How is heavy rainfall determined in National Climate Assessment
After contacting the author of the chart, I believe the following defines the process behind the chart.


Data source: NOAA's Global Historical Climatology Network Daily station data for "PCPT".
Use only stations in the Southeast states which have data for 90% of days from 1895 to 2011, to match stations used previous edition of the climate report.
Use only years with at least 300 days of data.
Remove data values that have been flagged in GHND for quality issues.
Count the number of days of heavy rainfall (3 or more inches) for each station and year.
Using a 1° by 1° lat/lon grid, average counts of stations in the same grid cell.
Report the average of all grid cell-years for a given decade.


I have still not been able to reproduce the chart because I have had trouble getting clean data. Will update with a chart when that happens.