Hurricane Iselle was forecast to make landfall on Hawaii's Big Island on Thursday night (local time) before passing south of the archipelago's smaller islands on Friday.
Residents were scurrying to gather supplies as officials warned of flooding and outages.
"Some weakening is forecast during the next 48 hours, but Iselle is expected to be near hurricane strength as it makes landfall on the Big Island tonight," the Central Pacific Hurricane Centre said.
Iselle was about 410km east of Hilo at about 8am Hawaiian Standard Time on Thursday, while Julio was upgraded overnight to a Category 2 Hurricane and forecast to pass just north of the Big Island on Sunday morning, according to the National Hurricane Centre.
State officials warned of the potential for flash floods, mudslides and power outages, and Governor Neil Abercrombie signed an emergency proclamation, freeing up funds and other resources, in anticipation of the storms' arrival.
Hawaii residents were stocking up on basics as authorities in Honolulu advised them to prepare seven-day disaster supply kits. Shoppers waited in lines at supermarkets with carts full of bottled water, batteries and nonperishable food.
While Iselle appeared to be weakening with winds of 130kmh Julio was packing maximum sustained winds of nearly 160kmh and could continue to strengthen on Thursday, but it was forecast to gradually weaken by late evening and into the weekend, the National Hurricane Centre said.
As they prepared, Hawaiians also had to contend with rumbling from underground on Thursday morning when a magnitude 4.5 earthquake struck the Big Island (19km) west of Waimea, according to the US Geological Survey.
