Why it rained when your weather app said there was only a 20% chance of precipitation

Types of Clouds During a Severe Thunderstorm

Different clouds that may occur during a severe thunderstorm

You’re hosting an afternoon cookout or playing a round of golf and need to know if rain will dampen your plans. The FOX Weather app shows there’s only a 20% chance of precipitation, so you decide to proceed with your event because those odds seem low enough to avoid any disruptions.

The only problem is: The chance of rain is not 0%. And while your hamburgers and hot dogs are on the barbecue grill, or you’re on the 12th hole of the golf course, the sky turns gray and rain starts pouring down from the clouds.

Summertime showers and thunderstorms are difficult to predict. It happens more often than you might think: A downpour soaks one town while the next town over doesn’t even see a single raindrop.

This is why forecasters can’t commit to a 0% chance or a 100% chance of precipitation in these types of situations. It’s nearly impossible to provide a simple yes or no answer to that burning question we all have: "Will it rain at my house today?"

HOW HEAVY IS IT REALLY GOING TO RAIN?

If the chance of rain is less than 50%, there’s a greater likelihood of it staying dry than not. For example, a 20% chance of precipitation means there’s an 80% chance of dry weather. On the other hand, an 80% chance of precipitation means there’s only a 20% chance of dry weather.

An isolated thunderstorm is seen dumping rainfall on one location while surrounding areas are dry.

(Arizona Fire & Medical Authority)

The official terminology used by the National Weather Service for the chance of rain is the "probability of precipitation," or "PoP" for short. The NWS said PoP simply describes the probability that any point in a given forecast area will receive at least 0.01 inches of precipitation during the time period in question.

So if the FOX Weather app is predicting a 20% chance of rain, the key takeaway is that there’s a 20% chance rain will fall at any point in the area the forecast is covering, and there’s an 80% chance it will remain dry in that forecast area.

There is forecaster confidence incorporated into PoP, so if the chance of rain is greater than 50%, the meteorologist who prepared that forecast is more confident than not that it will rain, and vice versa.

HOW MUCH IS LITERALLY A ‘BOATLOAD OF RAIN’?

A common misconception about PoP forecasts is that a 100% chance of rain means it will rain heavily all day long. This is not always true because PoP doesn’t portray anything about the intensity or the duration of the precipitation. A quick five-minute light shower that drops 0.02 inches of rain would still verify a 100% PoP forecast since: No. 1, it rained; and No. 2, there was at least 0.01 inches of precipitation.

PoP forecasts are most difficult in the summer when there is a lot of moisture and instability available to spawn showers and thunderstorms, especially the afternoon hit-or-miss variety on those hot and humid days, which aren’t always triggered by your typical cold or warm front.

This is when your neighbor a few blocks away might get soaked by a drenching thunderstorm, but your car windshield doesn’t even have a single drop of rain on it.

Note the widely scattered thunderstorms across parts of Texas in this Doppler radar loop.

(FOX Weather)

So instead of relying strictly on PoP, forecasters might use terms such as isolated or scattered to describe the chance of rain in the summer.

WHY HEAT FUELS SEVERE WEATHER

According to the NWS glossary, isolated is a precipitation descriptor for a 10% chance of measurable precipitation (at least 0.01 inches). Isolated can be used interchangeably with "few." Scattered is used when the NWS expects 30 to 50% of a given forecast area to be affected by convective precipitation such as thunderstorms. You might also come across a forecast of "widely scattered" showers or thunderstorms, which would correspond with 20% of a given forecast area receiving measurable rain.

"That kind of forecast is pretty typical for a summer day, especially in the southern U.S.," said FOX Weather meteorologist Aaron Barker, a resident of Houston. "The day can be going along just fine, and then a thunderstorm blows through with very little warning. You just have to be prepared to head inside at the first sign of a storm."

Now that you understand how to interpret PoP forecasts, we ask that you please don’t send us an angry tweet the next time you’re drenched on a day with only a 20% chance of rain.

You can download the FOX Weather app and track any potential showers or thunderstorms yourself using the built-in 3D Doppler radar feature, which you won’t find on any other weather app.