Long, low waves preceding the arrival of the visible swell from a storm have been recorded off Pendeen, England, and Woods Hole, Massachusetts, by means of new instruments for the measurement and analysis of ocean waves. These forerunners provide storm warnings of practical value. Expressions giving the fore-runner's distance from the storm system and its travel time as functions of recorded period and rate of change of period are derived from very general assumptions. The expressions are suitable for simple graphical representation. The application of the method to tracking storms across the ocean is illustrated by means of a few actual examples, and the computed storm tracks are shown to be in good agreement with the information contained on weather maps. Certain features of the wave records may eventually make it possible to compute not only the location but also the size, intensity, and general character of the storm.