How Hurricane Hilary impacted the Western United States

Hurricane Hilary shocked citizens in the Western United States this August, who were forced to prepare for the first tropical storm to reach California in decades. While West Coast hurricanes are not unheard of, they are rare, and they seldom make it to California. A 1939 storm called El Cordanzo was the only tropical storm to reach the U.S. in the 20th century.

Hilary was downgraded from a hurricane to a tropical storm before it crossed the border between Mexico and California. Despite this change, its effects were still significant within the Western United States. Weather authorities warned of heavy rain resulting in mudslides and flooding that “The New York Times” described as “life-threatening” and “catastrophic.” 

Areas across the desert were completely unprepared for the rain. Death Valley National Park is currently unable to reopen after damage devastated roadways within the park. Festival-goers at Burning Man were left stranded due to the mud that trapped their vehicles in the Black Rock Desert of Nevada. 

Why are these dangerous West Coast hurricanes so rare? What conditions led to Hilary making landfall in Southern California? What effects will this weather event have? What does an event like this mean for the West and for Utah? Aspen Manning, an assistant professor of geosciences here at SUU, agreed to help in answering some of the questions students may have about this unique weather event.

Most hurricanes only affect the East Coast because winds from the tropics blow from east to west. “Hilary had a different movement pattern because conditions in the Central U.S. helped change the course of the storm,” said Manning. 

An area of high pressure called an anticyclone in the center of the U.S. helped to modify the storm’s path. “With the anticyclone to the east, we had strong winds from the south, which caused Hilary to take a sharp turn north into Northern Mexico, California and the Western U.S.,” Manning explained.

As residents throughout California and Nevada observed, heavy rainfall was the main effect of Hilary. “Some locations in Southern California got the amount of rain in a day that they normally get in a year. Here in Utah, we got less rainfall than was forecasted,” said Manning. 

“One concern with big rain events in arid and semiarid regions is runoff. Because we have mountains and rocky substrate here in the West, heavy rains can flow into streams very quickly,” Manning added. 

Although one may assume this influx of rain may help to alleviate the drought throughout the West, Manning mentioned that last year’s snowfall was more effective. “Hilary may have eased the drought to some extent, but a single rain event can’t do much to end a lengthy drought like what we have been experiencing,” Manning said. 

Another factor to consider is the switch from a La Niña climate event to an El Niño year. These terms refer to climate events that take place off the coast of Peru and have an impact on weather all over the U.S. 

“Under neutral or ‘normal’ conditions, there is an upwelling of cold water off the coast of Peru, and the trade winds blow east to west,” said Manning. El Niño conditions mean that upwelling stops, resulting in warmer waters in the Pacific Ocean. La Niña conditions mean that upwelling grows stronger, and the waters are colder than normal.

Spring 2023 welcomed El Niño conditions after nearly three years of La Niña. “As far as tropical cyclones go, El Niño events typically suppress hurricanes in the Atlantic and strengthen or increase hurricanes in the Pacific,” Manning added. 

While Utah residents hopefully have little to worry about in regards to tropical storms, El Niño conditions may bring a warmer winter than Cedar City has experienced in the last three years. However, Manning warns against long-term weather forecasts. “For what it’s worth, this time last year, we had a similar forecast, so take it with a grain of salt. It’s important to keep in mind that our forecasting skill is limited to about 14 days,” Manning said.


Story by: Lily Brunson
Satellite image courtesy of NOAA