Understanding the Probability of Precipitation

What is the probability of precipitation? It's in our forecasts and, if you read our technical forecast discussions, you may see us refer to it as "POP". But what do we mean, when our forecast states "rain likely" or "a chance of snow"?

A POP expresses forecaster confidence that measurable precipitation (defined as 0.01" of liquid precipitation) will fall, at a given location, over a specific period of time (typically a 6 or 12 hour time frame). So, a 40% chance, of measurable precipitation, also means there is a 60% chance that no measurable precipitation will fall, at a given location.

Why is there so much uncertainty?

1) The nature of precipitation itself. Especially during the warm season, precipitation tends not to fall as widespread, continuous areas. Showers and thunderstorms develop in spots, but not everywhere. While there are times that we can identify general areas, that might be more favorable than others, we're not able to tell exactly where it will develop and where it will not develop. During the cool season, the track of low pressure systems determines when, and if, precipitation will fall. This is expanded upon in the next two reasons.

2) Disagreement among computer models. We use a suite of computer models to forecast the weather. When they disagree, it increases uncertainty for human forecasters. One of the most difficult things, for computer models to do, is to simulate precipitation. There are many factors, in the atmosphere, that can't be easily mimicked in the models.

3) Timing. Usually, before precipitation begins to fall, it has overcome ambient dry air. So, that adds uncertainty to when precipitation will begin.

Stated in terms of confidence, the table below represents the possible probabilities of precipitation, and their associated descriptors, that you could see in National Weather Service forecasts.

Range of POP

"Coverage" Wording

"Uncertainty" Wording

0-14%

None*

None*

15-24%

Isolated

Slight Chance

25-54%

Scattered

Chance

55-74%

Numerous

Likely

75-100%

Widespread

Definite

* With a 0-14% POP, a forecaster may choose to mention drizzle, flurries, or sprinkles as long as forecaster does not believe measurable precipitation will occur.

Coverage wording describes how much of an area is covered (or forecast to be covered) by precipitation. Isolated could be interchanged with "a few". So, the scale increases from a few to widespread, meaning that isolated to scattered coverage you could remain dry at your location. But, as coverage increases to numerous or widespread, the chances of your location, and those around you, getting wet becomes very high.

Uncertainty wording deals with "if" precipitation will occur at all.



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