Probability of Precipitation
Probability of Precipitation (POP)[edit]
The Probability of Precipitation (POP) is the most visible and most misunderstood component of a forecast. Most people are familiar with seeing numbers like “20% chance of rain” or “30% chance of showers,” but what those numbers actually mean is less widely understood. Because of this, misconceptions are common.
A “20% chance of rain” does not mean:
- It will rain over 20% of the forecast area.
- It will rain for 20% of the forecast timeframe.
- It will rain during 2 of the next 10 hours.
- The forecaster is only 20% confident in their prediction.
These are all common but incorrect interpretations. To use POP correctly—and to apply it to real decision-making such as whether a canyon is safe to descend—we need to rely on the official definition and understand how the number is intended to be used.
POP Definition[edit]
According to the National Weather Service (NWS), POP (Probability of Precipitation) is defined as:
- The probability (expressed as a percentage)
- that a measurable amount of precipitation (at least 0.01 inches of rain or snow equivalent) will occur
- at any point in the forecast area
- at any time during the specified time period (typically a 12-hour window unless otherwise stated).
So, a 20% chance of rain simply means there’s a 20% chance that any amount of precipitation–light or heavy–could occur anywhere in the forecast area during that period—not a guarantee that it will or won’t rain. It could rain briefly in one small part of the area, across the whole area for the whole time, in scattered spots at different times, or not at all. Any of those outcomes would fit within that 20% probability. To put it another way, it is the difference between saying there is a 20% chance it may rain anywhere in that area vs a 100% chance it will rain somewhere in that area.
Let's explore each aspect of this in a little more detail.
Probability[edit]
The central component of POP is the probability number, expressed as a percentage, representing the likelihood that measurable precipitation will occur. At its simplest, this is the number you see when the forecast says something like “20% chance of rain.” That percentage is an example of POP.
Measurable Precipitation[edit]
POP does not predict how much precipitation will fall. It only indicates the probability that a measurable amount—at least 0.01 inches—will occur. That threshold could be met by anything from a brief sprinkle to a heavy downpour; the POP itself does not describe intensity, duration, or accumulation.
This distinction is critical. A “20% chance of rain” does not mean “probably just a little rain.” It means there is a 20% chance that any measurable precipitation will occur, without saying how much might fall if it does.
Some forecasts may also show expected precipitation totals, but those are based on a different metric: the Quantitative Precipitation Forecast (QPF). POP and QPF measure different things—likelihood versus volume—and are produced separately. While a few National Weather Service products include both, it is rare to see precipitation totals included in everyday forecasts such as apps, TV, or radio. In most cases, you will only see POP.
For precipitation totals, see: QPF
Forecast Area[edit]
Probability of Precipitation (POP) values are never “free-floating” numbers. They always apply to a defined geographic area—whether that’s the grid cell you clicked on a map, the zone named in a text forecast, or the city/location your app is keyed to. Without that context, the percentage has no meaning. A “30% chance” for one location does not describe conditions everywhere in the region.
Every legitimate forecast makes the area explicit in some way, and that’s how you know what the POP actually refers to. On a map, the forecast area may be shown as a highlighted square. In a text product, it may be named as a city or forecast zone. In an app, it may be tied to your current GPS location. However it’s delivered, the area is always part of the forecast—if it isn’t, the forecast is incomplete or unreliable.
For a detailed explanation of how forecast areas work—including how they’re defined, the different types of geographic regions the National Weather Service uses, and why these boundaries matter—see: Forecast Area
Time Period[edit]
Probability of Precipitation (POP) values are always tied to a specific time window. Without knowing when the forecast applies, the percentage is incomplete and can’t be used meaningfully. A “30% chance this afternoon” does not describe the same thing as “30% chance tonight.”
Every legitimate forecast states its time period explicitly, and that’s how you know what the POP actually refers to. In an app, the times may appear as an hourly breakdown. On a website, they may be displayed as highlighted blocks. In a text forecast, they may be written as terms like today, this afternoon, or overnight. These terms are not vague—they have standardized meanings that correspond to specific hours of the day.
For a detailed explanation of how forecast time periods work—including the standard definitions used by the National Weather Service and how to interpret them—see: Time Period
Putting it All Together[edit]
Now that we’ve broken POP into its parts, let’s see how it applies in practice. Imagine a forecast that reads:
- “20% chance of rain in Hanksville this afternoon.”
Interpreted according to the official definition, this means there is a 20% chance that at least 0.01 inches of precipitation will fall anywhere within the Hanksville forecast area at any time between 12:00 p.m. and 6:00 p.m.. The forecast does not specify whether that amount will be light or heavy, brief or prolonged—only that the threshold for measurable precipitation may be met.
Understanding and applying the definition in this way is essential. POP forecasts are not vague guesses but standardized statements. Using them correctly allows us to make decisions based on what the forecast truly says, rather than on assumptions or common misconceptions.
A Note on the Calculation of Probability[edit]
When talking about the Probability of Precipitation it is common for people to share this formula: (P) Probability = (A) Area Coverage × (C) Forecast Confidence.
It says, for example, if a forecaster is 50% confident that 80% of the area will receive measurable rain, the result is a 40% POP.
While legitimate, this formula is of limited value because:
- It is not a strict rule that forecasters consistently follow today.
- Modern, data-driven models often render it unnecessary.
- The values of (A) and (C) are never released to the public.
- (A) and (C) cannot be reverse-engineered from a published POP forecast.
In a discussion on March 28, 2025, David Church (Science and Operations Officer at the NWS Salt Lake City office) confirmed that the formula is legitimate but downplayed its importance and emphasized that forecasters don't rely on it as much as they used to. Instead, they focus on interpreting data from forecast models. He said that even when used, (A) and (C) are rough estimates based on professional judgment, and also not values shared publicly.
Another point to understand is that even when (A) and (C) are used by forecasters, they are used to develop at POP value--which is the number forecasters want us to use to evaluate the probability it will rain. They don't want the public to diminish or undo that POP value by trying to break it down into the values used to devlope it.
In short, the formula may be interesting as background knowledge, but it is not useful for interpreting forecasts. Focusing on it risks obscuring the true purpose of POP and people may begin to second-guess the percentage or assume it’s less meaningful because it’s based on hidden inputs of (A) and (C). That’s not how POP is meant to function in public forecasts. Instead of circulating the (A × C) formula, discussions of forecasts should focus on the official POP definition and a clear understanding of its components and how to interpret them.
The bottom line is this: whenever a POP forecast is presented, it should be interpreted based on the official definition. That means identifying the forecast time window and the geographic area it applies to, and understanding that the forecast tells us:
- There is an X% chance
- that at least 0.01 inches of precipitation will occur
- at any time during the forecast window
- at any point within the forecast area.