PHOENIX, Arizona — Monsoon thunderstorms will return to Arizona and the other Four Corner states beginning Monday afternoon, bringing a flash flood threat as well as potential dust storms to the region through the week. This could bring beneficial rain to Colorado in particular, where much of the state is dealing with extreme and exceptional drought conditions, according to the U.S. Drought Monitor. The monsoon will peak across southwestern Arizona beginning Monday afternoon with heavy rain expected across the Interstate 19 corridor from Nogales through Tucson and into Casa Grande. However, a Level 2 flood threat will remain across the border region south of Tucson. Despite the flood threat, the monsoon will bring much-needed rain to the Denver metro area, the Front Range and the Rockies, which are experiencing areas of extreme and exceptional drought following a very dry winter.