CLIMATOLOGICAL PERSPECTIVE AND SYNOPTIC ANALYSIS OF LANDFALLING ATMOSPHERIC RIVERS IN CALIFORNIA DURING DECEMBER 2022 AND JANUARY 2023 (Invited Presentation)

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 European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric reanalyses of the global climate (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 cyclone development, and a persistent corridor of enhanced low-level poleward transport of water vapor over the Northeast Pacific. Several of the landfalling ARs occurred coincidentally with meteorological features such as mesoscale frontal waves, narrow cold frontal rain bands, the Sierra Barrier jet, and convection, which are known to affect precipitation intensity, precipitation distributions, and predictability.

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 meteorological characteristics of the nine landfalling ARs and their tandem beneficial and hazardous impacts to California.