(CNN) -- Hawaii residents hunkered down as Tropical Storm Iselle whipped the island chain with winds and heavy rains.
Iselle's top winds weakened to 60 mph as it made landfall at 2:30 a.m. Friday along the Kau coast of the Big Island.
Had the storm remained a hurricane, it would have been the first to hit Hawaii in decades.
Heavy rains are the biggest threat, with general rainfall predicted to be between 4 and 8 inches, although some areas could get more than a foot. Flash flood warnings cover much of the Big Island, while flash flood watches have been issued for the entire state.
As the storm approached, supermarket shelves were swept bare, while schools and government offices closed. Sandbags were placed around homes and hotels, and ports told to close.
In Hawaii County, 630 people have gone to shelters, Mayor Billy Kenoi told CNN affiliate KHON-TV in Honolulu.
Gwendolyn Hill, who operates a bicycle tour, said she usually goes to the store at 5:30 a.m. and is the only one there.
Not this time.
"People were buying water, rice, toilet paper and Spam," she said. "I don't eat Spam, but a lot of people here do and it was going fast."
She said she thought her family was prepared for the storm. They got out the camping gear, and had food and water for a week in case the power went out.
"The power goes out fairly often here on the Big Island, so losing power is not really a big deal," she said.
Shoppers had prepared for a potentially devastating one-two tropical cyclone punch, starting with Iselle and followed by Hurricane Julio, a Category 3 storm, about 900 miles behind it.
Hurricane Julio could affect the islands over the weekend, though forecasters expect it to brush the state only with its southern outer bands as it passes to the north as a weakened tropical storm.
Julio had strengthened to a Category 3 storm with top winds of 120 mph by Friday morning, when it was centered less than 1,000 miles away.
Flash flooding a threat
At many stores, such as KTA in Waimea, bottled water was sold out, leaving the seller scrambling to get more.
"We've been on the phone from very early (Wednesday) morning, working with our vendors trying to get more water. It's been a very difficult situation because everybody is trying to get water," store manager Colin Miura told CNN affiliate KGMB-TV, also in Honolulu.
Flash flooding on already saturated islands will be a main threat, along with mudslides from mountainous terrain into populated areas.
"Preparations to protect life and property should be rushed to completion," the Central Pacific Hurricane Center said in its public advisory on Iselle on Thursday.
Ports were taking no chance, and the U.S. Coast Guard on Thursday upped warning levels.
"All oceangoing commercial vessels and oceangoing barges greater than 200 gross tons are expected to make preparations to leave the ports," it said. Ships wishing to remain in port were required to file a safe mooring plan
A relatively rare event
Direct hits are rare for the state. Since the 1950s, two hurricane eyes have hit Hawaii -- and both approached from the south, where water temperature generally is warm enough to sustain the storms' strength.
That's not to say Hawaii hasn't had close calls. The central Pacific sees an average of about five tropical cyclones a year, and some have brushed the state in recent decades.
The cyclones generally approach from the east after forming in the eastern Pacific. But close to Hawaii, dry air, cooler water and wind shear combine to weaken approaching cyclones, dissipating them before they can become a significant threat, CNN meteorologist Indra Petersons said.
Hawaii's most damaging hurricane in recent decades -- Hurricane Iniki of 1992 -- came during an El Nino year, or a year of above-average sea surface temperatures. This year hasn't met the criteria for El Nino, but it could in the weeks ahead, Petersons said.
Iniki killed at least four people and caused about $2 billion in damage when it hit the western Hawaiian island of Kauai, according to the National Oceanic and Atmospheric Administration.
School's out; elections still on
The schools on the Big Island and Maui will be closed Friday as residents assess Iselle's impact.
Some airlines are making concessions to customers.
Hawaiian Airlines moved one flight, to Los Angeles, up by five hours to beat Iselle's arrival.
For people who had been scheduled to travel to or from Hawaii's airports on Thursday and Friday, United Airlines and Hawaiian Airlines said they won't charge fees to change reservations, and they'll waive differences in fares for those changes. United canceled flights to Hilo and Kona. American Airlines and US Airways also called off flights out of Kona on Thursday but expected Friday's flight schedule to operate normally.
Island Air will do the same for passengers ticketed from Thursday though Tuesday. Delta Air Lines said it would waive fees for reservation changes for Thursday and Friday, but fare increases could apply. It said two flights had been delayed in leaving the islands.
Hawaii's primary elections will go on as scheduled Saturday, KGMB reported. Local media reported that many turned up for early voting in expectation of severe weather on Election Day. Others are concerned that voter turnout could be affected.
Hawaii Gov. Neil Abercrombie has signed an emergency proclamation, his office said. It gives the government access to the state's disaster funds.