By Malia Mattoch McManus

HONOLULU (Reuters) - The center of Tropical Storm Iselle made landfall on Hawaii's Big Island on Friday with strong winds and heavy rain, knocking down trees and causing power outages ahead of a more powerful storm gathering steam behind it.

While Iselle has weakened to a tropical storm, according to the U.S. Central Pacific Hurricane Center, it is being closely followed by Julio, a Category 3 hurricane set to reach Hawaii on Monday.

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 areas of East Hawaii from the Puna area to the town of Hilo.

Hawaii Electric Light Company had about 5,000 customers without power, mostly in East Hawaii, a Hawaii County official said.

Iselle weakened from a hurricane to a tropical storm while its eye was still about 50 miles (80 km) south of Hilo, on the Big island, packing maximum sustained winds near 70 miles per hour (110 kph), with higher gusts.

The storm was expected to bring waves of up to 25 feet (8 meters) on southeast-facing shores on the Big Island before passing south of the state's smaller islands later on day.

Farther east, Hurricane Julio had gained momentum and was expected to pass just north of Hawaii by late Monday.

That hurricane was upgraded late on Thursday to a Category 3 storm, with maximum sustained winds increasing to near 120 mph (195 kph), the National Hurricane Center said. It was moving west-northwest at 16 mph (26 kph) and was expected to weaken through Saturday.

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 will be closed on Friday. Authorities planned to keep airports open so planes could land in an emergency although some airlines had canceled flights, officials said. Several shopping malls on Oahu will also be closed.

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.

Even before the center of the storm struck the Big Island, a downpour soaked residents on Thursday. At the Sunshine True Value Hardware store in Kapaau, 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 on Thursday, 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 Gunna Dickson in New York; Writing by Eric M. Johnson; Editing by Sandra Maler)