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. 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.