Heavy Cold-Season Precipitation in the Northwestern United States: Synoptic Climatology and an Analysis of the Flood of 17–18 January 1986

Abstract

Warm, moist southwesterly airflow into the northwestern United States during the cold season can result in rapid snowmelt and flooding. The objectives of this research are to document characteristic synoptic flow patterns accompanying cold-season (November–March) flooding events, and isolate flow anomalies associated with the moisture transport during a representative event. The first objective is accomplished through a 46-case composite spanning the years 1962–88; the second objective is addressed through diagnosis of a flooding event that occurred on 17–18 January 1986. The 46-case composite is constructed for a 6-day period centered at 1200 UTC on the day of heavy precipitation onset (denoted τ₀). Composite 500-hPa geopotential height anomaly fields reveal anomalous ridging over the Bering Sea preceding the precipitation event, a negative anomaly over the Gulf of Alaska throughout the composite evolution, and a positive anomaly over the southwestern Unites States and adjacent eastern Pacific Ocean during and after the event. The gulf trough and southwestern ridge lead to enhanced southwesterly geostrophic flow into the northwestern United States at τ₀. A positive temperature anomaly at the 850-hPa level advances northeastward into the northwestern United States by τ₀, and expands over much of the United States by τ₊₄₈. Piecewise geostrophic moisture transport computations for 17–18 January 1986, based on quasigeostrophic potential vorticity inversion, demonstrate that the transport of moisture into the northwestern United States is largely associated with a duo of mobile cyclones that track from the subtropical Pacific Ocean toward British Columbia. There is also a smaller contribution from a stationary anticyclone over the southwestern United States. These results indicate that the role of the planetary-scale flow, as depicted in the composite analyses, is to provide a persistent storm track, while the moisture flow within this storm track is modulated by cyclone-scale dynamics.