(Reuters) - Tropical Storm Darby bore down on Hawaii on Saturday, prompting its governor to declare an emergency over a storm expected to make landfall with strong winds and heavy rain later in the day.

Forecasters said Darby would reach the U.S. state's Big Island on Saturday when maximum sustained winds could near 50 mph (80 kph) and as much as 15 inches (40 cm) of rain could fall, the National Weather Service said in an advisory.

The torrential rainfall could cause life-threatening flash floods as well as landslides, it said.

"I urge residents and businesses to follow emergency instructions, prepare for the storm and take steps to protect your families, employees and property," Hawaii Governor David Ige said in an emergency proclamation statement.

The storm was about 100 miles (160 kms) southeast of Hilo as of 5 a.m. local time on Saturday, the National Weather Service said. It issued a tropical storm warning for Hawaii County on the Big Island, as well as for the islands of Maui, Molokai, Lanai and Kahoolawe.

Surf swells generated by Darby are likely to hit the Hawaiian Islands over the next couple of days.

(Reporting by Brendan O'Brien and Jon Herskovitz; Editing by Tom Heneghan)