WRF simulations of orographic precipitation and air mass transformation over the Central Idaho Mountains

As precipitating weather systems pass over complex terrain, the changes in water vapor distribution due to orographic precipitation can influence the atmospheric flows and precipitation processes downstream. In the Intermountain West, these processes play a potentially important role in determining the spatial distribution of precipitation, flooding, and associated mass wasting processes. Major flooding occurred in the Central Idaho Mountains during the January 1997 ‘Pineapple Express' event that affected much of the western United States. Numerical simulations of the event, computed with WRF, illustrate the orographic transformation of the air mass as it passed over Idaho and show how the Snake River Plain acted as a conduit for water vapor, delivering abundant precipitation to northwestern Wyoming and southeastern Montana. Model results will be used to construct moisture budgets for the region and will be used to estimate precipitation efficiency and the residence time for condensed moisture.