What made the rain in Hurricane Harvey so extreme?

Compared to other tropical cyclones, the rain from Harvey has been very hard

Fifty inches of rain. Nine trillion gallons of water. The Gulf Coast of Texas, and especially the Houston metropolitan area, has been inundated by rain produced by Hurricane Harvey. And as of this writing, the rain continues along a broad swath of the Gulf Coast, with a flood threat extending all the way east through New Orleans to the Florida Panhandle.

Even for one of the wettest and most flood-prone parts of the United States, the rainfall totals and flooding are breaking records. So, what has made Harvey such a prodigious rain producer?

A ‘train’ of rainstorms

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