The West Coast’s rainy season has arrived with a dramatic start as an atmospheric river carrying tropical moisture collides with a powerful bomb cyclone off the Pacific Northwest coast. This weather duo is unleashing widespread heavy rainfall, strong winds, and mountain snow from Northern California to Washington.
On Nov. 19, 2024, heavy wet snow began blanketing mountain regions while bursts of rain battered the coasts of Oregon and Northern California. Winds exceeding 70 mph have been recorded in parts of Washington as the bomb cyclone intensifies offshore.
This storm’s strength is driven by the meeting of two potent weather phenomena. The atmospheric river a “river in the sky” transporting moisture from the subtropics feeds the bomb cyclone, a rapidly intensifying low-pressure system. As the cyclone spins off the coast, it propels moisture inland, causing unpredictable and dynamic weather patterns.
Meteorologists liken the atmospheric river to a firehose aimed at the coast, with the bomb cyclone’s waves of energy pushing it north and south unpredictably. Forecasting how long specific areas will see rain, and how intense it will be, remains challenging.
Impacts and Concerns
The storm is forecast to last for several days, with some regions bracing for over a foot of precipitation. While early-season dry soils will absorb much of the rainfall, reducing the risk of widespread flooding, areas with wildfire burn scars face a heightened threat.
Burn scars often develop hydrophobic surfaces, preventing water absorption and increasing the risk of fast-moving debris flows during intense rain. Narrow cold-frontal rainbands within this storm may exacerbate these risks, particularly in hilly regions.
The National Weather Service advises residents in burn scar zones to stay alert for watches and warnings.
Category 4 Atmospheric River
The Center for Western Weather and Water Extremes has categorized much of this atmospheric river as a Category 4 on a 1-to-5 scale. This indicates significant potential for heavy rain and snow accumulation, particularly in Northern California.
An animation from the Cooperative Institute for Meteorological Satellite Studies shows how the bomb cyclone is directing the atmospheric river’s moisture to the West Coast, creating an extended and intense precipitation event.
What Does This Mean for Winter?
Despite this storm’s intensity, it doesn’t guarantee a wet winter. Early-season storms like this have occurred before without leading to above-average seasonal rainfall. For instance, a record-breaking storm in October 2021 was followed by a drier-than-average winter.
Meteorologists note that long-term forecasts, particularly seasonal trends, remain uncertain. The developing La Niña pattern may hint at drier conditions, but history has shown surprising exceptions, such as the wet winter of 2017.
For now, short-term forecasts suggest that this atmospheric river and bomb cyclone will continue to dominate West Coast weather into the following week. Residents are urged to stay informed and prepared for potential hazards.