More than 30 million people from Arizona to Iowa face a risk of flash flooding as Polo's remnants push inland. Heavy rain has already flooded roads in southern New Mexico, trapping drivers in their cars until the water receded.
In Dona Ana County, New Mexico, Tuesday's downpour pushed the McLeod Dam to the brink of failing, a sign of how fast a downgraded storm can still turn dangerous once it moves over land.
For the millions of people in Polo's path, the storm's weaker classification doesn't mean a weaker threat. Flash flooding can happen fast and without much warning, especially in areas already soaked by heavy rain. Communities stretching from the desert Southwest up into the Midwest now have to watch rainfall totals and dam conditions over the coming days, even as the storm itself gets smaller on paper.