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California and the dry desert states of Arizona and Nevada are used to breaking temperature records.

It was only in July that Phoenix, Arizona set a record for the most consecutive days of high temperatures of 110 degrees Fahrenheit (about 43.3 degrees Celsius) or higher. Across all 31 days in July, Phoenix beat the previous record set in 1974 by 13 days.

But during August 20 and 21, as a result of Tropical Storm Hilary, it was rainfall records now being broken in these states.

Residents on the U.S. West Coast are accustomed to coping with the last remnants of Pacific tropical storms and hurricanes, especially across California and the coastal southern cities, with high surf and rip currents, gusting winds, and intense periods of rainfall.

These Pacific tropical storms originate off the western coast of southern Mexico or central American countries such as Guatemala or El Salvador. Most head westward into the Pacific, but occasionally they track up the west coast of central Mexico, less frequently into Baja California, and much rarer still – crossing into the U.S. and the western states.

The History of Hurricane Hilary

Hurricane Hilary started life as a low-pressure area off the southern coasts of Mexico, Guatemala, and El Salvador on August 14, and as it strengthened and moved up the west coast of Mexico, on August 16, the National Hurricane Center named it Tropical Storm Hilary.

Gaining strength as a Category 1 hurricane on August 17, in just 18 hours it had rapidly intensified to a Category 4 storm by August 18, with sustained winds at 145 miles per hour (230 kilometers per hour).

By August 19, the hurricane was being steered by a trough of low pressure to its north near the California coast and a ridge of high pressure over the central United States. As a result, Hilary started to move to the north-northwest towards the Baja California Peninsula in Mexico on the morning of August 19.

Cooler air and wind shear then saw Hilary weaken to Category 3 and then to Category 1 strength, making landfall at 10 a.m. local time on August 20, in San Quentin, Baja California, with winds of 65 miles per hour (105 kilometers per hour), and then continuing in a north-northwesterly direction on its journey towards the U.S.

From August 17 until Hilary left the country, Mexico had experienced torrential rains and high winds along its west coast, causing damage in several central municipalities all the way up to the Baja California Peninsula, and at least two fatalities. 

First California Tropical Storm Since 1939

With Hilary the first tropical storm to hit southern California in 84 years since the 1939 California Tropical Storm, on August 18, the National Hurricane Center issued its first-ever tropical storm watch for southern California, with the Weather Prediction Center issuing an unprecedented Level 4 excessive rainfall warning for the desert valleys in the southwest U.S. states.

These warnings were taken seriously, and California Governor Gavin Newsom declared a state of emergency, as did mayors of the major cities. Schools were closed, flights diverted, recreational and entertainment venues were shut, and extra emergency personnel were deployed.

By late afternoon local time on August 20, Tropical Storm Hilary crossed the U.S. border into southern California. Hilary and its remnants brought widespread heavy rainfall and localized torrential rainfall to much of the southwestern U.S.

Desert Rains

This heavy rainfall included parts of the Los Angeles and San Diego metro areas, with one to four inches of falling (25 to 100 millimeters) in these areas, and four to nine inches (100 to 225 millimeters) across the mountainous areas east and north of Los Angeles and in parts of the Mojave Desert.

According to the National Weather Service, certain regions in California saw their wettest August day on record. Palm Springs in California (pop. ~45,000), for example, received over three inches (76 millimeters) of rainfall in 24 hours, breaking the previous record of 2.03 inches (50 millimeters) which was set on August 1, 1930.

The rainfall totals in this area were equivalent to 50 percent of its average annual rainfall which resulted in widespread street-level flooding.

Tropical Storm Hilary - rainfall accumulation
Total observed precipitation (inches) for Tropical Storm Hilary: Data Source: NOAA

Moving further north, the Death Valley National Park in California into Nevada received a full year’s worth of rain in a day, with flooding and mudflows rendering roads unpassable.

Further on from California, the Weather Prediction Center stated that Hilary could be one of, if not the wettest known tropical cyclone, post-tropical cyclone, or tropical cyclone remnant to impact Nevada, Idaho, and Oregon.

Impact of Hilary

Widespread flash floods are driving the impacts, exacerbated by the region's arid soil's inability to absorb such extreme rainfall intensities. River overtopping was minor but is unfolding as rainfall runoff moves through mountainous watersheds and downstream rivers.

The aging drainage infrastructure, initially designed under the assumption of climate stationarity, lacks the capacity to manage the intensity of these extreme rainfall events, which significantly contributed to the impacts.

There were nearly 50,000 power outages, and many business interruptions, with mud and debris flows resulting from the widespread floods. 

In Palm Springs, emergency crews conducted 46 rescues in the city late on Sunday night (August 20), as mud and standing water reached five feet (1.5 meters).

In nearby Cathedral City, a bulldozer was driven through the mud to a swamped care home to rescue 14 residents by scooping them up in the bulldozer scoop and carrying them to safety.

In the San Bernardino Mountains, the homes of about 800 residents were blocked in by mud, and in Desert Hot Springs, roads were impassable due to flooding.

Low Flood Insurance Penetration

Residential exposure in areas with the highest impacts from flooding sees single-digit flood insurance penetration rates for both the National Flood Insurance Program (NFIP) and private policies.

For certain counties, residential flood insurance penetration is less than one percent. For commercial and industrial exposures, flood insurance penetration rates are more varied and typically higher.

NFIP take-up rates in the Western U.S. states
National Flood Insurance Program (NFIP) flood policy take-up rates: Data source: Moody's RMS

With these single-digit percentages of residents having flood insurance, for residents and businesses who find themselves dealing with the aftermath of flooding there's a significant risk of financial devastation and long-term economic setbacks.

In addition, policies typically do not cover damages due to flooding/mudslides if originally caused by wildfires.

California saw a wet start to the year due to continued rainfall from a series of atmospheric rivers, a situation that can be attributed and made more likely due to climate change factors, but tropical cyclones such as Hilary impacting the western U.S. states as we have seen are very rare.

Residents in California and the western states, in addition to wildfire and earthquakes (with an Mw5.1 event on August 20, with an epicenter near Ojai and Ventura, California), need to also prepare for more frequent intense flood events and to consider flood insurance with their homeowner policies.

As always, Moody’s RMS event response team will keep you ahead of the latest updates on this and all major cat events, clients can visit the Support Center for more information.

Similarly, companies interested in improving their quantification of California and overall U.S. flood risk, across tropical and non-tropical-induced flooding, can take advantage of Moody’s RMS U.S. Inland Flood High Definition Model.

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Firas Saleh
Firas Saleh
Director of Product Management, Moody's RMS

Dr. Firas Saleh joined Moody's RMS as Director of Model Product Management in 2020. He oversees Moody's RMS U.S. Inland Flood and Wildfire HD Models and works closely with clients and across Moody's RMS functional teams on defining and executing the vision, strategy, and roadmap for Moody's RMS flood and wildfire products.

Firas holds a Ph.D. in Geosciences and Natural Resources from the University of Pierre et Marie Curie - Paris VI (Sorbonne Universités), France. He has a strong professional track record in the U.S. Federal Government, industry, and academia.

During his academic tenure at different institutes around the world, including the Paris School of Mines (Mines Paris-Tech), New Jersey Institute of Technology, and Stevens Institute of Technology, his research was focused on implementing quantitative forward-looking analytics to assess climate and weather-related physical risk and impacts on critical infrastructure resilience.

At Stevens, he was part of the team that pioneered and productized the coastal-inland operational flood forecast systems for Port Authority of NY-NJ critical facilities (JFK, LaGuardia, Newark, and Teterboro Airports) and NJ Transit. He has co-authored more than 30 publications in peer-reviewed articles, conference proceedings, and book chapters.

He also served as a Senior Commercial Specialist at the U.S. Embassy in Baghdad and Amman, the U.S. Commercial Service, and the U.S. Department of State. He is the recipient of the U.S. Department of Commerce Gold Medal Award for his distinguished federal service in fostering collaboration between government and Industry concerning water and construction. The Gold Medal is the highest honorary award granted by the U.S. Secretary of Commerce.

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