Moisture Pathways into the US Intermountain West Associated with Heavy Winter Precipitation Events

Abstract:

Two methods were used to identify the paths of moisture transport that reach the US intermountain west (IMW) during heavy precipitation events in winter. In the first, the top 150 precipitation events at stations located within six regions in the IMW were identified, then back trajectories were initiated at 6 hour intervals on those days at the four CFSR grid points nearest the stations. The second method identified the leading patterns of integrated water vapor transport (IVT) using the three leading Empirical Orthogonal Functions (EOFs) of IVT over land that were first normalized by the local standard deviation. The top 1% of the associated 6-hourly time series were used to construct composites of IVT, atmospheric circulation and precipitation. The results from both methods indicate that moisture originating from the Pacific that leads to extreme precipitation in the IMW during winter take distinct pathways through gaps in the Cascade (Oregon-Washington), Sierra-Nevada (California) and Peninsula Mountains (southern California through Baja California). The moisture transported along these routes appears to be the primary source for heavy precipitation for the mountain ranges in the IMW. The synoptic conditions associated with the dominant IVT patterns include a trough ridge couplet at 500 h PA, with the trough located northwest of the ridge where the associated circulation funnels moisture from the west-to-southwest through the mountain gaps and into the IMW.