* Iselle weakens to tropical storm from hurricane
* More than 1,200 people head to shelters
* Some 5,000 without power
* Hurricane Julio set to reach Hawaii on Monday (Adds center of the storm making landfall)
By Malia Mattoch McManus
HONOLULU, Aug 8 (Reuters) - The center of Tropical Storm Iselle made landfall on Hawaii's Big Island on Friday, bringing strong winds and heavy rain, knocking down trees and causing power outages ahead of a more powerful storm gathering strength behind it.
While Iselle weakened into a tropical storm before reaching the islands, it was being closely followed by Julio, a Category 3 hurricane set to reach the area as early as Sunday morning - although the latest forecasts showed it tracking just north of the archipelago.
As Iselle bore down, more than 1,200 people flocked to evacuation shelters across the Big Island, according to County of Hawaii Civil Defense, as heavy rains and strong winds pummeled eastern areas of the island from Puna to Hilo.
Hawaii Electric Light Company had about 5,000 customers without power, a Hawaii County official said. Flash flood warnings were issued for the entire island.
Iselle was expected to bring waves of up to 25 feet (8 meters) on southeast-facing shores on the Big Island, along with heavy rain and sustained winds near 70 miles per hour (110 kph), before passing south of the state's smaller islands later in the day.
About 870 miles to the east, Hurricane Julio had gained momentum, barreling west-northwest at 16 miles per hour (26 kph), with maximum sustained winds of 105 miles per hour (169 kph).
Prediction models showed Julio weakening before it reaches the Hawaiian Islands early on Sunday morning and tracking about 150 miles (240 km) north of the archipelago.
"Julio will be passing over cooler waters compared to Iselle and is forecast to weaken to a tropical storm before nearing Hawaii waters," Dan Kottlowski of Accuweather said. "Rainfall could still be locally heavy, especially in the higher terrain as a result of the tropical flow of moisture."
'RIDE IT OUT'
In anticipation of the rare back-to-back storms, Hawaii residents scrambled to stock up on supplies as state officials warned of the potential for flash floods, mudslides and power outages in the normally calm tourist haven.
Governor Neil Abercrombie signed an emergency proclamation freeing up funds and resources and authorities advised residents to prepare seven-day disaster supply kits and cautioned them against driving except in an emergency.
Hawaii's schools were closed on Friday but authorities planned to keep airports open so planes could land in an emergency, although some airlines had canceled flights, officials said.
Power was out at the Olinda Water Treatment plant in a rural area of Maui, and officials told some 700 water customers to conserve water, County of Maui spokesman Rod Antone said.
Emergency officials also told residents in the area of the Puna Geothermal Venture plant in Pohoiki to stay indoors or evacuate to safe zones after a spill of poisonous hydrogen sulfide. It was not immediately clear how serious the spill was.
At the Sunshine True Value Hardware store in Kapaau on Thursday, shelves were picked clean of batteries, flashlights, duct tape and plywood. Sales clerk Caryl Lindamood tried to stay cheerful.
"Mother Nature sure does like to stir things up for us, doesn't she?" she said, joking about both the storms and a small 4.5 magnitude earthquake that struck the Big Island 12 miles (19 km) west of Waimea on Thursday morning.
Robert Trickey, 56, an interior decorator, said he was worried about plate-glass windows that act as walls at his house near Pahoa on the Big Island.
Kailua-Kona resident Lisa Hummel, 44, said her family was filling water containers and stocking up on batteries, candles and flashlights, and planned to shelter in their basement when the hurricane arrives.
"We'll probably make a pot of chili and ride it out," she said. (Additional reporting by Ken Wills and Karin Stanton on the Kohala coast of the Big Island and Dan Whitcomb in Los Angeles; Writing by Eric M. Johnson; Editing by Sandra Maler and Bill Trott)