Mesoscale and Synoptic Scale Analysis of Narrow Cold Frontal Rainband During a Landfalling Atmospheric River in California During January 2021.

Narrow cold‐frontal rain bands (NCFR) often produce short‐duration and high‐intensity precipitation that can lead to flooding and debris flow in California (CA). On 27 January 2021, an atmospheric river (AR) associated with an intense surface cyclone made landfall over coastal northern CA, which featured a prominent NCFR. This study uses high‐resolution West Weather Research and Forecasting simulations to accurately resolve the gap and core structure of the NCFR and provide reliable precipitation estimations, compensating for limitations of radar and satellite observations. This NCFR was supported by robust synoptic‐scale quasi‐geostrophic (QG) forcing for ascent and frontogenesis. It propagated southward from Cape Mendocino to Big Sur in 12 hr before stalling and rotating counter‐clockwise in central/southern CA due to upstream Rossby wave breaking and an amplifying upper‐tropospheric trough. With the lower to middle tropospheric flow backed considerably to the south‐southwest over the NCFR, the increase of the vertical wind shear caused the transition from parallel to trailing stratiform precipitation. The stall and pivot of the AR and NCFR led to intense rainfall with a 2‐day precipitation accumulation greater than 300 mm over central CA. In addition, under the potential instability and frontogenesis, a moist absolutely unstable layer between 850 and 700 hPa was captured at the leading edge of the NCFR, which indicated slantwise deep layer lifting and high precipitation efficiency. This study reveals synoptic‐scale and mesoscale drivers of rainfall outside orographic lifting and reaffirms the importance of high‐resolution numerical modeling for the prediction of extreme precipitation and related natural hazards. Plain Language Summary: California often experiences short‐duration, high‐intensity rainfall associated with landfalling atmospheric rivers (ARs), which are long, thin corridors of moisture in the atmosphere. They can trigger post‐fire debris flows, shallow landslides, and flash flooding. This study examines characteristics of the high impact landfalling AR from January 2021 via high‐resolution model simulations. The landfalling AR, associated with an intense surface cyclone over the Northeast Pacific, moved southward through 27 January prior to stalling along the central California Coast on 28 January. Due to the stalling and pivoting of the AR, the coast of central and southern California experienced a long‐duration period of moderate precipitation, large‐scale forcing for ascent and short duration periods of intense precipitation. In addition, the intense precipitation along the cold front can be explained an effective dynamic lifting of a deep layer of atmosphere. This event caused a partial collapse of Highway 1 in Northern California and produced >375 mm of rainfall at Las Tablas, which led to a post‐fire debris flow ∼30 km south of Big Sur. High‐resolution weather modeling reveals the physical processes of precipitation and is necessary for the prediction of extreme precipitation and related natural hazards. Key Points: This atmospheric river caused sustained rainfall and short‐duration precipitation related to a narrow cold‐frontal rainbandThe narrow cold‐frontal rainband is mainly driven by synoptic‐scale quasi‐geostrophic forcing for ascent and frontogenesisHigh‐resolution modeling is necessary to improve the understanding and predictability of high‐intensity short‐duration precipitation [ABSTRACT FROM AUTHOR]