1 day ago

Atmospheric rivers (ARs) are long, narrow corridors of concentrated water vapor in the lower atmosphere that move tropical moisture poleward. The well-known “Pineapple Express” is one such moisture highway, linking near-Hawaii air masses to the U.S. West Coast. ARs can be beneficial, replenishing reservoirs and ending droughts, or agents of disaster when they unload huge volumes of rain and snow over hours or days.

To help communicate risk, scientists created an AR Category Scale that ranks events from 1 (primarily beneficial) to 5 (primarily hazardous). The scale combines two ingredients: how much integrated moisture the atmosphere is transporting and how long those conditions persist at a location. Duration matters: storms persisting over 48 hours are upgraded a category, while those under 24 hours are downgraded.

Climate change is already reshaping ARs, but not uniformly. A high-resolution modeling study led by NCAR (National Center for Atmospheric Research) found regional contrasts: southern California’s ARs, often subtropical Pineapple Express events, will grow stronger mainly because warmer seas boost evaporation and load more moisture into the air. By contrast, ARs hitting Northern California and the Pacific Northwest will be driven more by joint ocean–atmosphere warming, producing more intense storms that can push coastal water levels higher for days. In some computer model scenarios, northern coasts could see transient sea-level rises during ARs several times larger than today’s events.

Those changes translate to practical hazards: heavier runoff, compounded flooding when storms coincide with high tides or already saturated soils, more severe erosion and stresses on fisheries and coastal ecosystems. Improving forecasts requires coupled, fine-scale atmosphere–ocean models, richer observations (satellites, buoys, tide gauges) and operational tools that translate physical metrics into actionable warnings. While adaptation like updated flood maps, resilient infrastructure, and nature-based buffers is essential, limiting greenhouse gas emissions remains the most effective way to curb AR intensification and reduce long-term flood risk.