Breaking the previous mark of 48 inches, Harvey recorded a preliminary 51.88 inches at Cedar Bayou, Texas, about 30 miles from Houston, the NWS reported.
The storm, which dropped a foot of rainfall from Galveston, Texas to Beaumont, Texas, overnight, according to ABC News' meteorologists, is not expected to relent just yet.
Heavy rain is forecast to continue in the eastern part of the state, where an additional 6 to 12 inches of rain are possible in spots over the next 24 hours.
Rain could be heavy at times in Houston, where rescuers are already battling severe flooding, over that same span. An additional 6 inches of rain is possible there.
Houston will possibly see an end to the rain by Wednesday afternoon, ABC News meteorologists suggest.
ABC News' Daniel Peck and Max Golembo contributed to this report.