STORM is an empirical ionospheric correction model designed to capture the changes in F region electron density during geomagnetic storms. The model is driven by the previous 33 hours of ap, and the output is used to scale the quiet time F region critical frequency (foF2) to account for increases or decreases in electron density resulting from a storm. The model provides a simple tool for modeling the perturbed ionosphere. The quality of the model has been evaluated by comparing the predictions of the model with the observed ionospheric response during the six storms in the year 2000. The model output has been compared with the actual ionospheric response at 15 ionosonde stations for each storm. The comparisons show that the model captures the decreases in electron density particularly well in summer and equinox at midlatitudes and high latitudes but is less accurate in winter. The value of the model has been quantified by comparing the daily root mean square error of the STORM predictions with the monthly mean. The results of the validation show that there is a 33% improvement of the STORM model predictions over the monthly median during the storm days and that the model captures more than half of the increase in variability on the storm days, a significant advance over climatology.