In order to develop a better understanding of increases in demand and its implication on water supply development options, the Tampa Bay Water Authority commissioned the development of a long-term forecast of water demands. The forecasting effort uses a high-level of spatial disaggregation and explicitly incorporates uncertainties in long-term growth projections. The probabilistic model is derived from a recently developed GIS tool constructed at the traffic analysis zone (TAZ) level, providing reliable and precise estimates of water demand relationships for prominent user sectors. This approach produces a range of possible future water demands, including estimates of the chance of exceeding certain levels of demand over the forecast horizon. Includes table, figures.