For the second time in as many weeks, the Southwest is facing a major threat of flash flooding as more rain comes up from the tropics.

Tropical Depression 16-E is set to unleash 2-4 inches of rain across the region with up 5 inches possible in the mountains of Arizona. The National Weather Service has issued flash flood watches across parts of Arizona, New Mexico and California, including around Phoenix and Tucson and areas east of San Diego.


Utah, which experienced its deadliest flash flood in history last week, will likely be spared the brunt of the storm as will Los Angeles, which received near-record rain last week.

While the rains could provide some drought relief, that’s a secondary to the havoc they could wreak in a region with dry soil that’s prone to flash flooding. Just a little more than a year ago, Phoenix had its wettest day on record when 3 inches of rain fell. The downpours quickly flooded the interstate highways that connect the city, left thousands without power and caused Gov. Jan Brewer to declare a state of emergency not just in Phoenix but across Arizona.

Precipitation map for the Southwest U.S., September 2015
Precipitation map for the Southwest U.S., September 2015

Click to enlarge.


That rain came from a similar setup. Tropical Storm Norbert inched across the Gulf of California and sent moisture streaming up into the region. Dry surfaces then quickly funneled rains into torrents of water.

While it’s unclear if Tropical Depression 16-E will challenge any of Norbert’s records, it is likely to still pose significant risk to millions living across the region. This time of year is generally the tail end of the Southwest monsoon season, which starts in late June or July depending on the location. Afternoon thunderstorms tend to bring localized heavy rainfall, though, so Tropical Depression 16-E is a little different given its widespread influence.

El Niño could also be helping to influence the influx of moisture across the region. The Southwest tends to be wetter than normal during El Niño falls and winters.

And regardless of where you are in the U.S., heavy downpours are on the rise across all regions in the country. This trend is consistent with a warming climate, with a warmer atmosphere being able to hold more moisture.