Most military personnel planning models are deterministic steady-state models. This Note examines the impact of various types of uncertainties on projections of force structures using a Markov flow model of the first-term force. In particular, it addresses the impact of uncertainties related to the supply of enlisted Air Force personnel (stay/leave decisions by or about individual airmen, the makeup of accession cohorts, retention rate estimation, and recruiting shortfalls) on force planning factors such as accession requirements, reenlistment requirements, and personnel costs. The analysis indicates that projections of many force characteristics can involve sizable uncertainties. Individual stay/leave decisions comprise the largest source of this uncertainty. Another potentially larger contributor is uncertainty in the proportion of accession requirements that can actually be met. Uncertainties regarding estimates of flow rates, while important in projecting values for certain subsets of the force, appear to contribute little to uncertainty in overall force characteristics.