Mario rain to temporarily ease dryness in California and Southwest, but raise flood risk
Moisture from former Tropical Storm Mario will bring short-term rain to parts of California and the Southwest, but thunderstorms could trigger flash flooding, debris flows, and wildfire risks from lightning.
This “fogbow” was caught over Pacifica, California, in the San Francisco Bay area on Sept. 15. A fogbow is a rainbow with a white band appearing in fog according to the National Weather Service.
While winds associated with former Tropical Storm Mario have faded, moisture from the storm will stream into the southwestern United States to end the week. Some areas will benefit temporarily from rainfall, but excessive rain in others may lead to flooding and other hazards, AccuWeather meteorologists say. Lightning strikes could ignite new wildfires in areas with dry vegetation.
The bulk of the rain in the Interstate 5 corridor of Southern California will occur from Wednesday night to Thursday. In San Diego, the greatest amount of rain will occur on Wednesday night. In greater Los Angeles, it will be on Thursday.
This image of the southwestern United States, northwestern Mexico, and the eastern Pacific was captured at midday on Wednesday, Sept. 17, 2025. The low-level swirl from former Tropical Storm Mario can be seen in the lower center part of the image. (AccuWeather Enhanced RealVue™ Satellite)
Initially, rain mixing with oil residue on roads can create slick conditions, raising the risk of accidents. Then, as the rain pours down, runoff will lead to ponding on some streets and highways and potentially debris flows in hilly terrain. A general 0.50 of an inch to 1 inch of rain is forecast to fall. This is many times the historical average for September in the region.
Some showers can reach as far north along the California coast as San Francisco, while a thunderstorm or two could bring sporadic rainfall to California's Central Valley.
As the moisture spreads inland, heavy rain will be wrung out over the south- and west-facing mountainsides in Southern California, with 1-2 inches forecast. Downpours and thunderstorms will also extend into parts of Central California and the interior Southwest, including in Las Vegas and Phoenix, where there will also be a risk of flash flooding and debris flows.
The AccuWeather Local StormMax™ rainfall for this event into Friday is 5 inches.
Hazardous conditions are possible in the Sierra Nevada, including flash flooding, debris flows and wildfires.
In some areas, where little rain falls, lightning strikes from thunderstorms can spark new wildfires as drought conditions combine with dry fuels after a summer of intense heat and sunshine. Only higher humidity levels and lower temperatures may act as a deterrent for fires, except where it manages to pour.
A non-tropical storm and dip in the jet stream off the coast of California are helping to draw moisture into the Southwest. This setup could linger into next week and may continue to send periodic moisture into the region beyond the end of the week.
Additional showers may help ease drought conditions and temporarily reduce wildfire risk. However, once again, lightning with little accompanying rainfall can increase the risk of new wildfires. Where downpours are intense or repeat, the risk of flash flooding and mudslides will exist as well.
Despite Mario's downpours, which will lead to travel and outdoor disruptions and dangers in the Southwest, conditions are expected to be much less severe than those of Hilary in August 2023. The big difference between now and 2023 is that Hilary's center of circulation moved directly into the Southwest states, while Mario's is diminishing offshore and will stay at sea.
There is an additional dark side to the rainfall in the coming days that could spur new growth of grassy areas and brush, which will only dry out in the coming months and potentially lead to more fuel when Santa Ana winds howl.
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