In this presentation, we show that replacement of the 28-year with a newer 36-year climatology (1980-2015) has surprisingly strong impacts on the derived anomaly and (especially) percentile fields of key drought events, in particular for extremely severe drought events (D3 and D4 in USDM). In contrast, there is small impact on extremely wet events (W3 and W4 categories), possibly because there were relatively fewer extremely wet events during 2008-2015. Also, we analyze two mutually exclusive 18-year climatologies to further confirm our conclusions and investigate uncertainties from models and climatology lengths. At present, we are working with NASA scientists to investigate methods for deriving an “optimal climatology” (such as the method of Narapusetty et al., 2009, J. Climate), by means of which a new NLDAS climatology (e.g., up-dated routinely about every 5 years) would exhibit suitably less impact on the percentile fields in “before and after” comparisons for well-known historical drought episodes.