100 million people under flood alert in US after Polo criticizes Mexico. View forecast maps.

The remains of the old Hurricane Polo is expected to bring heavy rain and flooding to parts of the southern and midwestern United States after the powerful storm passes through Mexico, the National Hurricane Center said.

Polo was downgraded from hurricane to tropical storm before dissipating Tuesday evening over northwest Mexico. But as the storm continues to track inland, about 100 million people from Arizona to Iowa could be at risk of flash flooding from its remnants.

Heavy rain forecast

Forecasters said between 4 and 6 inches of rain could fall across several states in the South Plains and Midwest as the remnants of Polo move from Mexico into the center of the country, with up to 9 inches of rain possible in parts of Texas.

Heavy rains have already flooded some roads in southern New Mexico, trapping drivers in their cars until the waters recede.

Tuesday’s rains put McLeod Dam in Dona Ana County, southern New Mexico, on the verge of failure. The National Weather Service issued a flash flood emergency due to the dam, while county officials issued evacuation orders for the area. A county spokesperson told CBS News that pumps were being installed to pump water downstream of the dam and relieve pressure on it.

More than 60 flash floods were reported across the Southwest overnight Monday into Tuesday, with precipitation rates reaching 4 inches per hour in some areas.

Polo’s previous journey

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

Polo had maximum sustained winds of 35 mph Tuesday evening. Its remains are expected to continue on their way to the southwest of the United States, near El Paso, Texas, on Wednesday morning and move towards the northeast.

Flood alerts

Flood watches extended Wednesday for more than 1,200 miles across the central United States, with numerous warnings 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 have also been issued across the area, signaling an increased risk. Flash flood warnings were also issued for several counties in the Texas Panhandle.

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

The National Weather Service’s Prediction Center said there would be “a significant threat of flash flooding in the Four Corners region eastward into the Central Plains.”

CBS News meteorologist Rob Marciano expressed particular concern about the city of Albuquerque, New Mexico, where he said rain could be “saturating.” The desert city is expected to receive about 2 inches of rain. Marciano said the remnants of Polo combine with other weather systems to create a flood risk throughout the week.

In southern Colorado and other parts of the I-35 corridor, rain will continue through Wednesday. Parts of Texas, including Dallas, could see rain through Thursday. Some areas of Texas could receive 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, oscillating at times between Category 4 and category 5. Initially, it underwent a process called extremely rapid intensification, meaning wind speeds increased by 58 mph or more in just 24 hours.

Polo’s intensification was particularly dramatic, with wind speeds increasing from 45 mph to 150 mph between September 21 and 22. The NHC called the change “truly remarkable.”

The storm’s maximum sustained winds reached 180 mph over the weekend, placing 2009’s Hurricane Rick as the third strongest on record in the region. Its central pressure made it the second most powerful hurricane on record in the Eastern Pacific, behind 2015’s Hurricane Patricia.

Warmer waters, more humid air and less wind shear in the region have contributed to the formation of storms in the Pacific. Exceptionally strong El Niño conditions have also led to increased tropical cyclone activity in the region.

Another major storm, Hurricane Nolopassed the Big Island of Hawaii and brought heavy rain and strong winds to the state over the weekend.