Puerto Rico is a mountainous island. And to some extent, its highlands helped sap some overall strength from Maria. When it emerged out of the island, Maria downgraded to a Category 2. But the mountains might have also caused the storm to wring out more water and intensify winds while it was traversing the island.
“Watching the eye go over Puerto Rico, it was very unusual, unlike anything I’ve ever seen before,” Weber says. Usually, the eye takes a straight-line course through an island. But when the eye came into contact with the mountains, it bounced around like a pinball. “That could have exacerbated the damage, could have damaged a wider area with that eye wall dancing around like that,” he says.
Weber points out that the valleys could have acted like channels for wind, causing the speeds to accelerate as the storm passed through them.
The mountains also exacerbate the rain and flooding. “When you have a circulation of a tropical cyclone, wind and moist air are lifted up over the mountains,” says Stewart. Like a washcloth through a wringer, the mountains squeeze more rain of the system — “at least on the order of 10 to 30 percent compared to moving over flat terrain,” he says.
Then the mountainous terrain creates the problem of water pooling from the heavy rainfall. “Any small streams [and] valley areas will become flooded very quickly because the water has no place to go,” Stewart says.
Some areas of Puerto Rico saw around 9 feet of storm surge, the coastal flooding that is often the most deadly component of a hurricane. But here, Puerto Rico was slightly lucky. Because of its mountainous geography, the surge probably didn’t get more than a few hundred feet inland in many locations.