A powerful hurricane that formed in the Pacific Ocean last week weakened on Sunday to a tropical storm and was expected to continue to lose strength before coming close to Hawaii, forecasters said.

Tropical Storm Calvin had maximum sustained wind speeds of 45 miles per hour on Monday evening, forecasters said, making it a tropical storm. It had dropped from a Category 3 hurricane on Friday night to a Category 2 on Saturday.

The storm is moving west at 22 m.p.h. toward Hawaii and is expected to move in that same direction over the next several days, the National Hurricane Center and the Central Pacific Hurricane Center said.

It was about 655 miles from Hilo on the northeastern coast of the Big Island on Monday evening.

A tropical storm warning was issued for Hawaii County, which meant that sustained winds of 39 to 73 m.p.h. were expected within 36 hours. The storm was expected to be near the Hawaiian islands by Tuesday, with tropical storm conditions possible in some regions starting Tuesday night.

From Tuesday evening into Thursday, as much as 10 inches of rain was possible, which could lead to flash floods and mudslides. Swells caused by the storm were expected to begin reaching the main Hawaiian Islands over the next couple of days.

Maui County officials urged residents to stay vigilant and to be prepared “should storm conditions of high winds and heavy rains arrive as predicted,” Mayor Richard Bissen of Maui County said in news release.

Though the storm is on track toward Hawaii, it is likely to be downgraded to a tropical depression by the time it reaches the state in the middle of the week.

Whether a hurricane forms in the Atlantic or the Pacific, it generally moves west, meaning Atlantic storms pose a greater threat to North America. If a storm forms in the Pacific close to land, it can bring damaging winds and rain before pushing out to sea.

Hawaii is in the central Pacific but is occasionally affected by storms that form to the east. It is unusual, however, for a named storm to make landfall in Hawaii, given that the state’s land area is small and divided among several islands.

The last hurricane to make landfall in Hawaii was Iniki, in 1992. In 2020, Hurricane Douglas avoided a direct hit on the state but nevertheless produced damaging winds.

Hurricane season in the Eastern Pacific began on May 15, two weeks before the Atlantic season starts. Both seasons run until Nov. 30.

Complicating things in the Pacific this year is the development of El Niño, the intermittent, large-scale weather pattern that can have wide-ranging effects on weather around the world.

An El Niño reduces wind shear in the Pacific, which refers to changes in wind speed and direction. That instability normally helps prevent the formation of storms, so a reduction in wind shear increases the chances for storms.

In the Atlantic, El Niño has the opposite effect, increasing wind shear and thus reducing the chances for storm formation.

There is solid consensus among scientists that hurricanes are becoming more powerful because of climate change. Although there might not be more named storms overall, the likelihood of major hurricanes is increasing.

Climate change is also affecting the amount of rain that storms can produce. In a warming world, the air can hold more moisture, which means a named storm can hold and produce more rainfall, like Hurricane Harvey did in Texas in 2017, when some areas received more than 40 inches of rain in less than 48 hours.

Researchers have also found that storms have slowed down over the past few decades.

When a storm slows down over water, it increases the amount of moisture the storm can absorb. When the storm slows over land, it increases the amount of rain that falls over a single location, like Hurricane Dorian in 2019, which slowed to a crawl over the northwestern Bahamas, resulting in a storm-total rainfall of 22.84 inches at Hope Town.

Research shows that climate change might have other impacts on storms as well, including storm surge, rapid intensification and a broader reach of tropical systems.

Johnny Diaz, Christine Hauser, Rebecca Carballo, Lauren McCarthy and Livia Albeck-Ripka contributed reporting.