The pretropical storm Agnes rainstorm across western New York and Pennsylvania is analyzed using conventional surface and aerological data. Hourly precipitation maps and surface maps showed the north-eastward motion and intensification of a developing rain area over eastern Kentucky at 1200 GMT 20 June 1972. This area remained a separate entity from the main Agnes rainshield. Nonlinear balanced omega as well as kinematic omega computations suggest that a weak short wave in the mid and upper troposphere provided the initial triggering mechanism for the growth of the rain area. Plentiful moisture was available from the Agnes circulation to the south and the western Atlantic. Latent heat release then played a dominating role in modifying the resulting vertical velocity patterns. Finally, some possible general forecast considerations are suggested by these results.