Prolonged atmospheric river triggers High Risk (level 4/4) of Excessive Rainfall alert for Northern California coastline
Two powerful Pacific storm systems and an atmospheric river event will continue impacting the Pacific Northwest and northern California through the end of this week with heavy rain, flooding, strong winds, and higher-elevation mountain snow.
Image credit: NOAA/GOES-West, Zoom Earth, The Watchers. Acquired at 10:50 UTC on November 21, 2024
- The storms could deliver over 30% of the normal water year precipitation to parts of Northern California.
- Dangerous flooding, rock slides, and debris flows are expected, prompting a High Risk (level 4/4) of Excessive Rainfall to be issued across the northern California coastline today.
- The NWS Winter Storm Severity Index indicates a 40 – 70% probability of “extreme” impacts over the Klamath Mountains.
- This prolonged atmospheric river event poses significant risks of flooding, landslides, and travel disruptions across the Pacific Northwest and Northern California.
- Residents in affected areas should remain vigilant, adhere to local advisories, and prepare for potential evacuation orders in flood-prone zones.
Strong atmospheric river (AR) and remnants of a powerful Pacific cyclone, currently centered off the coast of British Columbia, Canada, are forecast to continue affecting the Pacific Northwest and Northern California until November 23.
In addition, a secondary surface cyclone, predicted to form off the Northern California coast on November 21, is expected to amplify impacts in the region, bringing additional widespread heavy precipitation and flooding risks.
The atmospheric river made landfall in the Pacific Northwest on November 19 and has gradually shifted south along the coast, according to the Center for Western Weather and Water Extremes (CW3E).
Strong winds and heavy rains were recorded across the region, resulting in the deaths of at least 2 people. Across the Pacific Northwest and B.C., Canada, an estimated 1.8 million people were left without power.
As of 11:20 UTC on November 21, 334 000 customers in Washington (estimated 835 000 people) and another 17 000 customers in Northern California (estimated 43 000 people) are still without power.
In their November 20 Quick Look update, CW3E reported AR Landfall tool shows over 95% confidence in moderate AR conditions (IVT >500 kg m⁻¹ s⁻¹) persisting over Northern California until November 22.
U.S. National Model (GEFS) model forecasts predict AR 3 – 4 conditions (moderate to strong) from the Bay Area to Southern Oregon, with several ensemble members indicating potential AR 5 (exceptional) conditions in Northern California.
The National Weather Service Weather Prediction Center (NWS/WPC) has issued a high-risk (70%) Excessive Rainfall Outlook (ERO) for Humboldt County from November 21 to early November 22, with moderate-risk (>40%) conditions across the Northern California Coast Ranges and Northern Sierra Nevada.
Total precipitation is expected to surpass 127 mm (5 inches) across much of Northern California, with localized totals exceeding 254 mm (10 inches) in the Coast Ranges, Klamath Mountains, and Northern Sierra Nevada.
CW3E forecasters said this storm could deliver over 30% of the normal water year precipitation to parts of Northern California.
“Impactful and for some place dangerous weather conditions will continue through early this weekend as two separate storm systems impact the Lower 48,” NWS forecaster Snell said on November 21.
The atmospheric river is expected to peak in intensity today but with moderate bouts of rain lingering through much of Friday and snow levels finally lowering somewhat on Saturday.
“In the meantime, dangerous flooding, rock slides, and debris flows are likely, which has prompted a High Risk (level 4/4) of Excessive Rainfall to be issued across the northern California coastline today. Be sure to check conditions before traveling and never drive across flooded roadways,” Snell said.
Rivers and streams in the region are rising, and the California Nevada River Forecast Center (CNRFC) anticipates at least two gauges to surpass the flood stage. Among them, the Eel River at Fernbridge is projected to reach moderate flood stage by midday on November 22.
Over 305 mm (12 inches) of snow accumulated in the Klamath Mountains within six hours on November 20 and the NWS forecasts an additional 457 – 610 mm (18 – 24 inches) of snowfall from November 20 – 23, resulting in hazardous travel conditions.
The NWS Winter Storm Severity Index indicates a 40 – 70% probability of “extreme” impacts over the Klamath Mountains.
References:
1 Quick Look at the Atmospheric River Impacting the US West Coast – CW3E – November 20, 2024
2 Short Range Forecast Discussion – NWS/WPC – November 21, 2024
I'm a dedicated researcher, journalist, and editor at The Watchers. With over 20 years of experience in the media industry, I specialize in hard science news, focusing on extreme weather, seismic and volcanic activity, space weather, and astronomy, including near-Earth objects and planetary defense strategies. You can reach me at teo /at/ watchers.news.
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