This article has been updated to fix a spelling error in the headline.

An atmospheric event known as the Pineapple Express is set to bring significant rainfall and snowfall to California and the Northwest this week.

So, what exactly is a Pineapple Express? It refers to a type of atmospheric river—a narrow band of moisture in the Earth’s atmosphere. This jet stream collects water vapor from tropical regions near the equator and transports it towards the poles, starting its journey near Hawaii.

Meteorologists at Accuweather are warning that this phenomenon may lead to intense winds, heavy rainfall, and potential landslides.

How the Pineapple Express Can Cause Disruption on the West Coast

On the West Coast, atmospheric rivers come from the Pacific Ocean, absorbing vast amounts of moisture. These occurrences are more frequent during the El Niño weather pattern in the Pacific.

These long and narrow corridors filled with moisture form when warm air from the tropics moves north ahead of strong storm systems. A 2017 study indicated that these “sky rivers” account for up to 65% of the region’s extreme rain and snow events.

Sky Rivers: An Overview

AccuWeather has issued warnings about potential flooding, avalanches, high winds, and travel delays as heavy rain sweeps across California. Some locations, including San Francisco and Redding, may experience a month’s worth of rain within a short time.

Meteorologist Heather Zehr from AccuWeather noted that landslides and mudslides could occur from the northern part of California down to San Francisco, which may result in road closures and disrupted travel.

Heavy Rainfall May Lead to Flood Risks in California

According to the National Weather Service, the valley and foothills are under a flood watch until Tuesday evening. By early Tuesday, snowfall was anticipated in the Sierra Nevada mountains at elevations above 6,000 feet, with travel likely hindered by strong winds and rapid snow accumulation.

The Climate Prediction Center of the National Weather Service forecasts below-average temperatures in California over the next eight to fourteen days. With the exception of Southern California, which is expected to see near-normal or below-normal rainfall, most of the state is forecasted for above-average precipitation.

CONTRIBUTING John Bacon

SOURE National Oceanic and Atmospheric Administration, AccuWeather and National Weather Service