Climatological Perspective and Synoptic Analysis of Landfalling Atmospheric Rivers in California During December 2022 and January 2023

The period between 26 December 2022 and 17 January 2023 featured several Pacific winter storms that led to nine landfalling atmospheric rivers (ARs) in California. These nine ARs produced a 21-day time-integrated integrated vapor transport (IVT) magnitude into northern California that exceeded all other 21-day periods in the 1959-present ECMWF ERA5 period of record, surpassing previous active periods that occurred during February 1986 and December/January 2005/2006. The full 23-day period featured rainfall records at Oakland (18.33”), San Francisco (15.28”), and Stockton (10.79”) with a maximum of 47.74” observed near Honeydew, California and 240” of snow at Mammoth Mountain. The development of the nine ARs was facilitated by a regime transition over the North Pacific in mid-December that favored the establishment of a zonally extended North Pacific jet stream, repeated cyclogenesis, a persistent corridor of enhanced low-level frontogenesis, and the poleward transport of water vapor over the Northeast Pacific. On the mesoscale, several of the landfalling ARs featured or occurred coincidentally with mesoscale frontal waves, narrow cold frontal rainbands, the Sierra Barrier jet, and convection that influenced precipitation intensity, precipitation distributions, predictability, and a myriad of hazardous weather. This presentation will (1) further place the events of late December 2022 and early January 2023 into climatological perspective within an AR and hydrometeorological framework; (2) illustrate the unique persistent large-scale flow associated with the zonally extended North Pacific jet stream; and (3) document the unique synoptic and mesoscale characteristics of the nine landfalling ARs and their tandem beneficial and hazardous impacts to California.