The Scale of California's Pineapple Express Phenomenon

A second atmospheric river rainstorm in a week is moving across California and has been causing landslides, downpours and high winds of hurricane strengths that toppled trees in some locations. Conditions could persist until mid-week as the storm is now projected to move across the state slower than originally expected.

The National Weather Service in Los Angeles warned that the storm was "dangerous" and posed "major risks to life and property." The service's office in San Diego meanwhile predicted the potential of flooding in Los Angeles and "locally catastrophic" flooding in Orange County as amounts of rain typical for a whole month could come down in just the span of days. The risk of excessive rain in Southern California was put at a 4 out of 4. More than half a million Californians were already without power early Monday due to wind damage. Some residents in Santa Barbara, Los Angeles and San José are currently under evacuation orders, according to CNN.

The phenomenon of atmospheric rivers is fed by the El Niño conditions that have taken over the climate of the Americas again since mid-2023. Atmospheric rivers are plumes of moisture that extend from the tropics all the way to more Northern reaches and are made possible by El Niño-related changes in the jet stream air currents that circle the globe. Normally, these jet streams travel in westerly directions but under certain conditions, plumes can escape and travel long distances, bringing tropical rains to more moderate locales. An atmospheric river that has been reoccurring is the one originating from the Pacific near Hawaii all the way to California - dubbed the Pineapple Express because of the fruit being a symbol of the U.S. island chain.

The latest Pineapple Express pummeling California was rated moderate to strong - a level 2-3 out of 5 possible - by the Center for Western Weather and Water Extremes at the University of California San Diego. Partially forecast as of early Monday, the current storm’s rating could increase since the system is now expected to move more slowly and duration is one of two levels the storms are rated on. The current weather phenomenon was initially expected to continue for around 30 hours into early Tuesday while carrying around half the moisture intensity of the most exceptional scenario still included in the rating system. Thursday's system that had already brought heavy rains as well as snowfall to the state was rated similarly, but opposed to Monday's forecast was a little shorter, yet a little more moisture-intense at a level 3 out of 5 on the rating scale.