The remnants of Hurricane Rosa are forecast to bring heavy rain and the threat of flash flooding to the Southwest over the next few days.

The now-tropical storm – which had been as strong as a Category 4 hurricane – is poised to make landfall along Mexico’s Baja California Peninsula on Monday evening, where heavy rain was already being reported.

Flood watches are in effect across portions of California, Arizona, Nevada and Utah, including the cities of Phoenix, Las Vegas, Flagstaff and Salt Lake City. More than 12 million people are at risk of flooding, the National Weather Service said.

Arizona should see the heaviest rain, the National Hurricane Center said, where up to a half-foot of rain is possible in some areas. This could bring “life-threatening flash flooding” and “dangerous debris flows and landslides,” the hurricane center said.

Flash floods are caused by heavy rain. They can turn otherwise calm dry washes, burn scars and urban areas with poor drainage deadly. Last year, a peaceful swimming hole turned into a raging waterway in Payson, Arizona, killing 10 family members.

In some desert areas, the rain that’s expected to fall from Rosa is as much rain that would typically fall in an entire year, AccuWeather said. Yuma, Arizona, the USA’s driest city with only about 3.6 inches of rainfall a year, could see nearly that much in the next couple of days.

Although flooding is a serious concern, some of the rainfall will probably be beneficial since the Southwest is experiencing drought conditions, weather.com said. According to the U.S. Drought Monitor, more than 40 percent of Arizona is experiencing at least extreme drought, the second-highest category.

As of 5 p.m. EDT, the center of Tropical Storm Rosa was 70 miles west-northwest of Punta Eugenia, Mexico, and was moving to the north-northeast at 10 mph. Rosa had winds of 40 mph. Several school systems in Mexico’s Baja Peninsula were closed Monday as the storm approached the region.

Tropical Storm Sergio was growing in the Pacific and could become a hurricane Monday, though it posed no immediate threat to land. In the Atlantic, Tropical Storm Leslie should become a hurricane, but it’s also far out to sea and away from any land areas.