AccuWeather forecasters are growing increasingly concerned that flooding downpours will reach parts of the Southwest as Hilary’s moisture arrives, and experts say there are some parallels to a historic hurricane that caused a flooding disaster for some communities.
Published Aug 16, 2023 1:04 PM EDT | Updated Aug 16, 2023 6:49 PM EDT
A newly formed tropical storm, named Hilary, is forecast by AccuWeather meteorologists to wander close enough to the southwestern United States to raise the likelihood of torrential rain and the potential for major flash flooding beginning this weekend and lasting into next week.
The tropical storm, with maximum sustained winds of 40 mph, developed in the East Pacific Tuesday morning less than 500 miles away from the southwestern Mexico coast.
Hilary is forecast by AccuWeather's tropical weather team to become a major hurricane and at least Category 3 on the Saffir-Simpson Hurricane Wind Scale while spinning just off the southern tip of Mexico's Baja peninsula this weekend. A Category 3 hurricane has winds of 111-129 mph.
The scope of Hilary's potential dangers
Direct impact from tropical storms and hurricanes in the eastern Pacific on the Southwest is rare but not unheard of, according to meteorologists. Most of these systems tend to turn out to sea or weaken long before reaching far enough to the north to bring heavy rain, severe thunderstorms and gusty winds in the zone from Southern California to Arizona. However, once in a while these conditions, especially the impacts from heavy rain, occur in the region.
Chances are increasing for Hilary to do just that starting this weekend, even if the system manages to weaken prior to reaching northern Baja California peninsula of Mexico or perhaps stays offshore.
The exact track of Hilary and its intensity will determine which areas in the Southwest are hit the hardest by rain and perhaps strong winds.
Due to the impacts of heavy rain and strong winds in Mexico, Hilary has been rated a 2 on the AccuWeather RealImpact™ Scale for Tropical Cyclones. In Mexico, an AccuWeather Local StormMax™ wind gust of 110 mph is projected with an AccuWeather Local StormMax™ rainfall of 20 inches. However, this depends on the track and strength of Hilary as it moves northward.
Most tropical systems tend to quickly lose wind intensity upon making landfall in this region due to mountainous terrain. On the other hand, impacts from heavy rain can persist for hundreds of miles after systems move inland.
At this time, AccuWeather meteorologists are analyzing a range of future scenarios for Hilary by the time its rainfall reaches the southwestern U.S., and the scope of impacts could be some beneficial rain with only localized flash flooding to a much more severe scenario where life-threatening conditions could evolve into a flooding disaster in some communities. In either case, most problems related to high winds may come from severe thunderstorms and possible tornadoes.
Rain, wind and other potential impacts in the Southwest
Impacts related to flooding rain, strong winds, pounding waves and coastal flooding will spread northward along Mexico's Baja peninsula late this week and this weekend.
Rain will precede the center of Hilary into the U.S. by two to three days and has the potential to be heavy enough to trigger major problems in the region, including the cities of Las Vegas and Phoenix and even in parts of the San Diego and Los Angeles metro areas.
Residents and visitors in the region should expect substantial travel delays at the very least.
Beginning Saturday, heavy rain will begin to affect Southern California, southern Nevada and western and central Arizona, AccuWeather Meteorologist Brandon Buckingham said. As the rain pours down, incidents of flash flooding, mudslides and debris flows will ramp up.
The intensity of the downpours will likely result in road closures over the mountains and deserts. Normally dry canyons and stream beds, called arroyos, may rapidly fill with rushing water and could pose a danger to hikers and motorists.
A general 1-2 inches of rain will fall on the region related to Hilary. However, in some areas, double that amount of rain may fall in several hours with a double-digit storm total, which is more than enough to rapidly runoff and collect over the arid, hard-packed terrain. The potential exists for historic flooding, perhaps on the order of a once-in-a-few-hundred-year return period or greater, depending on the track and intensity of Hilary.
Enough rain may fall to lead to major flooding in Palm Springs and Death Valley National Park, California.
“There could be more damage to infrastructure at Death Valley National Park due to flooding and washouts,” AccuWeather Meteorologist Brian Lada said.
Last summer, a heavy thunderstorm washed out roads and stranded more than 1,000 visitors. “Similar impacts may be felt from Hilary if heavy tropical downpours douse the park, which sits below sea level,” Lada added.
Severe thunderstorms could develop in response to Hilary's approach, causing ground stops at the airport hubs in the region.
Predicting Hilary's strength
In addition to perhaps some shift of the storm's track near the northern coast of Mexico, the speed with which Hilary will lose wind intensity is another wildcard.
"There is no doubt cool water just off the coast of the northern part of Mexico and Southern California will cause Hilary to weaken," AccuWeather Lead Long-Range Meteorologist Paul Pastelok said. The water off the coast of northern Mexico and California is cooler than it typically is during most El Niños, a pattern defined by warmer-than-average water in the central and eastern tropical Pacific.
El Niño officially developed in June and continues to evolve over the Pacific.
However, a fast-moving tropical system may not have time to unwind over the cool water or land. In such a scenario, Hilary, as a tropical storm, could make landfall in northern Mexico or could survive briefly in the southwestern U.S.
An outcome similar to that of Hurricane Kathleen from early September 1976 is one remote possibility, according to AccuWeather Senior Storm Warning Meteorologist William Clark. Interestingly, Kathleen occurred during an El Niño year.
Kathleen produced up to 15 inches of rain in Southern California and triggered major flooding in the southwestern U.S. The system reached the U.S. as a fast-moving tropical storm, took the lives of a dozen people and caused nearly $1 billion in damage converted to today's dollars, according to the National Weather Service and the San Diego Union-Tribune archives.
How far will Hilary's impacts be felt in the US?
Downpours associated with Hilary as a tropical rainstorm are not likely to be limited to just the Southwest.
Moisture will continue to stream northward through a large part of the western U.S. into next week, Buckingham said. Correspondingly, the flash flooding risk will extend northward through the Great Basin and into the Rockies. Rain is likely to reach parts of Northern California as well.
There is no doubt that drenching rain will go a long way in easing the wildfire threat in the Southwest, where rain falls. However, on the fringes of the rain, lightning strikes from thunderstorms could spark new blazes since forests and brush in the region have had all summer to dry out, Buckingham said.
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