100 million under flood alerts in U.S. after Polo slams Mexico. See forecast maps.

The remnants of former Hurricane Polo are expected to bring heavy rain and flooding to parts of the southern and midwestern United States after the powerful storm travels across Mexico, the National Hurricane Center said. 

Polo was downgraded from a hurricane to a tropical storm before dissipating Tuesday night over northwest Mexico. But, as the storm continued to track inland, roughly 100 million people from Arizona to Iowa could be at risk for flash flooding from its remnants. 

Forecast for heavy rain

Forecasters said between 4 and 6 inches of rain could fall on multiple states in the Southern Plains and Midwest as Polo's remnants move from Mexico upward into the middle of the country, with up to 9 inches of rain possible in parts of Texas.

Heavy rain has already flooded some roads in southern New Mexico, trapping drivers in cars until the waters receded. 

Those 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 evacuation orders for the region. A county spokesperson told CBS News that pumps were being brought in to pump water downstream of the dam and alleviate the pressure on it. 

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. 

Polo's earlier path 

Polo made its first landfall in Mexico's Baja California Sur as a hurricane. It crossed the Gulf of California and made a second landfall in Sonora, Mexico, the hurricane center said. 

Polo had maximum sustained winds of 35 mph on Tuesday night. Its remnants are expected to continue along a path into the southwestern United States, nearing El Paso, Texas, on Wednesday morning and moving in the northeasterly direction. 

Flood alerts 

Flood watches extended more than 1,200 miles across the center of the U.S. on Wednesday, with numerous alerts in effect for sections of Arizona, Colorado, Iowa, Kansas, Missouri, Nebraska, New Mexico, Oklahoma and Texas, according to the National Weather Service. 

Various flood warnings were also scattered across the area, signaling increased risk. Flash flood warnings were issued for several counties in the Texas Panhandle, too.

Several major cities could be impacted, including Austin, Texas; Chicago; Dallas; Oklahoma City; Omaha, Nebraska; and Tulsa, Oklahoma.

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 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 and then dissipating, 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.