Regional climate models often predict modest changes in winter precipitation on the western (windward) flanks of the Cascade and Sierra Nevada Mountains but large increases on the eastern (leeward) flanks, indicating a weakening of the climatological rain shadow under global warming. Such changes could have profound consequences for water resources and flooding in these regions, but their underlying causes are not well understood. Here we investigate the causes of rain-shadow weakening within a 30-year Pseudo-Global Warming simulation performed at 6-km resolution over the western United States. Preliminary analyses suggest an important role for static stability, which increases due to amplified warming aloft. This weakens the rain shadow in two ways: first, by inhibiting post-frontal convection that often brings heavy precipitation to windward slopes, and second, by weakening lee-side descent, thus allowing more precipitation to “spill over” the crest. Finally, we discuss other mechanisms that may also play a role, and describe ongoing numerical experiments that will allow us to quantify the relative importance of individual mechanisms.