In hydrology, “flashiness” refers to the frequency and rapidity with which stream flow changes over time. Flashiness varies with the size, geometry, geology, and vegetation of a given watershed, but it is also strongly influenced by local meteorology. To better understand these meteorological controls, we introduce a precipitation flashiness index, PFI, which is analogous to the Gini coefficient used in economics as a measure of income inequality. Analyzing daily precipitation data from across the western US, we find a clear orographic signature in the spatial pattern of PFI, with low flashiness on windward slopes of major mountain ranges and high flashiness in deserts and rain-shadowed regions in the lee. We find a similar pattern of PFI variability in regional climate model simulations, but also a tendency for this pattern to moderate under future warming, with PFI increasing where it is low (e.g., on windward slopes) and decreasing where it is high (e.g., in lee-side basins). This result reflects a shift toward less frequent, more extreme precipitation on windward slopes and more frequent, less extreme precipitation in the lee, which could have significant consequences for water resources and flood risk in these watersheds.