Author: Chris Castellano

CW3E AR Update: 10 March 2025 Outlook

CW3E AR Update: 10 March 2025 Outlook

March 10, 2025

Click here for a pdf of this information.

Multiple Atmospheric Rivers Forecast to Impact US West Coast This Week into This Weekend

  • A weak atmospheric river (AR) is forecast to make landfall over coastal Oregon late Tue 11 Mar.
  • As the upstream trough amplifies off the California coast, the AR is forecast to intensify over California, potentially bringing moderate AR conditions (IVT ≥ 500 kg m−1 s−1) to the Bay Area and Sacramento Valley.
  • A second and potentially stronger AR is forecast to make landfall over the US West Coast this weekend in association with a strong low-pressure system.
  • The GEFS control member is forecasting an AR 1 (based on the Ralph et al. 2019 AR Scale) over the Bay Area in association with the first AR.
  • While moisture will be somewhat limited with these ARs, strong dynamical forcing and a favorable direction of moisture transport will likely facilitate heavy precipitation over the US West Coast.
  • The first AR is forecast to produce 2–5 inches of precipitation in the Northern California Coast Ranges, Sierra Nevada, and eastern Transverse Ranges, and 1–3 inches of precipitation elsewhere in coastal California.
  • The NWS Weather Prediction Center (WPC) has issued a slight risk excessive rainfall outlook (ERO) for the Transverse Ranges and a marginal risk ERO for the rest of coastal California, the Sacramento Valley, and the Sierra Nevada foothills.
  • Low freezing levels during the first AR will facilitate heavy snowfall accumulations (at least 12–36 inches) above 4,000 feet throughout the Sierra Nevada. Major-to-extreme winter storm impacts are expected in these areas.
  • Model differences in the evolution of the second AR and associated upstream trough are leading to large differences in forecast precipitation over California during the next 10 days. The EPS is favoring much higher precipitation totals over coastal California and the Sierra Nevada compared to GEFS.

Click images to see loops of GFS IVT and IWV forecasts

Valid 1200 UTC 10 March 2025 – 0000 UTC 18 March 2025

Summary provided by C. Castellano and M. Steen; 10 March 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.

CW3E Subseasonal Outlook: 4 March 2025

CW3E Subseasonal Outlook: 4 March 2025

March 4, 2025

Click here for a pdf of this information.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Summary provided by Z. Yang, C. Castellano, J. Wang, M. DeFlorio, and J. Kalansky; 4 March 2025

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*Outlook products are considered experimental

CW3E Event Summary: 22-25 February 2025

CW3E Event Summary: 22-25 February 2025

4 March 2025

Click here for a pdf of this information.

Multiple Atmospheric Rivers Produce Heavy Rain and Flooding in the Pacific Northwest

The ARs:

  • The first AR made landfall on Sat 22 Feb and produced AR 2–3 conditions (based on the Ralph et al. 2019 AR Scale) in southern coastal Washington and coastal Oregon.
  • Significant inland penetration of the first AR also brought AR 2 conditions to interior portions of the northwestern US.
  • The second AR made landfall on Mon 24 Feb in association with a strong low-pressure system and produced AR 1–2 conditions in coastal Washington and Oregon.

Impacts:

  • At least 5–10 inches of total precipitation fell over much of coastal Washington and Oregon as well as the Cascades.
  • Unusually warm temperatures and high freezing levels created favorable conditions for snowmelt and rain-on-snow during the first AR in the Cascades and interior portions of the Pacific Northwest.
  • The combination of snowmelt and heavy rainfall led to significant riverine flooding in eastern Washington, northern Idaho, and eastern Oregon.
  • Strong winds during the second storm caused widespread tree damage and power outages in western Washington and northwestern Oregon.

MIMIC-TPW2 Total Precipitable Water

Valid: 1200 UTC 21 February – 1200 UTC 25 February 2025

Click images to see loops of GFS IVT/IWV analyses

Valid 1200 UTC 21 February – 1200 UTC 25 February 2025


 

 

 

 

 

 

 

 

 

 

 

 

 

Summary provided by C. Castellano, S. Roj, and J. Kalansky; 4 March 2025

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

CW3E Event Summary: 31 January – 5 February 2025

CW3E Event Summary: 31 January – 5 February 2025

12 February 2025

Click here for a pdf of this information.

Long-Duration Atmospheric River Produces Heavy Precipitation and Flooding in Northern California

The ARs:

  • A weak atmospheric river (AR) associated with a tropical moisture export (TME) made landfall over Washington, Oregon, and Northern California late 30 Jan.
  • A second AR associated with a much stronger TME made landfall early 1 Feb and stalled over Northern California for several days before finally dissipating early 5 Feb.
  • An AR 4 (based on the Ralph et al. 2019 AR Scale) was observed in coastal Northern California, with some locations experiencing > 100 hours of continuous AR conditions.
  • An AR 3 was also observed in northeastern California and northwestern Nevada due to inland penetration of the second AR.
  • CW3E’s AR Reconnaissance field campaign carried out eight IOPs that sampled both ARs, as well as nearby essential atmospheric structures and regions of forecast sensitivity.

Impacts:

  • At least 6–15 inches of total precipitation fell over the Northern California Coast Ranges, Klamath Mountains, southern Cascades, and Northern Sierra Nevada. A few stations recorded > 20 inches of precipitation.
  • While much of the precipitation in California fell as rain due to high freezing levels, these ARs produced an estimated 2–6 feet of snow in the highest elevations of the Klamath Mountains, southern Cascades, Northern Sierra Nevada, and Rocky Mountains in Central Idaho.
  • Heavy rainfall over multiple days caused riverine flooding in Northern California. The Pit River near Canby exceeded major flood stage.
  • The National Weather Service received numerous reports of landslides, debris flows, and flooding in Northern California.
  • Heavy snow and strong winds caused hazardous travel conditions in the higher terrain.

MIMIC-TPW2 Total Precipitable Water

Valid: 1200 UTC 29 January – 1200 UTC 5 February 2025

Click images to see loops of GFS IVT/IWV analyses

Valid 1200 UTC 29 January – 1200 UTC 5 February 2025


 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Summary provided by C. Castellano, S. Bartlett, L. DeHaan, S. Roj, M. Steen, S. Yoon, and J. Kalansky; 12 February 2025

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

CW3E AR Update: 10 February 2025 Outlook

CW3E AR Update: 10 February 2025 Outlook

February 10, 2025

Click here for a pdf of this information.

Atmospheric River Forecast to Impact California This Week

  • A weak pulse of moisture transport is forecast to move onshore late Tue 11 Feb and bring light precipitation to coastal Central and Southern California.
  • A stronger atmospheric river (AR) associated with a low-pressure system is forecast to bring heavy rain and mountain snow to much of California late Wed 12 Feb through Fri 14 Feb.
  • Ensemble and deterministic models are still showing some subtle differences in both the initial moisture pulse and the subsequent AR.
  • The GEFS control member is forecasting an AR 2 (based on the Ralph et al. 2019 AR Scale) over coastal Central California and an AR 1 over coastal Southern California, whereas the EPS control member is forecasting an AR 3 over San Diego County.
  • Storm-total precipitation amounts of 5–10 inches are possible in the Sierra Nevada, Central California Coast Ranges, and Southern California Transverse Ranges.
  • The heaviest precipitation and greatest impacts from this AR are expected Thu 13 Feb into Fri 14 Feb. The NWS Weather Prediction Center (WPC) has issued a slight risk excessive rainfall outlook (ERO) for much of coastal California, the Central/Southern Sierra Nevada, and portions of the Central Valley.
  • Compared to the deterministic GFS, the deterministic ECMWF is forecasting lower precipitation amounts over the Sierra Nevada and higher precipitation amounts over the Central California Coast Ranges and Southern California.
  • Heavy snowfall is also likely in the higher terrain of the Sierra Nevada, with >4 feet possible in the Central and Southern Sierra Nevada. Major-to-extreme winter storm impacts are likely in these areas.

Click images to see loops of GFS IVT and IWV forecasts

Valid 1200 UTC 10 February 2025 – 1200 UTC 15 February 2025

Summary provided by C. Castellano, J. Kalansky, and M. Steen; 10 February 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.