The Role of the North Pacific Jet in Two West Coast Atmospheric River Events during February 2017

In February of 2017, California experienced a billion-dollar disaster due to the extreme precipitation induced by a series of landfalling atmospheric rivers (ARs). This heavy precipitation resulted in extensive property damage and placed substantial pressure on the Oroville Dam, resulting in a multiday evacuation. ARs not only may have significant economic impacts, but they also may contribute disproportionately to precipitation anomaly statistics along the west coast of the U.S. Furthermore, recent research suggests that the frequency of landfalling ARs along the west coast of the U.S. may increase in a warmer climate. In view of these considerations, it is important to diagnose the physical processes and the antecedent environments that govern landfalling ARs in order to improve medium-range (i.e., 6−10 day) forecasts of extreme precipitation events along the west coast of the U.S.

This study examines a series of landfalling ARs that impacted the west coast of the U.S. on 4–10 and 15–21 February 2017. ARs are identified using two metrics: (1) total water vapor flux exceeding 5 × 10⁸ kg s⁻¹ over a depth of 3 km, and (2) the length and width of the corridor of total water vapor flux greater than or equal to 2000 km and less than or equal to 1000 km, respectively, such that the aspect ratio is greater than or equal to 2. Following the identification of the ARs, the large-scale upper-tropospheric flow pattern prior to their landfall will be examined in the context of the two leading EOFs of 250-hPa zonal wind over the North Pacific during September–May 1979–2014 in the CFSR. The first EOF corresponds to a zonal extension or retraction of the exit region of the North Pacific Jet (NPJ), while the second EOF corresponds to a poleward or equatorward shift of the exit region of the NPJ. The projection of 250-hPa zonal wind anomalies at one or multiple times onto these two leading EOFs subsequently provides an objective characterization of the instantaneous state or evolution of the upper-tropospheric flow pattern over the North Pacific, which may be represented on a two-dimensional phase diagram. The calculation of the two leading EOFs for the two landfalling ARs during February 2017 demonstrates that the NPJ is substantially retracted 5–7 days prior to the development of the ARs and that the NPJ rapidly undergoes a zonal extension prior to AR landfall.