An Unusual Case of Rapid Cyclogenesis in the Northeast Pacific Basin. Overview and Piecewise PV Inversion

Abstract A case of extremely rapid, record-setting extratropical cyclogenesis over the northeast Pacific Ocean in late November 2019 is examined. The development is of particular interest as much of the strengthening occurred in an unusual environment characterized by cold sea surface temperatures. Cyclogenesis began as a stationary upstream surface cyclone in the north-central Pacific ushered warm, moist tropical air poleward toward a preexisting surface frontal boundary, resulting in intense lower-tropospheric frontogenesis. The resulting thermally direct vertical circulation mobilized a diabatic Rossby wave (DRW) which moved eastward along the baroclinic zone. An intensifying upper-level jet/front system draping equatorward from Alaska became favorably aligned with the low-level DRW on its approach toward the California–Oregon border to force deepening rates as high as 6 hPa h −1 prior to landfall. The 3D Ertel potential vorticity (PV) structure associated with this storm is partitioned into separate upper-tropospheric, lower-tropospheric, and diabatically induced anomalies which are separately inverted to recover the flow associated with each piece. Analysis of this partitioned PV reveals that development followed a bottom-up sequence by which near-surface PV dominated early cyclogenesis, diabatically induced PV dominated a large period of subsequent intensification, and upper-tropospheric PV dominated the final period of development. Bottom-up developments of this intensity are rare. It is shown that diabatic influences in response to vigorous latent heat release are responsible for much of the lower-tropospheric cyclogenesis with an upper-level jet/front system becoming an important driver for the rapid cyclogenesis observed immediately before landfall. Significance Statement A rapidly developing low pressure system over the northeast Pacific Ocean in late November 2019 set all-time low pressure records and occurred in an unusual region of the world. The analysis shows that this development occurred from the bottom-up and midtropospheric latent heat release was the most important process leading to its record strength. It is very uncommon for low pressure systems of this intensity to follow a bottom-up development. More work is needed to determine how the upper- and lower-tropospheric features interacted with each other as they conspired to produce this record-setting low pressure system.