An Overview of the Dynamical Processes and Thermodynamic Drivers that Contributed to the Onset of Highly Impactful Heavy Rains in California during Winter 2022–2023

Drought-alleviating heavy and highly impactful rainfall occurred across California in late December 2022 and early January 2023. This highly impactful heavy rainfall was not anticipated in the subseasonal winter 2022–2023 outlook issued by the NOAA Climate Prediction Center in late October 2022. Noteworthy features that contributed to the heavy rainfall in California included: (1) high latitude Rex block across Alaska and the Chukchi Sea, (2) repeated cold surges from Asia into the western North Pacific (WPAC), (3) frequent cyclogenesis over the WPAC, (4) frequent eastward-directed trans-North Pacific (NPAC) subtropical atmospheric rivers (ARs), and (5) and the eventual formation of a trans-NPAC subtropical jet stream (STJ). These features collectively contributed to the formation of a sustained “fire hose” of deep tropical moisture directed toward California beginning in the latter part of December 2022 and continuing into the first part of January 2023. This presentation will provide an overview of how aforementioned noteworthy features and physical processes enabled the onset of sustained drought-alleviating heavy rainfall California in the latter part of December 2022.

The formation of a Rex block over Alaska enabled a series of amplified 500-hPa troughs to move eastward from eastern Asia into the WPAC equatorward of this block. Cold surges behind amplified, eastward-propagating WPAC troughs that reached the CPAC facilitated an eastward extension of the NPAC STJ. One of these eastward-propagating CPAC troughs deepened southeastward and developed into a very strong cutoff cyclone located near 30 N and 160 W. Collectively, these eastward-moving CPAC troughs enabled tropical moisture associated with them to surge northeastward along trans-NPAC ARs and reach the West Coast of North America by 19–20 December 2022. The associated trans-NPAC STJ served as a conduit for successive eastward-propagating trans-NPAC trough passages and associated ARs that produced episodic heavy rainfall in California.

Three testable hypotheses for the impactful California heavy rainfall that began in the latter part of December 2022 are as follows: 1) periodic Asian cold surges into the WPAC favor enhanced baroclinicity over the western NPAC that results in the formation of a strong trans-NPAC STJ, (2) the creation of a trans-NPAC STJ enables tropical moisture from the southwestern NPAC to reach California, and (3) the formation of a diffluent trans-NPAC STJ-exit region flow pattern in the EPAC creates a favorable environment for the formation of an axis of dilatation along the West Coast that serves as a locus of moisture flux convergence and low-level frontogenesis. The first and third hypotheses are supported by a strong time-mean NPAC jet stream with a diffluent jet-exit region near California. The second hypothesis is supported by anomalous tropical moisture that extends from the southwest NPAC toward California. This presentation will focus on the relative contributions of the physical processes mentioned in these three hypotheses to drought-alleviating heavy rainfall in California during winter 2022–2023 onset.