Most hurricanes that strike the U.S. are not major storms: only about a third of them are category 3 or higher. But a hurricane's category doesn't convey all of its risks.
Categories only measure wind speed. They don't account for the area a storm covers or measure rainfall or storm surge — which are deadlier than hurricane winds, on average.
And recent history shows that a low-category storm can do as much or more water damage than a storm with stronger winds.
Hurricane Katrina made landfall as a Category 3 storm. Sandy had Category 1 winds when it flooded New York City. Much of the flooding from Harvey came while it was a tropical storm — below Category 1 on the wind scale.
Forecasters can't say for sure yet whether future hurricanes will move slowly like Florence or Harvey did. But they are confident that the storms will drop more rain overall — which may increase flooding risks on the ground.