Worst-case scenario: how Harvey went from a little-noticed storm to a behemoth

Even if Harvey had been a milder storm, spinning lazily across the Gulf of Mexico, there would have been reasons for alarm early last week.

For starters, the jet stream - the air current that meanders across the continent, pushing storms along a familiar path - flowed far north of Texas, and thus when Harvey crashed into the state there was nothing in the atmosphere to shove it somewhere else. Harvey stalled.

After making landfall, the storm took a path that positioned it almost perfectly to drag huge bands of rain out of the Gulf and onto the metropolis of Houston, which is interlaced with rivers and bayous and paved over with impervious urban surfaces.

Essentially parked near the coastal town of Victoria, Harvey has dumped trillions of gallons of water across southeast Texas.