Mele Kalikimaka! Pineapple Express To Drench Bay Area Christmas Week

SAN FRANCISCO — A weeks-long winter dry spell is giving way to a wet stretch, with multiple rainstorms forecast to soak the San Francisco Bay Area from Sunday through Christmas Eve.

"The storm door is opening!" the National Weather Service said.

An atmospheric river flowing across the Pacific Ocean from Hawaii, also known as a Pineapple Express, will likely bring heavy rainfall to California starting this weekend, forecasters said.

The name comes from the buildup of moisture in the tropical Pacific around Hawaii, which can wallop the U.S. and Canada's West Coasts with heavy rainfall and snow. Prevailing winds cross over warm bands of tropical water vapor to form this "river," which travels across the Pacific as part of the global conveyor belt. When it reaches the West Coast, the Pineapple Express can dump as much as five inches of rain on California in one day, according to the National Weather Service.

Rainfall is expected to increase through Sunday as a conveyor belt of moisture reloads. Rainy and windy conditions are expected to continue into Monday and Tuesday. A firehose of moisture will slowly drift northward, then back toward the south. San Francisco is expected to receive as much rain by Tuesday as the entire December average, according to the Bay Area National Weather Service.

For now, the Bay Area can expect dense fog to develop overnight in inland valleys, especially in the North Bay and East Bay. Closer to the coast, fog and drizzle will combine to limit visibility overnight into early Thursday morning. The first wave of numerous rounds of rainfall will first move into the North Bay late in the day Thursday, with only light rain for the onset into Friday, according to the weather service.

While the Bay Area is in for a pummeling, meteorologists say there's plenty of uncertainty about the exact timing and intensity of the rainfall expected to impact the area. "What we do know...is the North Bay will see the most rainfall as several waves of moderate/heavy rainfall push through over the next week," National Weather forecasters said. The period of heaviest rainfall as of Wednesday looks to be late Saturday night through early Monday morning, which means Sunday may be a difficult travel day, according to the National Weather Service forecast. "Give yourself plenty of extra time to travel at anytime over the next week, but especially Sunday."

Here is a National Weather Service summary for the Bay Area.

- Mostly dry for tonight and Thursday with the exception of coastal drizzle tonight into early Thursday morning.

- Patchy to Areas of dense fog likely overnight.

- Rain, wind, and general thunderstorms Sunday.

- Rainy and windy conditions are expected to continue through the holiday into the end of next week.