Heavy rain and powerful winds are expected to bring flash floods and extreme surf to the Hawaiian islands over the weekend as Hurricane Nolo bears down on areas still recovering from previous storms.

Nolo was churning slowly south of Hawaii’s Big Island on Saturday with maximum sustained winds of 105mph (165km/h). The storm’s center was about 145 miles (235km) south of Hawaii’s southern-most point and could strengthen into a potentially life-threatening and catastrophic storm through the weekend.

Forecasters warned of possible flooding and mudslides, with parts of the Big Island expected to see rainfall totals up to 20in (51cm). Heavy rainfall also was expected on Hawaii’s other islands.

“Don’t venture out if you don’t have to into the dangerous flooding environment, but also high winds. We can have trees and power lines down,” Michael Brennan, director of the National Hurricane Center, said Friday.

The humanitarian non-profit group Community Organized Relief Effort (Core) and other partners spent the week packing and shipping supplies such as camp stoves and hygiene products by air cargo from Oahu to the Big Island for a community relief hub that has been active since August’s Hurricane Lala.

Residents are still recovering from Lala, which produced heavy rain and winds strong enough to sweep about 100 homes off their foundations on the Big Island.

Donald Trump on Friday approved an emergency declaration for the state of Hawaii, authorizing the Federal Emergency Management Agency to coordinate assistance from across the federal government and provide funding support for emergency measures.

Hawaii officials told residents to put together emergency kits, and the governor issued an emergency proclamation, activating hundreds of Hawaii national guard members.

Hawaii county officials on the Big Island were urging residents in flood-prone areas to consider taking refuge at shelters being set up or other locations.

Hawaiian Electric, the state’s primary utility, said crews that had been helping restore power in Kauai after this month’s Hurricane Lowell returned to Oahu, Maui and the Big Island in preparation for the storm.

Meanwhile, Mexico is under the threat of Hurricane Polo as it heads toward the Baja California peninsula after bringing storm conditions to the south-western coast. Polo was producing maximum sustained winds of 155 miles per hour (249km/h) Saturday but is expected to gradually weaken before making landfall Monday.

One of the towns in Polo’s projected path is San Juanico, a small fishing community on Baja’s Pacific coast where access to basic needs is already fickle.

“Our people are concerned about this event,” said Gloriely Robles Meza, a manager at a small hotel there. “We haven’t faced such a worrying situation in many years.”

The town has only had full-time electricity for a couple of years, and it still experiences frequent outages. Without power, residents can’t refrigerate food or pump water. The closest gas station is so far away that fuel is stored and sold from tanks in the town and frequently runs out if there is high demand.

Robles Meza said the community feels left to brace itself for the storm without the help of authorities.

“We must prepare on our own, each of us protecting our homes and belongings,” she said, adding that she wished officials would pre-position heavy machinery for clearing roads, along with emergency generators.

For now, she said, the only plan is to convert the local elementary school into a shelter.

“While this is important, we also have other impacts to consider,” she said, like what the town will do if it is cut off from help.

San Juanico has experienced late summer storms, but Polo feels different, said Jeffrey Westman, an American who was first drawn to the town’s world-class surf break 40 years ago and owns a property there.

“During the last couple of years, the storms have felt big and strong and scary,” said Westman, 63, who is in Europe right now but concerned for the locals in San Juanico, particularly about their ability to receive food deliveries or reach larger grocery stores if road access is cut off.

Washed-out roads could also block access to the closest medical facility, about 80 minutes away in Ciudad Constitución.

Westman said he is hopeful Polo behaves the way some past storms have, losing intensity before reaching the Baja coastline.

“A lot of times they do start petering out” as they approach, he said. “I’m hoping that’s the case.”