Heavy Rainfall: Hilary Drenches the American Southwest

Satellite data outlines Hilary’s precipitation on August 20, 2023.

Southern California, Nevada, and Mexico were subjected to heavy rainfall as a result of the storm, leading to inundated roads and power failures.

Initially a robust category 4 hurricane, Hilary lost strength and made landfall in northern Mexico on August 20, 2023, showering the Baja California peninsula and two states in the U.S. with rain.

Prior to reaching land, the storm brought heavy rain to Santa Rosalía in Baja California Sur, Mexico. According to authorities, the floodwater claimed one life. Before reaching the northern tip of the peninsula around 11 a.m. Pacific Time on the same day, Hilary was downgraded to a tropical storm. As it moved over Southern California, the storm caused floods, mudslides, and electrical outages.

Satellite Information and Historic Rainfall Levels

The provided map illustrates a satellite-derived approximation of the storm’s precipitation. (Click on the image for a detailed view.) The map’s data were gathered from 5 p.m. Pacific Time on August 19 to 2 p.m. Pacific Time on August 20, as the storm’s center neared Southern California.

On this map, the most intense reds indicate the greatest rainfall rates, with the Baja California peninsula experiencing the most substantial downpour. These figures are derived from remote sensing from the Integrated Multi-Satellite Retrievals for GPM (IMERG), a function of the Global Precipitation Measurement (GPM) satellite mission. Due to the data averaging, ground measurements may reveal considerably higher local rainfall rates.

According to the National Weather Service, August 20 set a record for the most rainfall in August for both downtown Los Angeles and San Diego, receiving 2.5 inches (6.3 centimeters) and 1.8 inches (4.6 centimeters) respectively. The storm additionally generated wind gusts reaching 70 miles (113 kilometers) per hour in parts of California, toppling trees and power lines, and leaving an estimated 40,000 inhabitants without electricity. Death Valley National Park was closed due to flooding, and roads in Las Vegas, Nevada were also damaged and flooded.

Effect on Typically Arid Desert Regions

Some of the worst-impacted locations in the U.S. have been Southern California’s normally dry desert areas. After receiving six months’ worth of rain in just six hours, Palm Springs’ streets were transformed into muddy rivers. Flooding also resulted in the closure of major roads in Coachella Valley, including I-10, the principal highway in that area.

In Southern California, August is usually a dry period, but the soil’s surface layers can react quickly to meteorological phenomena. Scientists at NASA’s Short-term Prediction Research and Transition Center (SPoRT) anticipated that the storm’s rainfall could saturate the soil in the area for a number of days.

On August 21, Hilary proceeded to move northwards into Oregon and Idaho, where it was expected to deliver up to 3 inches (7.6 centimeters) of rain in certain locations, with the possibility of flash floods.

The image was provided by NASA Earth Observatory and created by Lauren Dauphin, utilizing IMERG data from NASA’s Global Precipitation Mission (GPM) at NASA/GSFC.

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