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Hurricane Rosa weakened into a tropical storm Sunday as it headed toward northwest Mexico and parts of the Southwest, prompting storm warnings for the Baja California coast and flash-flood watches for parts of four American states.

The National Hurricane Center said Rosa should hit the Baja California Peninsula and Sonora state late Monday as a tropical storm with flooding rains.

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It's then expected to move quickly northwestward as it weakens, bringing 2 to 4 inches of rain to the Mogollon Rim of Arizona and 1 to 2 inches to the rest of the desert Southwest, Central Rockies and Great Basin. Some isolated areas might be more.

Here are the 2 AM PDT Oct. 1 Key Messages for Tropical Storm #Rosa. Latest information available at https://t.co/Oy8uoeRKme pic.twitter.com/1XsFQ05Wzl — NHC E. Pacific Ops (@NHC_Pacific) October 1, 2018

Rosa had maximum sustained winds of 70 mph Sunday evening and was centered about 235 miles southwest of Punta Eugenia in Mexico. It was heading north-northeast at 12 mph.

The National Weather Service announced flash-flood watches through Wednesday for areas including southern Nevada, southeastern California, southwestern and central Utah and the western two-thirds of Arizona.

Forecasts call for heavy rainfall in the watch areas, which include Las Vegas, Phoenix and Salt Lake City, with possible flooding in slot canyons and normally dry washes and a potential for landslides and debris flows from recent wildfire burn scars.

It’s all about #Rosa early this week as moisture and rainfall are already streaming into the southwest. pic.twitter.com/oprwGIh885 — Jim Cantore (@JimCantore) October 1, 2018

Meanwhile, Tropical Storm Sergio was growing in the Pacific and could become a hurricane force late Sunday night or early Monday, though it posed no immediate threat to land.

Sergio had winds of 65 mph Sunday afternoon and was centered about 535 miles south-southwest of Manzanillo, Mexico. The storm was moving west at 13 mph.