Author: cw3e-web

CW3E AR Update: 3 January 2023 Outlook

January 3, 2023

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Atmospheric River to Bring Significant Rainfall to Northern and Central California

• An atmospheric river in association with a strong low-pressure system over the North Pacific will bring significant precipitation to Northern California between Wednesday morning and Thursday afternoon
• IVT associated with this AR will exceed 750 kg m-1 s-1 along the coast of Northern California, making this an AR 3 based on the Ralph et al. 2019 AR Scale
• This AR will be the first of three landfalling ARs over the next 7 days, with the following two ARs also forecast to impact Northern and Central California with additional precipitation over the weekend and into early next week
• The NWS Weather Prediction Center (WPC) has forecast 7-day precipitation totals to exceed 15 inches along the coast of Central California and the Northern Sierra Nevada
• NWS WPC has issued a moderate risk for excessive rainfall with the potential for flash flooding along much of coastal California over the coming days, highlighting the enhanced risk due to saturated antecedent soil conditions
• Precipitation forecasts over the next 7 days for watersheds in northern California and the Northern Sierra are between 6–8 inches, with the ECMWF forecasting higher precipitation totals along the coast and less in the Sierra Nevada compared to the GFS
• NWS Weather Forecast Offices have begun issuing watches and warnings for flood, high wind, and winter weather hazards in association with this AR
• The NWS California Nevada River Forecast Center (CNRFC) has forecast multiple rivers to exceed flood stage in association with the precipitation from this AR
• Stay alert to official NWS forecasts, watches, and warnings at weather.gov and follow guidance from local emergency management officials

Click images to see loops of GFS IVT and IWV forecasts Valid 1200 UTC 3 January – 1200 UTC 10 January 2023

Summary provided by S. Bartlett, S. Roj, C. Castellano, J. Kalansky, and F. M. Ralph; 03 January 2023

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CW3E AR Update: 20 December 2022 Outlook

December 20, 2022

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Potential for Multiple Atmospheric River Landfalls Over the US West Coast

• A series of atmospheric rivers (ARs) is forecast to move across the Northeast Pacific and make landfall along the West Coast of North America during 22–27 Dec. Per Fish et al. (2019), these types of multi-AR events, known as “AR families,” are not uncommon during boreal winter.
• Current model forecasts suggest that AR landfall during this period is most likely to occur over the Pacific Northwest and British Columbia, but there is considerable uncertainty in the timing, location, and intensity of these ARs
• Models are also showing the potential for landfalling AR activity in California during 28–31 Dec, but there is lower forecast confidence given the longer lead times
• The 00Z GEFS is showing a high likelihood of AR 3 or greater conditions (based on the Ralph et al. 2019 AR Scale) over coastal Oregon during the next 7 days
• Some locations are forecast to experience AR conditions for more than 96 consecutive hours across multiple AR landfalls
• These ARs are forecast to produce more than 5 inches of total precipitation over the Pacific Coast Ranges and Cascades during the next 7 days, with more than 10 inches possible in the Olympic Mountains
• There are large model differences in forecast precipitation, with the 00Z ECMWF forecasting lower precipitation amounts in western Washington and higher precipitation amounts near the Oregon/California border compared to the 00Z GFS

Click images to see loops of GFS IVT & IWV forecasts Valid 0000 UTC 20 December – 0000 UTC 28 December 2022

Summary provided by C. Castellano, S. Bartlett, J. Kalansky, S. Roj, J. Rutz, and F. M. Ralph; 20 December 2022

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CW3E S2S Outlook: 16 December 2022

December 16, 2022

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Summary provided by J. Wang, C. Castellano, M. DeFlorio, and J. Kalansky; 16 December 2022

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CW3E Event Summary: 9-12 December 2022

December 14, 2022

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Atmospheric River with Strong Dynamical Forcing Produced Heavy Precipitation Throughout California

• An upper-level trough over the Gulf of Alaska on 9 December deepened along the US West Coast producing a landfalling atmospheric river (AR) and heavy orographic precipitation
• The landfalling AR was ranked as an AR2 (based on the Ralph et al. 2019 AR Scale) along the Oregon coast and an AR1 along the Central California coast
• The deterministic GFS struggled to forecast the AR location, magnitude, and orientation at lead times > 3 days
• The storm produced heavy precipitation along the Central California coast between the San Francisco Bay area and Santa Barbara
• Mining Ridge received 12.80 inches of precipitation over a 72-hour period ending 9 AM 12 Dec; an R-Cat 2 according to the Ralph and Dettinger (2012) classification
• The largest snowfall totals (> 6 feet) were observed across the higher terrain of the Sierra Nevada with the Klamath mountains receiving 1-3 feet of snowfall
• CW3E’s surface meteorology station at Seven Oaks Dam observed 1 inch of rain during the event. Approximately 20% of water year normal precipitation has fallen at the station
• Recent storms in California have been cold, efficient snow producers leading to a discrepancy between water year to date precipitation and current snowpack in the Sierra Nevada
• The AR resulted in multiple rainfall-related swift water rescues, snowfall-related road closures, and dangerous backcountry avalanche conditions
• As part of CW3E’s ongoing Atmospheric River Reconnaissance research field campaign, two flights originating from Hawaii sampled atmospheric conditions upstream of this AR in the North Pacific

Click images to see loops of NAM IVT/IWV analyses Valid 0000 UTC 8 December – 1800 UTC 12 December 2022

Summary provided by Shawn Roj, Samuel Bartlett, Matthew Steen, Jay Cordeira, Jon Rutz, Chris Castellano, J. Kalansky, B. Kawzenuk, F.M. Ralph; 14 Dec 2022

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