What does a “30% chance of rain” tell you?

As a way to deal with inherent weather uncertainty, communicating forecasts have necessarily taken on a probabilistic approach, meaning that a measure of forecast confidence becomes intricately tied to a range of potential future outcomes. For example, asking will it rain tonight is a common question one might ponder. A quick check of a weather forecast indicates there happens to be a “30% chance of rain" between 6 PM and 6 AM. What does that really mean, though? It may be surprising the different interpretations that such forecast language conveys!

I have been asked this question before over the years, so here is what is being communicated to you (at least from a National Weather Service perspective). Probability of precipitation (PoP for short) is the likelihood expressed as a percentage from 0% to 100% that measurable precipitation (≥0.01 inches) occurs at any location in a forecast zone at any time during a designated period of time. A PoP value considers both the confidence that precipitation either develops or enters a forecast zone and the expected areal coverage for precipitation falling in that forecast zone. For example, between 6 PM and 6 AM a forecaster is 100% certain rain develops, but coverage may be restricted to only 30% of the forecast zone. The final communicated forecast PoP is the “confidence” multiplied by “expected areal coverage”, which equals 30% (i.e., 1.00 x 0.30 = 0.30 or 30%). Keep in mind that a forecaster can arrive at the same PoP, but for different reasons. Perhaps the forecaster is not as confident that rain occurs (say 30%), but if the environment does become conducive for rain development it would cover the entire forecast area (100%). The final forecasted PoP would be equivalent in both scenarios. 

Also keep in mind that a PoP value does not imply potential precipitation intensity! So, a 10% of rain for the day may still carry a localized heavy rain threat to be aware of. Such a forecasting scenario would actually be quite common in the southwestern U.S. during the summer North American Monsoon convective season when isolated yet impactful thunderstorms are often a daily occurrence. This is why as an end user (public or professional) of precipitation outlooks it is critical to know the context for the provided forecast. Any forecast caveats or uncertainties are often clarified in an accompanying weather forecast or outlook discussion. Furthermore, you should be aware that different forecasting services (government or private) may have different approaches to defining communicated precipitation probabilities and forecast zone boundaries. As a best practice, forecasts should provide these details to the intended customer.

Photo Credit: Jonny William Malloy