Hurricane Hilary Forecast: Storm Will Bring Rain to California This Weekend

Tropical Storm Hilary became a hurricane early Thursday morning as forecasters warned it would continue to rapidly strengthen through the day and could potentially bring “significant impacts” to Mexico and the Southwestern United States this weekend.

As of Thursday afternoon, the storm, the eighth named storm of the Eastern Pacific hurricane season this year, had sustained winds of 110 miles per hour, with higher gusts, according to the National Hurricane Center. Tropical disturbances that have sustained winds of 39 m.p.h. earn a name. Once winds reach 74 m.p.h., a storm becomes a hurricane, and at 111 m.p.h. it becomes a major hurricane.

Hilary formed 470 miles off the coast of Manzanillo, Mexico, on Wednesday and was moving west-northwest toward Baja California. The hurricane strengthened into a Category 2 on Thursday, and it was expected to further intensify later on Thursday and become a major hurricane of Category 3 or higher later Thursday. On Saturday and Sunday, before making landfall, it will rapidly weaken as it moves over colder waters.

Because of the storm’s angle to the coast, it is difficult to pinpoint an exact landfall location, but forecasters are fairly confident that Hilary will continue on its current trajectory, turning north on Friday and moving parallel to the coast.

Hilary will bring up to six inches of rain, with isolated higher amounts, across portions of the Baja California Peninsula through Monday morning, with the possibility of flash flooding, meteorologists said Thursday. Rain will begin Friday in portions of the Southwest and will peak Sunday into Monday.

Stefanie Sullivan, a forecaster with the National Weather Service in San Diego, said a worst-case scenario for Southern California would be if the track shifted farther west and made landfall in California, which could produce much stronger winds and larger surf. The only tropical cyclone to truly make landfall in Southern California was an unnamed storm in 1939 that made landfall in Long Beach, she said.

The better scenario for California could be worse for Arizona and Baja California. If the storm tracks farther east into the Baja California peninsula over the next couple of days, the moisture and heavy rainfall would be shifted east.

A difference of just 100 miles or so in the track of the storm could mean a large change for the expected weather, forecasters with the Los Angeles weather office said.

A tropical storm warning was issued by Mexico’s government for the southern portion of Baja California Sur from Cabo San Lazaro southward and Los Barriles southward. A tropical storm watch also stretched from north of the west coast of the Baja California peninsula to Punta Abreojos and north of the east coast of the Baja California peninsula to Loreto.

Waves generated by the storm could also form life-threatening surf and rip current conditions.

The Eastern Pacific hurricane season has been very active over the past few weeks, but most of these recent storms have tracked west toward Hawaii, including Hurricane Dora, which helped enhance extreme winds that led to the devastating wildfires on Maui.

It is “exceedingly rare” for a tropical storm to come off the ocean and make landfall in California, Ms. Sullivan said. However, storms have come close or weakened before coming ashore, still causing flooding and dangerous winds, like Kay, a post-tropical cyclone, last year. Sometimes storms even move across the state from Mexico; in 1997, Hurricane Nora made landfall in Baja California before moving inland and reaching Arizona as a tropical storm.

Hurricane season in the Eastern Pacific began on May 15, two weeks before the Atlantic season started. The 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.

In the Pacific, an El Niño reduces wind shear, a term that 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.)

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.

An average Eastern Pacific hurricane season has 15 named storms, eight hurricanes and four major hurricanes. The Central Pacific typically has four or five named storms that develop or move across the basin annually.

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, as 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 it can absorb. When the storm slows over land, it increases the amount of rain that falls over a single location, as with Hurricane Dorian in 2019, which slowed to a crawl over the northwestern Bahamas, resulting in 22.84 inches of rain at Hope Town over the storm’s duration.

These are just a few ways that climate change is most likely affecting these storms. Research shows there may be other impacts as well, including storm surge, rapid intensification and a broader reach of tropical systems.

Derrick Bryson Taylor and Jesus Jiménez contributed reporting.