Southern California’s Halloween festivities could turn into a soggy mess as a powerful storm threatens to drench trick-or-treaters, unleash thunderstorms and deliver the region’s first rainy Oct. 31 in years.

The spooky weather system is expected to sweep across the Southland Saturday evening, bringing widespread rainfall just as costumed children hit the streets in search of candy. 

Los Angeles could experience its first wet Halloween since 2014, putting a damper on a holiday that has largely escaped rain for more than a decade. 

And Mother Nature isn’t waiting until Halloween night to make her presence known.

Scattered showers began creeping into parts of Southern California Friday morning, offering an ominous preview of the heavier downpours expected to arrive over the weekend.

The National Weather Service warned that the storm’s most significant rainfall will likely coincide with peak trick-or-treating hours Saturday night before intensifying overnight into Sunday.

Forecasters expect widespread rainfall totals between half an inch and 1.5 inches, with some mountain and foothill communities potentially getting soaked with as much as 2.5 inches.

“This has been a challenging system to get a hold of and even more challenging to describe,” the National Weather Service said in its forecast discussion.

The agency warned that rainfall predictions remain uncertain, with a 20% chance that some areas could receive either double or half the currently projected amounts.

The storm is being fueled by a collision between chilly northern air and a surge of moisture moving into the region from the southwest.

“This upper-level low is going to pick up all the moisture related to this system down to our southwest, and all that moisture is clashing with the colder air to the north,” FOX 11 meteorologist Maria Quiban explained.

That atmospheric clash could also produce a frightening addition to Halloween night — thunder and lightning.

Forecasters estimate a 10% to 20% chance of isolated thunderstorms Saturday night into early Sunday, particularly across southern portions of the region.

“If you are headed out to trick-or-treat, make sure you have your umbrella and, of course, rain boots,” Quiban warned.

Temperatures are also expected to take a tumble, with daytime highs hovering in the upper 60s to low 70s under mostly cloudy skies.

Saturday morning and early afternoon should offer a brief reprieve for families hoping to squeeze in Halloween festivities before the storm arrives.

Most areas will see only scattered sprinkles during the day, with rainfall accumulations generally staying below one-tenth of an inch.

But conditions are expected to deteriorate after sunset.

San Bernardino could receive around an inch of rain, while Woodland Hills and Burbank may see approximately four-tenths of an inch, according to Quiban.

Santa Clarita could receive slightly more, while Palm Springs may also get drenched with nearly an inch.

The National Weather Service’s San Diego office issued its own Halloween-themed warning on X.

“A spooky storm system will move through the region Halloween night into Sunday morning,” the agency wrote. “Widespread rain is expected with the heaviest rain in SD County.”

Meanwhile, coastal communities are facing a separate weather headache.

A coastal flood advisory remains in effect through 2 p.m. Friday for beaches in Los Angeles, Orange and Ventura counties, where elevated tides and surf reaching 4 to 5 feet could trigger minor flooding.

High tides between 6 and 7 feet are expected Friday morning, although forecasters say coastal flooding should be less severe than earlier in the week.

The Halloween storm is expected to gradually lose its grip on Southern California Sunday morning, with most rainfall clearing out by the afternoon.

A weak high-pressure system should usher in warmer, drier conditions early next week, giving residents a chance to dry out their costumes and recover from the soggy holiday.

Another cooling trend could arrive toward the end of next week, although additional rainfall currently appears unlikely.