The latest in an unrelenting run of severe rainstorms is pushing its way into the Southwest this weekend, following deadly floods that have transformed some neighborhoods into hazmat zones requiring costly cleanups.

Traditionally, the heaviest rains associated with an El Niño phenomenon arrive in the U.S. during the winter months. But warming oceans and an ongoing parade of tropical storms and hurricanes enhanced by El Niño have sent moisture pouring into the Southwest since September. Monthly rainfall records have been threatened in parts of Arizona and New Mexico.

Starting late Saturday, a severe “atmospheric river” storm packing tropical moisture may drop more than 4 inches (10.2 centimeters) in some areas. “Our main concern is just how saturated soils are already,” said Joshua Schank, lead forecaster for the National Weather Service office serving Flagstaff, Ariz.…