The remains of Hurricane Rosa are expected to cause heavy rains and the threat of sudden floods in the southwest during the next few days.

The now tropical storm, as powerful as a category 4 hurricane, is about to face Monday night the lands of the Baja California peninsula in Mexico.

Flood watches are in effect in parts 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 being flooded, the National Weather Service said.

Arizona is expected to experience the heaviest rains, said the National Hurricane Center, where it is possible to rain up to half a foot in some areas. This could result in "flash floods threatening for life" and "dangerous debris and landslides", said the hurricane center.

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

In some desert areas, the rain that should fall from Rosa is as much rain that would usually fall in a whole year. AccuWeather said. Yuma, Arizona, considered the driest city in the United States with only about 3.6 inches of precipitation a year, could almost see as much in the next few days.

Although floods are a very serious concern, some of this precipitation will probably be beneficial as the southwest is currently experiencing drought conditions, said weather.com. According to the US Drought Monitor, just over 40% of Arizona experiences at least one extreme drought, the second largest category.

At 11 am ET, the center of tropical storm Rosa was located 90 miles southwest of Punta Eugenia, Mexico, and was traveling north-northeast at 12 mph. Rosa had winds of 45 mph. Several school systems on the Baja Peninsula, Mexico, were closed on Monday as the storm approached the area.

Meanwhile, tropical storm Sergio was developing in the Pacific and could become a hurricane on Monday, although it poses no immediate threat to the land. In the Atlantic, tropical storm Leslie is expected to become a hurricane, but it is also far from the sea and far from land.

Contributor: The Associated Press, Republic of Arizona

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