California in a wet, white Christmas from the Pineapple Express storm

A powerful Pineapple Express storm could bring a wet, white and possibly violent Christmas to California, with possible snow in the Sierra Nevada and heavy rain across the South.

While the forecast remains to be seen, the upcoming river system will be the strongest in years to reach the Los Angeles area for the holidays — and it threatens a tough slog for those hitting the road to visit friends or family.

The most likely scenario for Los Angeles, Ventura, Santa Barbara and San Luis Obispo counties involves a “high amount” of rain: 2 to 4 inches along the coast and valleys between Dec. 24 and 26, according to Robbie Munroe, a meteorologist with the National Weather Service office in Oxnard.

There is a 50% chance that that scenario will come true.

There is also a 30% chance of moderate rainfall, 1 to 2 inches along the coast and valleys, and a 10% chance of “very high” amounts, 4 inches or more, in those areas.

The last time it rained on Christmas Eve or Christmas Day in downtown LA was in 2021, when 0.83 inches fell over the two days. The last time more than 2 inches of rain fell in those two days was back in 1971, according to meteorologist Bryan Lewis of the weather service’s office in Oxnard.

It’s still unclear whether the storm will be bad – with storms strong enough to cause flooding and mudslides or debris flows – or good, with mild rain spread over the days.

“Even if we see very high rates, the impact is still going to be on the small to moderate side,” Munroe said at a press conference on Wednesday.

But, more troublingly, the Christmas week storm could be the first in a row, and one or two more will appear.

“This could be the first of two or three hurricanes,” Munroe said. “And if that happens, the impacts could be more felt at that time, maybe continue as we approach the new year, even.”

Beyond the Christmas storm, there’s a lot of energy and moisture in the Pacific Ocean, Munroe said, “but it’s hard to know if it’s going to come in as one big storm or maybe two or three” moderate programs.

While the storm promises to bring Southern California’s first significant rainfall in a month, shortly after one of the wettest Novembers on record, Northern California has already been hit by storms this week.

It was expected to be relatively quiet on Thursday, with storms forecast to begin that evening. Localized flooding is possible along the North Coast Thursday night into Friday, with light to moderate rain expected in San Francisco and Sacramento.

The San Francisco Bay Area is expected to see the first of a series of Christmas week storms on Sunday, which could make for a hectic holiday season.

“Early next week, we may see rivers begin to swell and the possibility of rock and land slides disrupting travel plans,” said the National Weather Service office in Monterey, which issues forecasts for the Bay Area.

“This atmospheric river system will bring heavy rain,” the weather service office in Sacramento said. Snow levels could drop to 5,500 feet above sea level on Tuesday and Wednesday, suggesting “potential mountain holiday impacts” on Christmas Eve.

The storm is expected to develop over the tropical southwest coast of California. It’s expected to start out as a warm storm, and some resorts in Los Angeles County may see rain instead of the snow they’re looking for.

“There’s a chance they could get a fair amount of snow in the resort later on Christmas Day … or the 26th, or even beyond, if the active weather pattern continues,” Munroe said.

In the Sierra, where resorts have been hurt by warm weather and snow drought so far this season, it was unclear whether there would be enough cold air to lower snow levels.

“This system will be pulling a lot of warm water south,” writes Bryan Allegretto on the Palisades Tahoe weather blog.

Allegretto said he was nervous about the low-pressure system being so far from California. To get more snow in the California mountains, “we need cold air in the center of the trough and the bottom to move inland,” and computer forecasting models are split on whether that will happen, he wrote.

“Let’s hope the trend is towards a storm that moves inland and drops snow levels quickly on Christmas Eve,” Allegretto wrote. “If we get cold air, we can see a lot of snow.”

Storm temperatures can disrupt travel in critical corridors. A very cold storm could cause snow to pile up on the Grapevine portion of the 5 Freeway – as well as Tehachapi Pass, which connects Bakersfield to the Mojave Desert.

In Orange County, the Inland Empire and San Diego County, the heaviest rain is expected later on Christmas Eve and Christmas Day, according to the National Weather Service office in San Diego.

“However, if the system remains strong or stays offshore, the heaviest rainfall may be delayed for a time when rainfall may continue into next Friday or beyond,” the weather service said. “Confidence continues to grow that heavy rains will lead to increased flooding and the risk of debris flows, as well as disruption to holiday travel across the region.”