Defining the Difference between Isolated & Scattered Thunderstorms

With the summer months now in full swing, “pop-up” rain showers are beginning to occur with increased regularity. In many portions of the US, these storms can appear almost daily and have strong impacts on outdoor summer festivities. If you open any weather app or tune into any weather station to see the upcoming forecast, you are likely to see the terms “isolated” and “scattered.” Meteorologists often use words such as these to describe storm coverage on any given day. But what do these terms really mean? Let’s explore.

As defined by The National Weather Service (NWS) the term “isolated” is used to describe less than a 10% chance of measurable precipitation in any given location within a forecast area. A forecast area (or zone) is simply the geographic region for which a particular weather forecast is issued. When isolated storms are forecasted, only a small number develop and at a distance from other storms. Therefore, the exact location of an isolated thunderstorm is incredibly difficult to predict. Isolated storms can be intense in nature, often associated with downpours of heavy rain and in some cases even lightning. While they can certainly put a damper on outdoor activities, most isolated storms are short in duration. If you do get caught in a storm, take shelter and wait it out. Chances are the storm won’t be long before you can resume your plans as scheduled.

On the other hand, “scattered” thunderstorms are more widespread in coverage. While varied definitions of the term exist, The NWS defines scattered as “area coverage of convective weather affecting 30 percent to 50 percent of a forecast zone.” In contrast to isolated storms, scattered storms often come one after another or in a multitude of “rounds”. In other words, one specific location may see “on and off” periods of rain and sunshine over the course of an entire day. While it is still difficult to plan for such weather, outdoor plans are more likely to be affected and may even need to be rescheduled if scattered storms persist.

It is important to note that both of these terms describe the coverage of thunderstorms over a given area. However, they say nothing about the intensity of the storms present. Both scattered and isolated storms can become strong or even severe at any given time. While many severe weather days can be forecasted up to days in advance, the spontaneity of these storms make them even more dangerous. In any situation, make sure to follow the old saying “when thunder roars, stay indoors” and take shelter immediately if severe storms roll your way.

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©2019 Weather Forecaster Dennis Weaver

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