Digitalizing precipitation data from the image of weather map I have a set of very old precipitation anomaly maps, from 1995-2006 for Brazil. The data of these maps, is to the best of my knowlegde, lost.

I would like to have them digitalized, ie, to NetCDF or another similar gridded format. Is there any tools recommendend for this job?

The figure below is a recent plot, similar to my files.
You can convert the image to NetCDF data using gdal_translate. The command line looks something like:

gdal_translate -ot Int16 -of netCDF jpeg_filename nc_filename


You can use ncl example: click here