HONOLULU — Hawaii is no stranger to natural disasters. The state has endured tsunamis. Lava flows almost constantly from its volcanoes. An earthquake shook the Big Island on Thursday morning.

And now, after more than two decades without a hurricane hitting the state, two tropical cyclones are barreling toward it. Hurricane Iselle was expected to slam into the island of Hawaii by nightfall Thursday. Hurricane Julio, with winds of over 100 m.p.h., was close behind and could reach the islands over the weekend.

The storms have interrupted the endless summer spirit that usually prevails on the islands, prompting a run on bottled water at supermarkets as residents stock up in case they lose power or water.

But the storms also threatened to disrupt the primary election here on Saturday, when two incumbent Democrats, Gov. Neil Abercrombie and Senator Brian Schatz, face close, bitterly contested primary challenges. Though voting remained scheduled for Saturday, Democrats said it was difficult to know what effect the storms might have on these races.