In their own way, meteorologists are ‘PoP’ artists

It didn’t rain at my house today. Did it rain at yours? I am sure we have all experienced this situation at one time or another, where we hear about the downpour that happened on the other side of town, while our lawn remains dry. So which forecast would be correct, the one that said rain or the one that didn’t? Welcome to the wonderful world of “PoP,” the meteorological abbreviation for “probability of precipitation.” It is seen by the public, as a percentage, meant to convey the chances that a specific location will see rain or not. Unfortunately, the PoP number is one of the most misunderstood numbers in weather – even among meteorologists. That is why you will never see them in any of my forecasts on TV.

When the National Weather Service issues a forecast for the Spokane/Coeur d’Alene area, the two cities are usually part of a “forecast zone” that includes most of Spokane county, a small piece of Lincoln County in Washington, and about half of Kootenai county. A PoP is issued specifically for Spokane, another one for Coeur d’Alene, and then those numbers are averaged to produce a PoP for the entire forecast zone. According to the National Weather Service office in Peachtree City, Ga., here is the formula used to produce the “PoP”:

PoP=C x A, where “C” is the confidence that precipitation will occur somewhere in the forecast area and “A” is the percent of the area which will receive measurable precipitation (at least 0.01 inches), if it occurs at all.

So if a forecaster is 100 percent sure it is going to rain somewhere in the zone forecast area (C=1) and thinks it will rain over 40 percent of the area (A=0.4), then the “PoP” would be 40 percent. But many times the meteorologist does not have 100 percent certainty. In another example, the forecaster may only be 50 percent (C=0.5) sure that measurable precipitation will occur, and if it does it is expected to cover only 80 percent (A=0.8) of the area. In that example, the “PoP” would also be 40 percent (0.8 x0 .5=0.40).

The above mentioned calculations and phrases are only standard for NWS forecasts. There is a good chance that the weather person on TV will use his or her own numbers and phrases, that do not necessarily adhere to NWS guidelines. That leaves you, the viewer, to make sense of it all.