DISCUSSION: On September 18, 2016, the National Weather Service (NWS) Louisville, Kentucky forecast office released an Area Forecast Discussion at 3:18 p.m. EDT, which forecasted a potential isolated shower/sprinkle for far eastern parts of the state. Now, you often hear weather forecasters use the term “isolated showers”. But, what exactly does the NWS consider “isolated”?
A shower or thunderstorm that is “isolated” refers to rainfall/thunderstorms that will potentially cover approximately 10%-20% of a county viewing area or region. More specifically, very few locations will possibly receive rainfall while majority of the viewing area might remain mostly dry of precipitation. If a forecast anticipates isolated showers/thunderstorms, then a meteorologist will often highlight more specific locations (i.e. small cities, neighborhoods, small towns) that may receive rain or thunderstorms.
The NWS Louisville forecasters illustrated the perfect definition of “isolated” showers by exhibiting radar-estimated rainfall amounts for two separate isolated shower systems (radar-estimated rainfall amounts can vary from actual ground rainfall estimates). The blue-green coloring on the radar imagery (above) displays the sparse regions that received rainfall at 8:37 p.m. EDT on September 18, 2016. In the left-most image, the neighboring areas located east of Green Grove received a maximum radar-estimated rainfall amount of 1.3 inches. The right-most radar imagery specifies a radar-estimated amount of 0.8 inches in a secluded location just north of Nonesuch, Kentucky. Therefore, only these two areas received “isolated” showers since the precipitation coverage occurred in a small vicinity of the state.
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~Weather Forecaster Aisha Murphy