isolated thunderstorm

The meaning of the words isolated, scattered and widespread is a common source of confusion in weather forecasting. Let’s clear the air.

National Weather Service Terms

These terms are based on the work of the National Weather Service (NWS). The precipitation probability or probability of precipitation (POP), refers to how likely it is that measurable rain will fall. This is the 20 percent chance of rain style forecast you’re used to hearing. That might include snow, and “measurable” means 0.01 inches or more. The time period is typically 12 hours, and the forecast zone or area is usually county-wide.

Along with the POP, forecasters use an expression of uncertainty and equivalent areal qualifier. It is the equivalent areal qualifier that causes the confusion.

NWS forecasters use categorical terms such as intermittent and occasional over periods of 12 hours to describe precipitation events that are likely to be on and off, but have a high probability of occurrence—meaning 80 percent or higher.

What Forecasts Mean

When a local meteorologist forecasts isolated showers or storms, if they follow the NWS, they typically mean there is a 10 percent or less chance of getting wet at any given location. When the forecast calls for isolated storms, it’s smart to watch radar and the sky, but you have a great chance of staying dry.

According to the NWS, widely scattered is actually the next equivalent areal qualifier. Widely scattered means the POP rises to 20 percent, and there is a “slight chance” expression of uncertainty (up from none at all). It’s likely that you will avoid storms with this forecast, but there’s still a fair amount of risk.

When the POP rises to 30, 40, or 50 percent, forecasters say there will be scattered showers or storms and a “chance” expression of uncertainty. Whether or not you escape the storm is essentially a toss up.

Once the chance of precipitation rises to 60 or 70 percent, the expression of uncertainty is “likely,” and showers will be called numerous.

Once the POP hits 80, 90, or 100 percent, there’s no more uncertainty, obviously. In these situations meteorologists might refer to “widespread” storms, or instead might refer to periods of or occasional sun or breaks in the storm.

In other words, when the forecast calls for widespread storms and showers, chances are better than even that most of your local area will be affected. This is a bad time for a picnic, and a great time for a Plan B.

Final Thoughts

Remember, although these forecasts include lots of valuable information, there’s a lot of detail that they don’t include. For example, these terms really only refer to the coverage or area of the storm, not how hazardous or intense it is. They also can’t tell you about duration or timing, and a complete forecast needs to inform the public about all of those things.

However, we hope this makes it a little easier to understand what to expect from that forecast!