MEXICO CITY — Tropical Storm Rosa neared Mexico’s Baja California on Monday, spreading heavy rains that were projected to extend into a drenching of the U.S. Southwest.

The U.S. National Hurricane Center said the centre of Rosa, which was a hurricane until late Sunday, was should hit Baja California and Sonora state late Monday, bringing 3 to 6 inches (7.5 to 15 centimetres) of rain.

It’s then expected to move quickly northwestward as it weakens, bringing 2 to 4 inches (5-10 centimetres) of rain to central and southern Arizona and 1 to 2 inches (2.5-5 centimetres) to the rest of the desert Southwest, Central Rockies and Great Basin. Some isolated areas might be more.

Rosa had maximum sustained winds of 50 mph (85 kph) early Monday and was centred about 105 miles (170 kilometres) west-southwest of Punta Eugenia in Mexico. It was heading northeast at 12 mph (19 kph).

The National Weather Service earlier 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.

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

Sergio had winds of 70 mph (110 kph) early Monday and was centred about 590 miles (950 kilometres) south-southwest of Manzanillo, Mexico. The storm was moving west at 14 mph (22 kph).