Scattered vs. Isolated Thunderstorms: What’s the Difference?

Meteorologists look to the “probability of precipitation” to forecast scattered or isolated storms.
A major thunderstorm is about to blow over this vineyard.
A major thunderstorm is about to blow over this vineyard. / Frank Bienewald/GettyImages

Checking the weather forecast can sometimes feel like you’re reading a statistics textbook. Meteorologists convey the chance of cloud cover or rain in percentages, but what those numbers mean in the real world isn’t always clear, so you’ll often hear terms like scattered and isolated to describe thunderstorms.

Say there’s a 20 percent chance of thunderstorms this afternoon. This is called the probability of precipitation, and it means that there’s a 20 percent chance that rain will fall somewhere in the forecast area. If that prediction is accurate, you’ll likely escape thunderstorms but there will be a few towns in your region that get a good soaking. If it rains on you, you were just in the lucky spot that happened to see a thunderstorm. This principle applies to all types of precipitation, whether it’s light rain showers, heavy thunderstorms, sleet, or snow.

Some forecasts will opt to say there’s an “isolated chance” of thunderstorms, rather than throw out a percentage. The National Weather Service will call for an isolated chance of storms if there’s a 10 to 20 percent chance of one happening, meaning any storms that do form will be few and far between. A 30 to 50 percent chance of precipitation is called a chance for “scattered” storms—like when it’s storming at your work but not at your home. Above 50 percent, the odds that you’ll see rain that day are substantial, so the forecast may say “likely rain” or just predict rain without qualifying the statement.

Partly Cloudy vs. Mostly Sunny

Another portion of forecasts that can get confusing is the difference between “partly cloudy” and “mostly sunny” skies. These terms also refer to percentages; in this case, it’s the percentage of the sky that’s expected to be covered by clouds.

The National Weather Service considered 10 to 30 percent cloud coverage to be “mostly sunny.” A sky that is just under half-covered by clouds is “partly cloudy,” and one that is over half-covered with clouds is “partly sunny.” A sky that’s more than three-quarters obscured is considered “mostly cloudy.” And if it’s more than 90 percent obscured, it’s straight-up “cloudy.”

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A version of this story was published in 2016; it has been updated for 2024.