Posted: Jun 19, 2014 11:54 AM by Meteorologist Robert Gauthreaux
Updated: Jun 19, 2014 11:54 AM
With summertime officially beginning on Saturday (one could easily argue that our Louisiana summer has long been underway), you may begin to hear the terms "isolated" or "scattered" being used more frequently. What is the difference between the two? The answer parallels Wednesday's article. If you hear that there is a chance of isolated storms, and you stay dry all day, that doesn't necessarily mean the forecast was incorrect.
It is not difficult to understand when you think about the situation that each word describes. Isolated, comes from the French word isolé which itself stems from the Latin word "insula" meaning "island." Isolated thunderstorms tend to be solitary, acting alone without any real interaction or connection with other storms. They sort of pop-up randomly at different locations at different times and don't necessarily cover a lot of ground. In fact, the National Weather Service officially defines this as a 10 percent chance of rain, while 10 to 20 percent is commonly used. Saying that there is a chance of "a few" storms may also fit into this category, as well as "in-and-out," "spotty," or "pop-up."
Scattered implies something a little more distributed, across a larger area. Not necessarily organized, but not completely random either. I'm no linguist, but I find that the word probably comes from a northern English variant of the Norse word "schateran," similar to shatter. You can think of a sheet of glass that when it is shattered; it is in "many pieces," and leaves you with only partial obscurity. The NWS uses "scattered" to describe a 30 to 50 percent chance of rain in the forecast.
Overall, these terms are used to express aerial coverage whether it's a spritz, or a torrential downpour as discussed yesterday. Sometimes we see thunderstorms in the form of a "squall line." As a cold front pushes through, a literal line of thunderstorms brings heavy rainfall across an area. Typically the entire area sees heavy thunderstorm activity as a line, neither scattered, nor isolated. Under these circumstances, rain chances are usually higher, even if rainfall is only expected for a brief time. When we expect over a 50 percent chance of thunderstorms, we typically don't add an adjective.