The M3WNDW program extracts a windowed set of data from a gridded CMAQ file and outputs this subset to a new gridded file for a specified time period. The definition of the sub-rectangle is user-specified by the entries of: LOCOL, LOROW, HICOL, and HIROW. One application of this utility would be to create an initial concentration file for a nested grid from the output of a parent grid.