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.
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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.
