Hawaii's Big Island is under a tropical storm warning as Tropical Storm Calvin is forecast to pass over or just south of the island, threatening to bring dangerous coastal surf, heavy rain and gusty winds to the area, according to the National Hurricane Center.

Calvin is expected to remain a weak tropical storm as it swiftly presses westward and passes by the state. As of late Monday night, the storm was about 570 miles east of Hilo with maximum sustained winds of 45 mph.

The heavy rainfall poses the most significant threat, prompting flood watches for the islands of Maui, Molokai, Lanai, Kahoolawe and the Big Island from Tuesday evening through Wednesday afternoon.

"Excessive rainfall may lead to flooding conditions and landslides over steep terrain, especially along east and southeast facing slopes," the National Weather Service in Honolulu said.

The storm is also expected to generate swells and rapid surf along east-facing shorelines on Tuesday and into Wednesday, the weather service said in a tweet. The agency warned the elevated surf could become life-threatening in some areas.

Between 4 to 8 inches of rainfall are forecast through Thursday, mainly along windward areas of the Big Island, and 1 to 4 inches are expected elsewhere across the state, the hurricane center said.