Monday evening could bring heavy rainfall to the western part of the state as trade winds begin weakening tonight, allowing southerly winds to draw up deep tropical moisture over the islands, the National Weather Service said.

The shift in winds as a high pressure area moves away from the islands and increased moisture will expand the potential for heavy showers and thunderstorms over western parts of the state from Monday night through Wednesday.

The weather service issued a flash flood watch for Oahu, Kauai, and Niihau in anticipation of the wet weather. The watch is scheduled to take effect 6 p.m. Monday and continue through late Tuesday night.

There could be more than two inches of possible rainfall in the atmosphere above the islands. Showers are expected to begin late Monday over Kauai and gradually spread eastward through Tuesday.

Trade winds could return in the second half of the week.