CW3E Event Summary: 29 December 2022 – 1 January 2023

January 4, 2023

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Atmospheric River Brings Record-Breaking Precipitation to California

• A strong atmospheric river (AR) made landfall over Northern California on 29 Dec
• AR 1/AR 2 conditions (based on the Ralph et al. 2019 AR Scale) were observed across much of coastal California
• While the initial pulse of moisture transport produced little precipitation, two additional stronger pulses produced heavy precipitation across portions of Northern and Central California on 30–31 Dec
• More than 5 inches of storm-total precipitation fell across the San Francisco Bay Area, while more than 10 inches fell over portions of the Sierra Nevada
• More than 3 feet of snow fell in the higher terrain of the Central and Southern Sierra Nevada
• Warm air associated with this AR limited snowfall accumulations in the Northern Sierra Nevada
• The combination of heavy rainfall and high antecedent soil moisture and streamflow conditions led to riverine and urban flooding
• Multiple levee breaks along the Cosumnes River caused major flooding on Highway 99 and led to water rescues and at least one fatality
• Heavy rain falling on nearly saturated soils triggered mudslides and rockslides in Northern and Central California
• Strong winds downed trees and power lines, resulting in numerous power outages in the Sacramento area
• The severe flooding along the Cosumnes River was poorly forecast at lead times < 48 hours due to large errors in precipitation forecasts, which substantially underestimated the observed precipitation in the Upper Cosumnes watershed

Click images to see loops of NAM IVT/IWV analyses Valid 0000 UTC 29 December 2022 – 0000 UTC 2 January 2023

Summary provided by C. Castellano, S. Bartlett, J. Kalansky, S. Roj, and F.M. Ralph; 4 January 2023

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*Summary products are considered experimental