So how often do hurricanes like Harvey come along? A quick look at the record of Atlantic coast hurricanes reveal that there have been 94 that have achieved Category 4 status since 1851. That means there’s at least one every two years. As a point of comparison, there’s a Category 5 hurricane once every three years.
The impact of each hurricane can’t just be dictated by its categorization, however. It depends where it hits, when it hits, and what its path is – and, as Hurricane Katrina revealed so grimly, it also depends on how effective the response of the authorities are. This means that weaker hurricanes can end up doing far more damage than stronger ones.
In any case, the fact that the NWS – a normally conservative scientific group when it comes to rhetoric – is describing Harvey as “unprecedented” is worth paying attention to.
Its assessment is likely based on the fact that Texas, despite having experienced at least six Category 4 hurricanes since 1900, has never undergone such rapid flooding before. This is primarily because this storm has stalled over the state; instead of spreading out the harm, it’s concentrating it in one place. This has culminated in what some are calling a one-in-1,000 year event.
It’s too early to tell what the overall outcome of the storm will be, but at this rate, it will likely be the most costly natural disaster in American history. When the Federal Emergency Management Agency (FEMA) were asked how long they’d need to be there to help the state recover, its administrator told reporters that they’re “going to be there for years.”
IFLScience has reached out to the NWS for comment.