Using existing data sets of spaceborne soil moisture retrievals, streamflow and precipitation for 26 basins in the United States Southern Great Plains, a 5-year analysis is performed to quantify the value of soil moisture retrievals derived from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) X-band (10.7 GHz) radiometer for forecasting storm event-scale runoff ratios. The predictive ability of spaceborne soil moisture retrievals is objectively compared to that obtainable using only available rainfall observations and the antecedent precipitation index (API). The assimilation of spaceborne observations into an API soil moisture proxy is demonstrated to add skill to the forecasting of land surface response to precipitation.