[PHOTO GALLERY] Hawaii island preps for Hurricane Iselle

Iselle weakened rapidly off the Kau Coast and slowed its westward motion just before it moved over the Big Island and Maui early Friday morning, bringing strong winds and heavy rains.

The rains prompted a flash flood warning for Hawaii island until 5:30 a.m. as heavy rain from Iselle continued, even as the center of the storm moved west over Maui.

The rains and winds downed power lines and trees, closing roads and sending hundreds of people to evacuation centers on the Big Island.

The Central Pacific Hurricane Center said the storm, with maximum sustained winds of 60 mph, was 45 miles south of Hilo, headed west at 7 mph toward the island’s southeast coast at 1 a.m. Friday.

Tropical storm force winds of 39 mph or more extend outward up to 175 miles from the center, forecasters said.

"A band of heavy rainfall rotating around the center of Tropical Storm Iselle has moved onshore over the southeastern slope of the Puna District with rain rates of 2 to 3 inches per hour. Heavier rainfall at rates of 2 to 3 inches per hour were offshore and moving toward the coast," forecasters said.

Other locations in the warning include Waipio Valley, Laupahoehoe, Ookala, Honokaa, Waimea, Hilo, Keaau, Pahoa, Mountain View, Volcano and Glenwood.

Iselle spent most of Thursday as a weak hurricane, with maximum sustained winds of 75 mph. Its outer bands began drenching the island starting about midday.

The winds picked up through the afternoon, sending at least 850 people into evacuation shelters earlier in the day. 

Hawaii County Civil Defense officials urged particular caution for people living along coastal areas and suggested they move to higher ground.

At about 9:10 p.m., Hawaii Electric Light Co. reported that more than 18,300 people on Hawaii island were without power. Multiple road closures due to downed trees were reported.

Iselle was expected to continue to lose strength into Friday, but even as a tropical storm it was expected to dump five to eight inches of rain — and as much as 12 inches — on the island. "These rains could cause life-threatening flash floods as well as rock and mud slides," forecasters said.

Maui, Oahu and Kauai were expecting heavy showers and gusty winds Friday, but nothing like the direct hit that Tropical Storm Iselle was delivering to the Big Island.

"So much wind, so much rain," Mayor Billy Kenoi said Thursday night at the Hawaii County Civil Defense headquarters in Hilo.

Hawaii island was still under a hurricane warning despite the late Thursday downgrade. Although maximum sustained winds were at 70 mph, hurricane-force gusts were still possible, according to the hurricane center.

All other islands remain under a tropical storm warning and the entire state is covered by a flash flood watch.

Because the storm was hit with strong northerly winds and the core is passing over Hawaii island, wind shear is expected to significantly weaken Iselle over the next 24 hours, officials said.

But the storm is still dangerous, officials emphasized.

"Gusts will be strongest over mountainous terrain, though passes and where winds blow downslope. Winds affecting the upper floors of high rise buildings will be significantly stronger than those near ground level," forecasters said.

Hawaii island received waves from Iselle ahead of the storm starting Wednesday night. 

Forecasters are predicted a storm surge of 1 to 3 feet above sea level for the east coasts of the Big Island.

"The combination of a dangerous storm surge and the tide will cause normally dry areas near the coast to be flooded by rising waters," forecasters said.

Forecasters said those under a hurricane and tropical storm warning should rapidly complete preparations to protect life and property. A warning is issued when a hazardous weather event is occurring, imminent or likely.

The flash flood watch lasts until 6 a.m. Saturday. 

Iselle is the first of two hurricanes heading toward the Hawaiian islands.

Julio strengthened into a category 3 hurricane Thursday as it approached the Central Pacific and is following a similar path as Iselle, although its track is a little more northerly. Forecasters expect Julio to weaken and be a tropical storm when it nears Hawaii Sunday.

At 11 p.m. Thursday, Julio was 970 miles east of Hilo, moving west-northwest at 16 mph. Maximum sustained winds strengthened to 120 mph, up from 105 mph at 2 p.m., and making Julio a major Category 3 hurricane.

Hurricane-force winds extended outward 40 miles from the storm’s center at 11 p.m., with tropical storm-force winds extending 125 miles.

The improvement in the weather after Iselle passes is likely to be brief as Julio is expected to pass north of the islands on Sunday.

At the White House on Thursday, President Barack Obama was briefed by his homeland security adviser on preparations for the storms that are threatening his birthplace. Spokesman Josh Earnest said administration officials will remain in close contact with the Federal Emergency Management Agency and other agencies that are that are preparing to help with response and recovery efforts as the storms near.

Gov. Neil Abercrombie signed an emergency proclamation ahead of the storms, which allows for easier access to emergency resources at state and federal levels and activates the major Disaster Fund for disaster relief.

Forecasters caution that wind and rain projections are only estimates.

"It is vital that you do not focus on the exact forecast track," forecasters said. "Even small errors in the forecast track can mean major differences in where the worst conditions will occur. Damaging effects can extend far from the center.

Hawaii has been directly hit by hurricanes only three times since 1950, though the region has had 147 tropical cyclones over that time. The last time Hawaii was hit with a tropical storm or hurricane was in 1992, when Hurricane Iniki killed six people and destroyed more than 1,400 homes in Kauai, Lau said. 

"We’ve been lucky so far. So we just need to really take this threat seriously and make sure everybody is prepared," he said.  

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The Associated Press contributed to this story.