Hawaii on Wednesday braced for a walloping by a rare duo of storms headed for the vacation paradise, with local residents rushing to stock up on water and flashlights.

Hurricane Iselle, predicted to weaken to a tropical storm, was expected to bring strong wind, heavy rains and possibly damaging swells to the archipelago's main islands by Thursday.

The Central Pacific Hurricane Center (CPHC) said in a bulletin early Wednesday that Iselle was "closing in" on Hawaii's Big Island, and could trigger flash floods and mud slides.

"Heavy rains associated with Iselle are expected to reach the Big Island Thursday and spread to the remainder of the state Thursday night and Friday," the bulletin warned.

"These rains could cause life-threatening flash floods and mud slides."

A tropical storm watch was in effect for Hawaii and Maui counties, the CPHC said, adding that "watches will likely be required for additional islands later today."

At 1700 GMT, Iselle was about 695 miles (1,120 kilometers) east of Hilo, Hawaii, and some 900 miles east-southeast of Honolulu.

While currently packing maximum sustained winds near 85 miles per hour (140 kilometers per hour), forecasters predicted the system would be downgraded to a tropical storm by Thursday.

But local media reported that meteorologists believe Hawaii's famously rugged mountain terrain could help dissipate the effects of Iselle.

"The large mountains could tear the system apart - potentially," weather service meteorologist Eric Lau was quoted as saying by the Honolulu Star-Advertiser.

"We've never had a direct hit before," Lau said. "We just don't know."

Swirling on Iselle's heels in the eastern Pacific is Hurricane Julio.

With maximum sustained winds of nearly 75 miles per hour, at 1500 GMT it was 1650 miles east of Hilo and advancing west-northwest.

"Slow strengthening is expected during the next 24 to 36 hours," the National Hurricane Center said.

As the two storms approached, authorities urged Hawaii residents and tourists alike to prepare, sparking long lines at stores as people snapped up basics such as water, flashlights and batteries.