Leftover tropical moisture from Mario to move into Southern California, southwest US
Leftover tropical moisture from Mario will surge into Southern California and the Southwest this week, raising the risk for showers, storms and flash flooding.
Tropical moisture is on the way to the Southwest, with unusual September rain expected in Southern California.
Tropical storm impacts in Southern California from the eastern Pacific are rare, but AccuWeather meteorologists forecast that leftover moisture from Mario, which dissipated Wednesday morning just off the western coast of Mexico, will move into the southwestern United States later this week.
It is not uncommon for New Mexico and parts of Arizona to occasionally experience rain from a tropical storm that makes landfall in northwestern Mexico. During early September, Lorena formed and ramped up to a hurricane in a similar zone to Mario. Even though Lorena never made landfall in Mexico, some of its moisture was drawn northward, enhancing showers and thunderstorms in Texas, New Mexico and Arizona — but not in Southern California.
Rainfall from a tropical system in Southern California is even rarer. One of the most dramatic examples was Hurricane Hilary in August 2023. Hilary was once a powerful hurricane over the eastern Pacific, tracked into northwestern Mexico and unleashed tremendous flash flooding as far west as parts of Southern California.
The magnitude and extent of rain that occurs in the Southwest states during the second half of this week will depend on the strength of high pressure west of the Rockies.
"Right now, this high pressure area is very strong but is likely to weaken a bit this week," AccuWeather Lead Hurricane Expert Alex DaSilva explained. "If the high weakens substantially, a greater surge of tropical moisture could stream in from the Pacific," DaSilva said.
If the high remains strong, there may only be scattered showers and thunderstorms, similar to activity seen earlier this summer with the North American monsoon.
“Any lightning could spark new wildfires in the Southwest,” DaSilva said.
Showers are likely from Wednesday into Thursday in San Diego and from Wednesday night through Thursday night in Los Angeles. Thunderstorms are possible in Palm Springs, California, on Thursday and Friday and may reach Las Vegas by Thursday night and Phoenix on Friday.
As rain sweeps across the region, the heaviest storms are expected to be confined to the mountains of Southern California. However, downpours can lead to localized flash urban flooding in the metro areas, mountainsides and some desert locations.
The combination of water and oil residues on the roads can make for extra-slick conditions.
Motorists will also need to be on the lookout for mudslides that can push debris onto secondary roads. Rainfall totals will range from a general 1-2 inches with an AccuWeather Local StormMax™ of 5 inches.
Historically, coastal areas of Southern California receive only 0.10 to 0.30 of an inch of rain during the entire month of September. However, up to 10 times that amount may fall within just a few days.
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