Hawaii residents on Sunday were bracing for the arrival of Hurricane Douglas, which was threatening to bring sustained winds near 85 miles per hour and up to 15 inches of rain in some areas, forecasters said.

If Douglas, which was downgraded to a Category 1, reaches the islands, it would be only the third hurricane in modern times to do so.

Hurricane warnings were in effect for the counties that include the islands of Kahoolawe, Kauai, Lanai, Maui, Molokai, Niihau and Oahu, the National Hurricane Center said in an advisory on Sunday.

Hurricane conditions were expected in portions of Maui County and Oahu by Sunday afternoon and Kaui on Sunday night. Conditions, including heavy rainfall, could continue into Monday, the center said. A tropical storm warning was in effect for the Island of Hawaii earlier Sunday.

“Douglas will pass dangerously close to, or over, the islands today, bringing a triple threat of hazards, including, but not limited to, damaging winds, flooding rainfall and dangerously high surf,” the center said on Sunday.

It is rare for hurricanes to hit the state of Hawaii because of the islands’ size compared with the expanse of the Pacific Ocean. Other conditions in Hawaii, including lower water temperatures and wind shear, also weaken hurricanes.

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Only two storms since modern record-keeping starting in 1900 are known to have struck the islands: In 1992, Hurricane Iniki hit Kauai as a Category 4 storm, killing six people and causing about $3 billion in damage. In 1959, Hurricane Dot caused about $5.5 million in damage.

While Douglas has weakened, the storm was expected to remain a hurricane as it moved through the islands on Sunday.

The combination of high water levels, storm surge and large breaking waves could raise water levels by as much as three feet above normal tides near the center of the storm, the center warned.

There can still be hurricane-force winds in an area with a tropical storm warning because of the islands’ steep terrain, including mountains, the center said.

From the counties of Maui to Kauai westward, there may be five to 10 inches of rain, with up to 15 inches in elevated areas, possibly contributing to flash flooding and landslides.

Earlier in the day, Maui County asked residents to shelter in place or move to an emergency shelter immediately if they lacked a safe place to weather the storm. Much of the county has already been through the brunt of the storm with minimal damage, said Mayor Michael Victorino of Maui County.

Tourism has been severely affected by both the storm and the coronavirus pandemic, Mr. Victorino said. On any given day, the county received about 8,000 visitors. Over the last few days, there have been less than 50 visitors per day, which has made the job of emergency management much easier.

“That was the blessing,” Mr. Victorino said. “We didn’t have to work so hard and concern ourselves with the visitors.”

Upon entering the state, all travelers have a mandatory 14-day self-quarantine. Maui’s visitors are sheltering in place at hotels, Mr. Victorino said. If it becomes dangerous, local shelters can separate those who are self-quarantining or who have the coronavirus.

Thirteen shelters opened on Sunday in Honolulu, including the Hawaii Convention Center, which can hold 1,600 people, with social distancing, Honolulu’s mayor, Kirk Caldwell, said at a news conference on Saturday.

Officials have warned that space at the shelters may be constricted because of social distancing policies. Gov. David Ige of Hawaii said the authorities would monitor capacity at the shelters and open more if necessary.

In Maui County, which has about 167,000 residents, fewer than 30 people have gone to a shelter, Mr. Victorino said.

In a statement on Sunday morning, the hurricane center emphasized that residents should not focus on the exact forecast track or intensity of Douglas.

“Due to Douglas’s angle of approach to the islands, any wobble in the track could lead to significant differences in where the worst weather occurs,” the center said. “Even if the center remains offshore, severe impacts could still be realized over the islands, as they extend well away from the center.”

Mr. Ige on Thursday issued a pre-landfall emergency proclamation that authorized state funds for quick disaster relief.

“We don’t just focus on the wind,” Mr. Feltgen said on Friday about the storm. “You have to look at the water impacts on this thing as well. Very heavy rainfall.”

Marie Fazio contributed reporting.