CW3E AR Update: 17 March 2023 Outlook

March 17, 2023

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Atmospheric River Forecast to Bring Precipitation to Southern California

• An atmospheric river (AR) developing in the Eastern Pacific is forecast to make landfall in Southern California on 20 March and persist in the region until 21 March
• AR2 conditions (based on the Ralph et al. 2019 AR Scale) are forecast near San Diego, CA with AR1 conditions possible northward to San Luis Obispo, CA
• AR3 conditions are forecast in Southern Arizona as a result of the inland penetration of this AR
• There is still uncertainty between the GEFS and ECMWF EPS in the timing and intensity of AR conditions, including direction of IVT, which is leading to uncertainty in watershed precipitation forecasts over the next 7 days
• The 00Z ECMWF is forecasting 3.10 inches of precipitation over the Santa Ana watershed over the next 7 days, while the 00Z GFS is forecasting 1.81 inches; a difference of 1.29 inches
• The NWS Weather Prediction Center (WPC) is forecasting >1.5 inches of precipitation for Southern California coastal regions, >3 inches in the San Gabriel Mountains, and 1 inch over Central Arizona
• Experimental excessive rainfall outlooks have been issued by the WPC for 21 March with a marginal risk (at least 5%) of rainfall exceeding flash flood guidance for much of Southern California and most of the Central Valley as well as Central Arizona

Click images to see loops of GFS IVT and IWV forecasts Valid 0000 UTC 17 March – 1200 UTC 22 March 2023

Summary provided by M. Steen, S. Roj, and S. Bartlett; 17 March 2023

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*Outlook products are considered experimental