how we forecast rain chances for these seasons as we head toward fall.
