Why Hurricane Hilary is so strange — and how it could impact California

The storm should hit Mexico Sunday and could cause historic rainfall in the Southwest.

In a summer of extreme weather events, Hurricane Hilary is yet another atypical occurrence — a tropical storm headed for the West Coast of the United States.

What it will likely mean for Southern California and the southwest is potential heavy flooding — and even flash flooding — with up to seven inches of rain forecast in some areas and tropical storm force winds up to 73 mph as it moves over land.

Though California has had hurricanes before, it’s extremely rare because cold water flows from Alaska typically make the Pacific coast an unsuitable environment for them, which rely on water surface temperatures higher than 26 Celsius to form and grow powerful. Hilary, a Category 4 storm as of Saturday afternoon, is expected to make landfall on Sunday morning, likely in northern Mexico around Baja California, according to the National Hurricane Center.

While people on the southeastern coast — particularly in Florida up through the Carolinas, and along the coast of the Gulf of Mexico — are likely well-versed in hurricane preparation, and live in states with storm-hardened infrastructure, that’s not so for Southern California and parts of the southwest where the hurricane is expected to hit. Though Hilary is expected to weaken as it heads northward and makes landfall, it could still bring several inches of rain — as many as ten inches are forecast in some parts — and heavy winds.

California Gov. Gavin Newsom issued a press release Friday outlining the government’s preparation for the storm and urging Southern Californians to prepare themselves for “the wettest tropical cyclone in state history and the first-ever Tropical Storm Watch issued for California.” According to Axios, 43 million people in California and Mexico are under tropical storm warnings, and 27 million under flash flood warnings, in an area stretching all the way to Idaho.

California has already had an extremely wet year, though that’s unrelated to Hilary’s development and unusual path. Thirty-one atmospheric rivers hit California last winter and this spring, many of them quite strong. California’s atmospheric rivers provide much of the state’s precipitation, both as rain and as snowfall, which accumulates in high elevations as snowpack and melts in the warmer months.

But this year’s atmospheric rivers were severe in both intensity and duration, erasing some drought restrictions, but also causing devastating flooding and record-breaking snowfall.

They were also concentrated in California’s Central Coast and in Southern California, where Hilary is expected to hit, too. “That’s where we’re really seeing a lot of our larger anomalies in terms of overall precipitation,” Chad Hecht, a research and operations meteorologist at the Center for Western Weather and Water Extremes at UC San Diego’s Scripps Institution of Oceanography, told the LA Times in April. “This year, the Central Coast saw four strong atmospheric rivers, where it typically averages less than two.”

Though Hilary won’t have the power of many hurricanes on the East and Gulf Coasts like Ida in 2021 and Ian in 2022, it still has destructive potential. The amount of rainfall in typically arid locations will likely cause flash flooding, mudslides, and landslides — which could be especially dangerous with the addition of debris from recent forest fire seasons, Axios reports.

Hilary is unusual, but it doesn’t necessarily portend things to come

Although Hurricane Hilary’s path is strange, it’s not unprecedented, as Paul Miller, an assistant professor in Louisiana State University’s Department of Oceanography and Coastal Sciences told Vox in an interview.

“It’s certainly unusual,” Miller said. “There is historical precedent for it, though — it’s the kind of thing that has happened before. We can cite examples from our lifetime, but it is certainly unusual.”

Hurricane Nora impacted the American southwest from the Pacific Ocean in 1997, and Lester in 1992 before that. Still, Miller points out, “We’re looking at 25 years since the last time we can point to a similar case.”

Though many unusual or extreme climate events are linked to climate change, there isn’t much to directly connect Hilary’s growth and path to the phenomenon. Higher water surface temperatures could account for some of the storm’s strength, but “There’s nothing about what’s happening right now to make me think that climate change is so dramatic that Southern California is now in the crosshairs of tropical systems,” Miller said.

The Eastern Pacific has its own hurricane season.It’s longer than the Atlantic hurricane season, starting on March 15 and ending on November 30. As the New York Times reported Saturday, it’s been an active season in the Eastern Pacific, though none of the storms have come as far west as Hilary has. “We typically don’t talk about storms in the Eastern Pacific as much because they tend to be less impactful to land,” though that’s not always the case, Miller said. Storms like Hurricane Dora, for example, have tracked much further west, adding to the heavy winds which helped the deadly fire in Maui spread.

Hilary’s path has to do with two specific weather factors, Miller told Vox, including a heat dome over the central US. Though the warm temperatures and the heat dome Miller described didn’t cause Hilary to form — that happened thousands of miles away in the Eastern Pacific — it is helping determine Hilary’s path.

High-pressure air moves clockwise, Miller explained, acting like a conveyor belt bringing Hilary up the West Coast and in toward California and Nevada. A trough in a jet stream over California is also “grabbing Hilary and pulling it northward,” he said, as well as helping cause the storm’s precipitation.

Strong winds are another concern for those in and around the hurricane’s path. “Generally speaking, the strongest winds will cling to the right side of the hurricane,” Miller said. “So if this same storm was hitting [North Carolina’s] Outer Banks or something, the strongest winds would be over ocean,” instead of a populated area like San Diego.

As of Saturday afternoon, Hilary had yet to make landfall and was weakening as it headed northward — much as experts expected. But as with any major weather event, there are still unknowns, Miller said. “The biggest question from this point forward is going to be where it makes landfall.”