Because of limitations in sampling of atmospheric moisture, surface humidity is used to estimate atmospheric moisture availability for probable maximum precipitation (PMP) estimation. In practice, a pseudoadiabatic moisture profile is assumed to estimate precipitable water from surface 12-hour persisting dew point. This assumption was evaluated for the central United States using surface and upper air observations. The results show that the pseudoadiabatic assumption systematically overestimates precipitable water for PMP estimation. The overestimation occurs for extreme rainstorms and for climatological maximum conditions because the surface humidity is high but the upper air is drier than predicted by a pseudoadiabatic profile. To better represent the moisture relationship for the central United States, an empirical power law moisture profile was derived from a 23-year climatology of the monthly maximum precipitable water and monthly maximum 12-hour persisting dew point. Moisture maximization concepts were then used to recompute PMP estimates for the Chicago area. The results suggest that the pseudoadiabatic assumption overestimates PMP by about 6.9% on average. An analysis of atmospheric conditions for extreme rainstorms also raises concerns regarding the PMP moisture maximization assumption that such storms could occur under maximum atmospheric moisture conditions.