Climatology of the Snake River Plain Convergence Zone

Thomas Andretta

National Weather Service

Pocatello-Idaho Falls, ID

The climatology of Snake River Plain Convergence Zone (SPCZ) is examined using observational and high resolution model data sets. This convergence zone is confined to the planetary boundary later and is formed by the merger of low-level north to northeast winds in the Upper Snake River Plain with low-level southwest to west winds in the eastern Magic Valley and Lower Snake River Plain of eastern Idaho. The SPCZ is initiated by the passage of a synoptic scale cold front which aligns the flow to a northwest direction through the Big Lost, Little Lost, and Birch Creek Valleys in central Idaho. This presentation examines a six-year climatology of this mesoscale feature based on mean antecedent synoptic characteristics obtained from 27 SPCZ cases covering the period November 1995 to November 2001. The prevailing wind patterns for different stations in the path of the mesoscale boundary are analyzed. The evolution and structure of the SPCZ is documented for Type A and Type B synoptic patterns based on wind and reflectivity signatures. Finally, a multiple variable linear regression formula was derived to calculate (from MESO ETA model parameters) the amplitude (Z coefficient) of the convergence, probability of occurrence, and estimates of the precipitation and snowfall for Idaho Falls and Pocatello. The scheme was applied to an SPCZ episode on 04 January 2002. The MESO ETA and MM5 model fields provided a realistic simulation of the event with these results: Z coefficient of approximately 1.0; probability of occurrence of 35 to 40%; and model precipitation and snowfall amounts verifying close to the observations.