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An analysis of 3,104 stations in the United States shows virtually every station exhibits a positive correlation between precipitable water (PW) and extreme daily precipitation (EP) with over one-third statistically significant. To first approximation, EP scales linearly with PW, but there is nonlinear scaling at the lower and upper ends of the PW distribution. On average, EP is amplified by twice the amount of PW, but there is substantial seasonal and spatial variability caused by dynamically forced vertical velocity with stations ranging from a one-to-one relationship to over three-to-one. These latter stations are generally found in elevated terrain or near coasts and in regions and seasons affected by strong synoptic-scale weather systems. The results also point to PW, not vertical velocity, as the key limiting factor in the most intense EP events. This has important implications for projecting changes of the most intense EP events in a warmer world.

Plain Language Summary As the climate warms, water vapor is increasing in the atmosphere. Analysis of the historical observations of extreme daily precipitation shows that precipitation amounts increase with more water vapor in the atmosphere. On average, extreme daily precipitation amounts equate to twice the amount of water vapor. There is, however, considerable spatial and seasonal variability affected by the predominant weather types in the region, orography, and proximity to oceans. The analysis indicates that when the amount of atmospheric water vapor is high, as occurs during the warm season and in the south and eastern portions of the United States, the increase in extreme precipitation for a given increase in water vapor becomes disproportionately large compared to the same increase at low amounts of water vapor. Water vapor is a limiting factor in the amount of extreme precipitation more often than the intensity of the weather causing the event.