Long time readers know my dislike for the “probability of precipitation” parameter used by the National Weather Service, and some other private agencies. Very few people understand it, and it only leads to forecast confusion. Here is an e-mail I received from a reader this afternoon:
“Regarding the “probability of precipitation” issue: In my opinion, for a television weathercast, the probability of precipitation should be the chance that any one spot within the viewing area will receive measurable rainfall within a given period of time. I guess this amounts to “precipitation coverage.” So if there is a 60% chance of rain, there is a 60% chance that my backyard will get wet. Couldn’t a percentage approach be applied in this way, and the viewing public educated as to what it means (or is there just not enough accuracy in a 24-hour forecast to put a number on it that will likely be close to the actual rain coverage the next day)? To me, “50-60% rain coverage” is more informative than “there likely will be several showers in the area.” By the way, Dave Baird is a funny guy.”
Some great points, and Baird is very funny.
I like the idea of using the idea of “precipitation coverage”, but that is far from the official definition from the National Weather Service:
“Probability of precipitation is the likelihood, expressed as a percent, of a measurable precipitation event (1/100th of an inch) at a grid point during the indicated valid period.”
As long as that is the “official” definition, it wouldn’t make sense for us or anyone else to buck the system and make another definition since confusion abounds anyway. Of course, the problem with the NWS definition is that nobody knows the grid resolution of a forecast, and nobody knows the valid period.
So… for now we will continue to stay away from the dreaded P.O.P. and use the best descriptive terms possible in our forecasts….
