Tropical moisture brought September rains and thunderstorms to parts of Southern California Thursday, making for treacherous driving and fueling fears of flash floods and mudslides in recently burned areas.
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The first rains of the fall caused the usual slicking of roads and several crashes followed on Los Angeles-area freeways Thursday morning.
The region was in the grip of a muggy heat wave that yielded to monsoonal moisture early Thursday, according to the National Weather Service. Some areas saw up to a tenth-of-an-inch of rain, with additional storm activity possible later in the day with warming temperatures, forecasters said.
Temperatures were expected to be down from Wednesday, but humidity will remain elevated, as high as 69% in Los Angeles County, forecasters said.
Remnants of Tropical Storm Mario could dump up to 2 inches of rain in some areas. A flood watch was in effect Thursday for a wide stretch of the region, including Pacific Palisades, which was devastated by January's wildfire, but it was canceled in most western portions of the L.A. area by late morning.
The watch will remain in place until late Thursday in the San Gabriel Mountains, the Antelope Valley and Antelope Valley foothills and the Golden State (5) and Antelope Valley (14) freeway corridors. Forecasters said flash flooding from excessive rainfall will remain possible in those areas
"An additional round of heavy showers and thunderstorms could bring local rainfall rates to one-half to one inch per hour, capable of flash flooding and debris flows," according to the NWS. "The risk for high-impact flooding will steadily decrease after 7 p.m., ending by 2 a.m."
Los Angeles Mayor Karen Bass' office said the city was monitoring the forecast, and coordinating with the Emergency Management Department, the fire and police departments, the recreation and parks department and the Los Angeles Department of Water and Power to ensure all are ready to respond as needed.
"As the system is convective, this storm will be different from our normal winter storms and monsoonal flow induced afternoon convection," according to the NWS. "While most areas will get at least some rain today, some areas may get little to none. Still, others just a short distance away could have flooding."
Highs are expected to reach 81 in downtown Los Angeles on Thursday and 80 on Friday, with similar numbers in most inland county areas.
Overnight lows will be warmer than normal into early next week as the moisture aloft will trap a lot of the heat from the daytime. Overnight temperatures in the upper 60s and low 70s are expected to be the norm in the inland areas.
The high humidity levels will continue into early next week, with a slight chance of showers as additional moisture from the south arrives.
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