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.
- There is a 20% chance it will rain at your exact location.
- 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).
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 chance of at least 0.01 inches occurring. That threshold could be met by anything from a brief sprinkle to a heavy downpour—the POP itself does not specify which.
This distinction is critical. A “20% chance of rain” does not mean “probably not much.” It only means there is a 20% chance that any measurable amount will fall. That could be a trace, or it could be a significant storm—the number alone does not reveal the volume. To estimate how much precipitation is expected, a separate forecast product must be used: the QPF (Quantitative Precipitation Forecast).
For precipitation totals, see: QPF
Forecast Area[edit]
POP percentages are always tied to a defined geographic area. Without knowing the boundaries of that area, the number cannot be applied meaningfully. For a detailed explanation of how forecast areas are defined and used, see: Forecast Area.
Time Period[edit]
POP percentages are also tied to a defined time window. Without knowing when the forecast applies, the number alone is incomplete. For more on how these time periods are standardized, 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 to the public:
- 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.
- They cannot be reverse-engineered from a published POP.
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.
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. 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 issued, interpret it based on the official definition:
- 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.