Hilary Becomes First Tropical Storm to Impact Southern California Since 1939

SAN DIEGO, CA – While the Tropical Atlantic is beginning to stir, the Eastern Pacific has had a very active season to date.  Two weeks ago, Hurricane Dora, a powerful Category Four Hurricane with 140 mph winds combined with high pressure to the north of the Hawaiian Islands to help funnel winds that fueled deadly and devastating wildfires to Maui.  Now, Tropical Storm Hilary is producing tropical trouble for California.

Once a powerful Category Four Hurricane riding northward along the West Coast of Mexico and Baja California, Hilary has since weakened to a strong tropical storm with 60 mile per hour winds.  Barometric pressure has risen to 990 millibars or 29.23 inches of Hg (Mercury), but the big concern with Hilary will be the rain.  According to NCEP’s Weather Prediction Center, there is a high risk (at least 70 percent) of flash flooding in much of the Desert Southwest including Palm Springs, Death Valley, and even Las Vegas, Nevada.

Raging Rivers in Baja California

Hilary has already created devastating flash floods and mudslides in the Baja California region of Mexico.  As it trekked northward, the storm produced torrential rains that turned roads into raging rivers in places like Santa Rosalia.  One person died from the devastating floods in the mining town of 14,000 according to the New York Times.  According to the National Hurricane Center, Hilary is forecast to produce up to 10 inches of rain in portions of Northern Baja California.

Rainfall amounts range anywhere from 3 to 6 inches with localized amounts as high as 10 inches in portions of Southern California and Nevada.  Rain could even reach as far north as Oregon and Idaho, where rainfall totals could fall anywhere between one and three inches with localized amounts as high as five.  Keep in mind that Hilary still contains an abundance of tropical moisture that is running into mountainous terrain, which will result in orographic lifting and torrential rain.

Devastating Flash Floods for Southwestern U.S.

In order for the rainfall from a tropical storm or hurricane to become a problem, the storm system itself usually has to be a slow mover.  However, in the case of Hilary, the storm is moving in upwards of 25 miles per hour.  So, the torrential rains will not last long, but due to the fact that the rain is falling in an area that gets very little in the way of rain during an average year, they will still have the potential to produce devastating flash floods.

The last time a tropical storm directly impacted Southern California was in 1939 when a storm made landfall in Long Beach, California.  The storm was so devastating and caught many by surprise.  As a result, the National Weather Service created an office in Southern California.  Nearly 100 people were killed while coastal homes were destroyed, power was disrupted, and crops were destroyed according to the New York Times and “A History of Significant Weather Events in Southern California,” by Weather Type.

Long Beach Tropical Storm of 1939

The 1939 storm was one of four tropical cyclones that impacted Southern California in September of that year.  One of the key ingredients in these tropical cyclone impacts was a strong El Niño.  A moderately strong El Niño has been developing during the course of the summer of 2023 in the Eastern Pacific.  The last time a tropical storm came close to impacting Southern California was during another El Niño year, 1997.  Prior to 1939, California had not been directly impacted by a tropical storm or hurricane since 1858.

There were actually two instances where a tropical cyclone threatened the Southwestern United States in 1997.  The first time was in early September when powerful Hurricane Linda threatened Southern California.  At one point, Linda was the most powerful storm in recorded history in the Eastern Pacific.  The Category Five Hurricane had winds estimated to be as high as 180 mph with gusts upwards of almost 220 miles per hour.  The storm would eventually weaken in the cooler waters off Southern California and turn away.

Hurricanes Linda and Nora of 1997

Several weeks later, Hurricane Nora threatened the Southwestern United States in late September 1997.  Nora was not as powerful a hurricane as Linda was, but still attained major hurricane strength with 130 mph winds and a minimum central pressure of 950 millibars, or 28.05 inches of Hg (Mercury).  Like Linda, the storm would weaken, but it eventually brought remnant rains into Arizona, California, Nevada, and Utah.  Almost a foot of rainfall was recorded in the Harquahala Mountains in Arizona.

Most of the time, storms in the Eastern Pacific will form off the Southwestern Mexican coast or Central America, and head westward without threatening land.  Sometimes though, these tropical cyclones will hug the Western Mexico coast and go into its Baja California region.  Fortunately, due to the cooler waters of the California Current, an offshoot of the Humboldt Current, tropical storms and hurricanes usually die out before reaching Southern California and the Southwestern United States.  Tropical storms and hurricanes need sea surface temperatures of at least 80 degrees in order to grow and thrive.

El Niño provides tropical cyclones in the Eastern Pacific an opportunity to reach Southern California and the Southwestern United States because it causes sea surface temperatures to be above normal.  In turn, the above-normal ocean water fuels tropical storms and hurricanes to reach at least major hurricane status.  In some instances, they can become as strong as a Category Five storm as Linda did in 1997.  

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