Tropical Storm Polo is expected to impact the U.S. after slamming Mexico. See its path.

The effects of former Hurricane Polo, now a tropical storm, are expected to bring rain and flooding to the southwestern United States after the powerful storm travels across Mexico, the National Hurricane Center said Tuesday. 

Polo was downgraded from a hurricane to a tropical storm Tuesday afternoon local time. As the storm's path continues inland, more than 30 million people from Arizona to Iowa could be at risk for flash floods from Polo's remnants. Heavy rain has already flooded some roads in southern New Mexico, trapping drivers in cars until the waters receded. 

Heavy rains Tuesday put the McLeod Dam in Dona Ana County in southern New Mexico on the brink of failing. The National Weather Service issued a flash flood emergency due to the dam, while county officials issued voluntary evacuation orders for the region.

More than 60 flash floods were reported across the Southwest from Monday to Tuesday night, with rainfall rates in some areas as high as 4 inches per hour. 

Maps show Polo's path 

Polo made its first landfall in Mexico's Baja California Sur as a hurricane and was moving to the northeast at about 21 mph. It crossed the Gulf of California and made a second landfall in Sonora, Mexico, the hurricane center said. 

Polo had maximum sustained winds of 45 mph Tuesday night. The storm is expected to dissipate late Tuesday night, forecasters said. But its remnants are expected to continue along a path into the southwestern United States, nearing El Paso on Wednesday morning and moving on in the northeasterly direction. 

Flash flood watches 

Flash flood watches extend more than 1,200 miles and are in effect from Phoenix, Arizona, to Des Moines, Iowa. More than 14 million Americans are facing the risk of major flooding. 

The National Weather Service Prediction Center said there will be a "significant flash flood threat across the Four Corners Region eastward into the Central Plains." 

CBS News meteorologist Rob Marciano expressed a particular concern for the city of Albuquerque, New Mexico, where he said the rain may be "saturating." The desert city is forecast to receive about 2 inches of rain. Marciano said that Polo's remnants are combining with other weather systems to create flooding risk throughout the week. 

In southern Colorado and other parts of the I-35 corridor, the rain will stretch into Wednesday. Parts of Texas, including Dallas, may see rain through Thursday. Some parts of Texas may see more than 7 inches of rain.  

An active Pacific region 

Before being downgraded to a tropical storm, Polo had been a powerful hurricane, at times fluctuating between Category 4 and Category 5. Early on, it underwent a process called extreme rapid intensification, which means wind speeds increased by 58 mph or more over the course of just 24 hours. 

Polo's intensification was particularly dramatic, with wind speeds jumping from 45 mph to 150 mph between Sept. 21 and Sept. 22. The NHC called the change "truly remarkable." 

The storm's maximum sustained winds were as high as 180 mph over the weekend, tying 2009's Hurricane Rick as the third-strongest on record for the region. Its central pressure made it the second-strongest hurricane on record in the Eastern Pacific, only behind 2015's Hurricane Patricia.

Warmer waters, moist air and less wind shear in the region have helped enable storm formation in the Pacific. Unusually strong El Niño conditions have also led to increased tropical cyclone activity in the region.

Another major storm, Hurricane Nolo, moved past Hawaii's Big Island and brought heavy rain and winds to the state over the weekend.