Hurricane Lala roared ashore near Hawaii's Big Island on Saturday, strengthening into a Category 1 storm with maximum sustained winds of 75 mph just hours before its core was forecast to pass the island. The National Hurricane Center said hurricane-force winds were most likely across the southern half of the Big Island Saturday afternoon, with the storm's strongest bands pushing directly over land even though its eye was tracking just offshore.

But wind isn't what has emergency officials worried. Forecasters are warning that Lala's torrential rainfall is the storm's real threat, with totals of 10 to 20 inches expected across the Big Island and isolated pockets that could see as much as 25 inches. A Flood Watch is in effect for the Big Island and Maui County through late Saturday night, expanding to Oahu, Kauai and Niihau from Saturday evening through early Monday morning, according to the Hawaii Emergency Management Agency.

LIFE-THREATENING FLOODING AND MUDSLIDES ARE POSSIBLE, ESPECIALLY IN AREAS OF STEEP TERRAIN.

A storm that keeps shifting

The National Hurricane Center cautioned that Lala's movement has been "erratic," complicating forecasts for exactly which communities take the worst of it. The current track has the center of the storm passing just south of the Big Island before turning west on Sunday and picking up forward speed, keeping its core south of the rest of the main Hawaiian Islands into early next week. Still, officials stressed that even a storm passing offshore can dump devastating rainfall on windward slopes, and leeward communities face a real flood risk of their own as saturated ground gives way.

Hawaii County activated emergency shelters ahead of the storm's arrival, and utility crews pre-staged equipment anticipating downed trees and power outages. Hilo International Airport and several inter-island routes saw scattered delays and cancellations Saturday as airlines braced for the worst of the weather to pass through. Residents in flood-prone gulches and near streams were urged to move to higher ground before nightfall, when the heaviest rain bands were expected to arrive.

Lala's rapid strengthening — from tropical storm to hurricane in about a day — mirrors a pattern meteorologists have flagged with increasing frequency in recent Pacific storm seasons, as warmer ocean water gives storms more fuel to intensify quickly and with less warning time for coastal communities.

What's next

Forecasters say the worst of Lala's rain and wind should clear the Big Island by early Sunday as the storm continues west, though flood watches remain in effect for the rest of the island chain into Monday as rain bands sweep across Oahu and Kauai. The National Weather Service in Honolulu will continue issuing updated advisories through the weekend, and officials are urging residents across the state — not just on the Big Island — to have an emergency plan ready given the storm's unpredictable track.