Tropical Storm Humberto is expected to strengthen into a Category 4 hurricane as it moves between the U.S. and Bermuda.

Tropical Storm Humberto is spinning in the central Atlantic, but the bigger focus for the United States is what’s brewing behind it and how the two could interact.

The few storms that have formed this Atlantic hurricane season have largely taken a similar path — curving away from the United States. Hurricane Gabrielle is the latest example. The storm began in ...