The National Weather Service says trick-or-treaters may want to add an umbrella to their costumes because Halloween can be dark and stormy.
The storm will begin arriving on Friday, with isolated showers expected and high temperatures 5 to 10 degrees below normal, according to the National Weather Service.
Forecasts say the storm system could peak Saturday night into Sunday morning, with a 70 percent chance of rain. Ryan Kittle, a meteorologist with the National Weather Service in Oxnard, said that while forecasters are still unsure about rainfall amounts, a half-inch to an inch of rain is likely.
However, he said the region still has about a 20% chance it will double and a 20% chance it will be halved. Rainfall amounts are expected to be relatively even throughout Los Angeles County, with totals toward the high end of the range in mountainous and foothill communities such as La Cañada and Azusa, he said.
Highest temperatures along the coast and valleys are forecast to be above 60 degrees.
Forecasts indicate that some models are predicting rain to arrive later, so a dry Saturday night is still possible.
The latest forecast for rainfall will peak between 9 p.m. Saturday and 9 a.m. Sunday, “which is good news for Halloween events, at least going forward,” Kittel said.
“I think people should still prepare for Halloween events, but right now it’s looking like the rain will be light rather than mild,” he said earlier in the evening.
Kittle said the storm’s impact is expected to be minimal, with increased traffic possible and minor flooding of roads possible. “Probably the biggest impact is the Halloween event,” he said.
The last recorded rainfall on Halloween in Los Angeles occurred in 2014, when a quarter of an inch fell in downtown Los Angeles, according to weather service records.
Times staff writer Grace Toohey contributed to this report