'Pineapple Express' Storm Barrels Toward SoCal With First Halloween Rain In Decades

A weather pattern known as the “Pineapple Express” threatens to bring heavy rain to Southern California over Halloween weekend, potentially dampening trick-or-treating and causing flooding in some areas, forecasters said Thursday.

Two Pacific storms could draw tropical moisture toward the western United States over Halloween weekend, with a third weather system near Hawaii potentially contributing to the developing pattern, according to AccuWeather.

"A storm from Alaska and another off Washington's coast will draw tropical moisture into the western U.S. this weekend," AccuWeather Meteorologist Lydia Burroughs said. "Tropical Rainstorm Olana near Hawaii will help influence that moisture plume. Rainfall amounts in Southern California will depend on how quickly the storms and moisture come together."

The weather pattern could develop into an atmospheric river, a long band of concentrated moisture that can bring heavy rain and mountain snow. When that moisture stretches from near Hawaii to the West Coast, it is commonly called a Pineapple Express.

However, AccuWeather said the more likely scenario would keep the storms and their moisture somewhat separated, limiting the heaviest rainfall around Los Angeles, San Diego and Palm Springs.

Under that scenario, most major Southern California cities would receive less than an inch of rain, while some mountain slopes could see up to 3 inches, according to AccuWeather. Heavy downpours could trigger localized flash flooding and flows of mud and debris in areas recently burned by wildfires.

The National Weather Service said rain could begin as early as Friday, with the greatest chance of heavier rainfall arriving late Saturday into Sunday morning.

In Los Angeles County, forecasters said rain could peak Saturday evening, potentially disrupting Halloween festivities. The latest forecast calls for about a quarter to a half-inch of rain across most areas, with up to three-quarters of an inch in southeastern Los Angeles County.

There is also a 30 percent chance of a wetter storm producing 1 to 2 inches of rain across much of the region, according to the National Weather Service.

In San Diego County, Orange County and parts of the Inland Empire, light showers could develop Friday morning before rain chances increase Saturday afternoon and evening.

The National Weather Service said there is a 75 percent to 85 percent chance of at least a tenth of an inch of rain from Saturday morning through Sunday night across coastal and mountain areas, with a 35 percent to 65 percent chance of an inch or more, depending on location.

Forecasters cautioned that the exact timing and rainfall totals remain uncertain, with some projections suggesting the heaviest rain could arrive later than expected.

Cooler temperatures are also expected through the weekend. Inland areas of San Diego, Riverside and San Bernardino counties could see Sunday highs 10 to 15 degrees below average, according to the National Weather Service.

If rain falls Saturday, it could mark the first wet Halloween in decades for some Southern California communities. The National Weather Service in San Diego said Tuesday that many cities in the region have gone 30 to 40 years without rain on Halloween.

The approaching storm follows days of unusually high tides that have flooded coastal communities across Southern California.

On Wednesday, floodwater spilled into streets in Huntington Beach's Sunset Beach neighborhood, while seawater also flooded parts of Malibu, Redondo Beach, Long Beach and Newport Beach, according to City News Service.

Coastal flood warnings remain in effect until 1 p.m. Thursday for Orange and San Diego counties, followed by coastal flood advisories through 2 p.m. Friday. Coastal flood advisories also remain in effect for portions of the Los Angeles County coastline through 2 p.m. Friday.

The National Weather Service warned of additional flooding in low-lying coastal areas, including beach parking lots, bike paths and harbor walkways.

Rain is expected to taper off Sunday, with drier and warmer weather returning early next week.

City News Service contributed to this report.