Linked Extreme Weather Events over the Western United States during February 2019 Resulting from North Pacific Rossby Wave Breaking

Abstract:

February 2019 was a stormy and cold month over much of the western CONUS. Record-breaking snowstorms and rainstorms were observed in parts of Washington, Oregon, California, and Arizona during this month. The upper-level flow pattern over the North Pacific Ocean was characterized by a retracted western Pacific jet stream, anticyclonic wave breaking (AWB) over the eastern Pacific, and a persistent northern storm track from the central Pacific to Alaska. Individual cyclones moving northeastward along this storm track supported sequential ridge building events, strong warm-air advection, a parade of moisture-laden atmospheric rivers, and frequent heavy precipitation. A deep trough downstream of the region of AWB enabled Kona low development near Hawaii, subtropical jet stream (STJ) formation, and an active southern storm track from near Hawaii toward the West Coast. The purpose of this presentation is to show how the formation of this storm track enabled the occurrence of multiple extreme weather events over the western CONUS associated with individual northeastward-moving cyclones.

Kona low formation near Hawaii occurred along the southwestern end of a deep, positively tilted trough over the eastern Pacific. This deep trough allowed anomalously cold air to overspread the Pacific Northwest, California, and the subtropical eastern Pacific. A prominent baroclinic zone at the southern end of this deep trough along with a strong STJ allowed frequent landfalling atmospheric rivers (ARs) on the West Coast and supported widespread inland storminess. The interaction of eastward-moving moisture-laden air masses associated with the STJ and midlatitude upper-level baroclinic disturbances moving southward in the deep trough off the West Coast resulted in the occurrence of multiple extreme weather events (EWEs) over the western CONUS during a two-week period in February 2019. Notable EWEs included a record-breaking snowstorm in Seattle, WA, on 9 February, the biggest snowstorm in Redding, CA in 50 years on 13 February, the third wettest day on record in Palm Springs, CA, on 14 February, a record-breaking snowstorm in Flagstaff, Arizona on 24–25 February, and a record-breaking snowstorm in Oregon on 25 February. We will show how these individual EWEs can be related to the configuration of the large-scale North Pacific flow.