Isolated vs. Scattered Thunderstorms

Source: www.pilotworkshop.com
Featuring Scott Dennstaedt

Subscriber Question:

"What is the difference between isolated and scattered thunderstorms when they appear in an area forecast?" - Neil R.

Scott:

"That's one of more frequent questions I get from pilots reading the area forecast (FA) and other NWS forecast discussions.

To better describe the overall threat, forecasters add a qualifying term to the forecast. That includes scattered (SCT) or isolated (ISOL). From a flight perspective, which are worse, scattered thunderstorms or isolated thunderstorms?

If a forecaster mentions scattered thunderstorms, this implies that 30, 40, or 50 percent of the area for which the forecast is valid is expected to experience measurable precipitation (0.01 inch or more) from thunderstorms during the valid time. Isolated, on the other hand, still implies a chance of thunderstorms, but less than 30 percent of the forecast area will receive measurable precipitation from thunderstorms.

Let's look at an example from the Chicago area forecast. The forecaster at the Aviation Weather Center is predicting that the northern two-thirds of Indiana will experience scattered thunderstorms. This means that 30 to 50 percent of this area will likely experience measurable precipitation from thunderstorms.

On the other hand, in the southern third of Indiana, the forecaster is predicting isolated thunderstorms. This implies that the thunderstorms in southern Indiana will be less numerous so that less than 30 percent of the area will experience measurable precipitation from thunderstorms."

IN
NRN 2/3...SCT015 BKN030 OVC100 TOPS FL280. SCT TSRA. POSS
SEV. CB TOPS FL450. OTLK...IFR CIG SHRA TSRA BR.
SRN 1/3...BKN040 TOPS 150. ISOL TSRA. POSS SEV. CB TOPS
FL450. OTLK...MVFR CIG SHRA TSRA BR.