Author: Chris Castellano

CW3E AR Update: 8 December 2025 Outlook

CW3E AR Update: 8 December 2025 Outlook

December 8, 2025

Click here for a pdf of this information.

Major Atmospheric River to Bring Heavy Rain and Flooding to the Pacific Northwest

  • An atmospheric river (AR) has made landfall over the Pacific Northwest early today and will continue to impact the region through Thu 11 Dec.
  • After the initial AR landfall today, a second and potentially stronger pulse of moisture transport will move onshore late Tue 9 Dec/early Wed 10 Dec.
  • CW3E’s West-WRF ensemble is showing >90% probability of an AR 4 or greater between the Olympic Peninsula and central coastal Oregon, and >50% probability of an AR 5 over much of coastal Washington.
  • An AR 3/AR 4 is also forecast in the western Cascade foothills and in central Washington and Oregon due to substantial inland penetration of moisture transport.
  • The NWS Weather Prediction Center (WPC) has issued a slight risk of excessive rainfall across much of western Washington and northwestern Oregon today through early Thu 11 Dec.
  • More than 10 inches of total precipitation are currently forecast in portions of the Washington Cascades over the next 5 days, with more than 5 inches possible in the lowlands of western Washington.
  • Multiple days of heavy rainfall and relatively high snow levels will likely result in significant riverine flooding, especially west of the Cascades, where 10 stream gages are currently forecast to reach major flood stage.
  • While much of the precipitation is expected to fall as rain, areas above 5,000 feet in the Washington Cascades are forecast to receive >2 feet of total snowfall.
  • As part of the CW3E-led Atmospheric River Reconnaissance program, flights are being planned and conducted to sample this AR in partnership with the US Air Force 53rd Weather Reconnaissance Squadron.

Click images to see loops of GFS IVT and IWV forecasts

Valid 1200 UTC 8 December 2025 – 1200 UTC 13 December 2025

Summary provided by C. Castellano, S. Bartlett, J. Kalansky, and M. Warner; 8 December 2025

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*Outlook products are considered experimental

For any unfamiliar terms, please refer to the American Meteorological Society Glossary.

CW3E Publication Notice: Lightning Characteristics of Atmospheric Rivers over the Americas Observed by GOES-16

CW3E Publication Notice

Lightning Characteristics of Atmospheric Rivers over the Americas Observed by GOES-16

November 26, 2025

CW3E collaborators Bin Guan (UCLA), Duane Waliser (NASA/JPL), along with CW3E director Marty Ralph and researcher Minghua Zheng, recently published an article in the American Geophysical Union’s Geophysical Research Letters, titled “Lightning Characteristics of Atmospheric Rivers Over the Americas Observed by GOES-16.” The research was mainly supported by NASA, with additional support from the California Department of Water Resources. The work stemmed from a NASA-supported 1-year pilot study in 2018–2019 conducted by the same team of researchers.

Figure 1. (Left column) Mean flash density [(100 km)−2 h−1] at each grid cell during wet days in (a) November–March, (b) May–September, and (c) all months for the period of 2020–2023. (Center column) Same as the left column but for AR wet days. (Right column) The ratio of the center column to the left column, where hatching indicates locations with AR frequency below 5%. Wet days are defined as the days with >0.1 mm precipitation. In all panels, the values were smoothed by a 9-point smoother before being plotted. In the bottom-right panel, the green circles mark the three locations further analyzed. From Figure 3 in Guan et al. (2025).

Using lightning observations from the Geostationary Operational Environmental Satellite (GOES)-16 and an atmospheric river (AR) database derived from the MERRA-2 reanalysis, the new study investigates the influence of ARs on lightning and its seasonal and geographical variations. This collaborative effort supports the “Novel Observations” priority among the four identified in the CW3E 2025-2029 Strategic Plan.

Results show that, compared to the annual AR frequency of below 15% among all days, AR frequency is above 30% during lightning days in a number of locations. Also, compared to all wet days, mean flash density during AR wet days can be higher by a factor of 2 or more depending on the locations (Figure 1). Although long-lived, traveling ARs have the strongest moisture transport, short-lived ARs are found to be associated with the highest flash density. The study demonstrates that the new-generation GOES observations, in particular the Geostationary Lightning Mappers, provide useful information for better monitoring and further understanding AR structure and impacts on hemispheric scales.

By improving our understanding of the broader impacts of ARs, this study contributes to a growing number of efforts highlighting that AR-related hazards extend beyond the traditional focus on precipitation and flooding.

Guan, B., Waliser, D. E., Ralph, F. M., & Zheng, M. (2025). Lightning characteristics of atmospheric rivers over the Americas observed by GOES-16. Geophysical Research Letters, 52(22), e2025GL118477. https://doi.org/10.1029/2025GL118477

CW3E AR Update: 10 November 2025 Outlook

CW3E AR Update: 10 November 2025 Outlook

November 10, 2025

Click here for a pdf of this information.

Atmospheric River Forecast to Bring Heavy Rain and Snow to California This Week

  • An amplifying shortwave south of Alaska is forecast to interact with a cutoff low and a region of poleward moisture transport in the subtropical Northeast Pacific, eventually leading to the formation of a strong atmospheric river (AR) along the US West Coast late Wed 12 Nov.
  • This AR is forecast to bring strong AR conditions (IVT >750 kg m−1 s−1) to the Bay Area and then quickly move through southern California on Thu 13 Nov.
  • The GEFS and ECMWF ENS control members are forecasting an AR 2/AR 3 (based on the Ralph et al. 2019 AR Scale) over portions of coastal northern and central California. Overall, the ECMWF ENS is forecasting a higher peak IVT magnitude and a slightly longer duration of AR conditions over much of coastal California.
  • Marginal risk excessive rainfall outlooks (EROs) have been issued for the Klamath Mountains, Northern Sierra Nevada, Northern California Coast Ranges, and much of coastal central and southern California due to the potential for high rainfall rates in the core of the AR.
  • The NWS Weather Prediction Center is forecasting 2–4 inches of storm-total precipitation in portions of the Klamath Mountains, southern Cascades, Sierra Nevada, California Coast Ranges, and Transverse Ranges, and 1–2 inches elsewhere in coastal California and the Central Valley.
  • Decreasing freezing levels will likely facilitate significant snowfall accumulations in the higher terrain of the central and southern Sierra Nevada. Moderate winter storm impacts are expected in these areas.
  • These ARs will likely bring beneficial precipitation to areas that are currently experiencing moderate-to-severe drought in southern California.

Click images to see loops of GFS IVT and IWV forecasts

Valid 0000 UTC 10 November 2025 – 0000 UTC 15 November 2025

Summary provided by C. Castellano, S. Bartlett, J. Kalansky, B. Moore, and M. Steen; 10 November 2025

To sign up for email alerts when CW3E post new AR updates click here.

*Outlook products are considered experimental

For any unfamiliar terms, please refer to the American Meteorological Society Glossary.