Light rain fell in the Southland Wednesday morning and showers are likely Wednesday afternoon, when there also will be a chance of thunderstorms — a prospect causing worry among people living below areas denuded by wildfires, according to the National Weather Service.

Showers are also likely Wednesday evening, and there will be a slight chance of thunderstorms, forecasters said.

The chance of measurable precipitation Wednesday has been set at 70 percent in both L.A. and Orange counties.

The day’s first rains were reported after 6 a.m., with the NWS reporting light rain in Santa Ana and Fullerton. Light rain was falling in Van Nuys an hour later

The remnants of Hurricane Rosa — now a tropical depression — will not be a factor in the Southland’s rain episode, having largely migrated to Arizona and New Mexico, NWS forecasters said. Instead, the rain will be largely a product of an upper-level low-pressure system that originated in the eastern Pacific .

Although not a monster, Wednesday’s storm will be unusually strong for this time of the year, said NWS meteorologist Dave Bruno, adding that the lion’s share of the precipitation will appear between noon and midnight.

Between a quarter-inch and a half-inch of rain is expected in L.A. County, said weather specialist Bonnie Bartling at the NWS monitoring station in Oxnard. In Orange County, a 10th to a quarter of an inch of rain is expected near the coast and a half-inch is possible in higher areas, such as in the Santa Ana Mountains, said meteorologist Jimmy Taeger who monitors Orange County conditions from the NWS station in San Diego.

The approaching system is raising some fears of mud slides and debris flows over areas previously denuded by wildfire — including near the site of the Holy Fire in Orange County and, in L.A. County, the La Tuna Fire near Burbank and the Creek Fire north of Sylmar — but those concerns are tempered by the fact mostly light rain is expected, according to Bruno. However, the risk of slides will increase considerably if thunderstorms appear since they can generate heavy downpours, and thunderstorms are possible, given an infusion of cold air and instability in the atmosphere, Bartling said.

Between two-tenths and a half-inch of rain is possible over the Holy Fire burn scar, Taeger said.

Orange County warned residents of the possibility of mud and debris flow.

“Residents of Trabuco Canyon are advised that the Holy fire burn area is susceptible to mud and debris flows due to heavy rainfall and can occur with little or no warning,” the county said in a statement. “Mud and debris flows are fast-moving landslides that can be several feet deep and life-threatening. Flooding in nearby areas is also possible.”

Taeger said that in the area monitored by the NWS in San Diego, the greatest volume of rain expected will be over San Bernardino coastal mountain slopes, where six-tenths of an inch is possible.

The NWS forecast highs of 61 degrees on Mount Wilson; 69 in Avalon; 71 at LAX; 73 in Downtown L.A., Burbank, Saugus and Lancaster; 74 in Long Beach; 73 in Burbank, 74 in Long Beach and Pasadena; and 75 in San Gabriel and Woodland Hills. Temperatures in the county will remain in the low-to-mid 70s through at least Tuesday.

In Orange County, a high of 70 was expected in San Clemente; 72 in Laguna Beach; 74 in Newport Beach; 76 in Anaheim; 77 in Fullerton, Irvine and Mission Viejo; and 78 in Yorba Linda. Highs will be in the mid-to-high 70s and low 80s through at least Tuesday.

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