Predicting path and intensity of Tropical Storm Humberto and Invest 94L is difficult due to something called the Fujiwhara effect.

The Fujiwhara effect happens when two hurricanes passing close to each other "dance" around a common center.

Several scenarios are possible if that happens, making forecasts difficult.

Weather forecasters and storm-wary residents are keeping an eye on two tropical systems southeast of Florida and the United States: Tropical Storm Humberto and what's now Invest 94L.

While they may stay over the Atlantic and away from the U.S., appearances can be deceiving and predictions fleeting, especially to those familiar with how quickly conditions can change.

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