Response of Precipitation Increases to Changes in Atmospheric Moisture and Its Flux in the Columbia River Basin: WRF Model-Based Precipitation Maximization for PMP Studies

US probable maximum precipitation (PMP) estimation guidance fundamentally relies on the assumption that any change in precipitable water is consistent with the change in precipitation. While this assumption is theoretically sound in extreme storms that are convective in nature with lifting so vigorous as to convert all available atmospheric water vapor into precipitation, this type of storm rarely occurs in the Pacific Northwest of the United States; the assumption may be invalid. This study investigates the relationship between changes in precipitable water and changes in precipitation using high-resolution model-based precipitation maximization for a large number of atmospheric river (AR) events impacting the Columbia River Basin (CRB) in the Pacific Northwest. Analysis indicates that the relationship between changes in precipitable water and changes in precipitation cannot be simply approximated as either 1: 1 (i.e., consistent) or a linear relationship (R=0.13; p=0.17). Our analysis on the precipitation maximization results showed that the horizontal wind speed at 10m plays an important role in determining the relationship between these changes. The relationship between integrated water vapor transport (IVT) change and precipitation change was found to be stronger (R=0.47) and statistically significant (p<0.01) for storms impacting the CRB, which was approximated as precipitation change = 2.0 x IVT change -0.6. Our finding underscores the importance of considering not only atmospheric water vapor amounts but also the accompanying flows transporting atmospheric water vapor, in maximizing precipitation depths over a target region.