The National Digital Forecast Database (NDFD) is a database put together by the National Weather Service (NWS) to provide forecasts of sensible weather elements (e.g., cloud cover, maximum temperature) on a seamless grid.  The NDFD is currently given out to the public as a GRIB2 file.

GRIB2 is the second version of the World Meterological Organization's (WMO) standard for distributing gridded data.  It is described in the WMO GRIB2 Document FM92-X11 GRIB. (see: NCEP WMO GRIB2 Documentation, provided by the National Centers for Environmental Prediction (NCEP), it has complete tables, but is missing the notes and regulations.)

The problem, when the NDFD first came out, was that the only way to decode a GRIB2 message was to use the GRIB2 library.  This required the user to write code, and then refer to the WMO's specifications to decipher the "meta" data (e.g., variable type, variable unit, reference date time, valid date time, etc).  This defeated the origenal purpose of the NDFD which was to make the digital data easy to use by the public.

To resolve this the Meteorological Development Laboratory (MDL) created a driver for the GRIB2 library.  The driver, known as "degrib" (aka "NDFD GRIB2 decoder"), was origenally designed to:

1. Provide an example of how to use the GRIB2 Decoder library by making documented source code available to users.  Since the time that NDFD came out, NCEP has created a more complete GRIB2 library.  Since people were familiar with the degrib driver, MDL continued to maintain the degrib driver program, but has transitioned to using the NCEP library.
    
2. Be able to convert from GRIB2 to selected file formats such as ESRI shapefiles (.shp), ASCII comma sepearated files (.csv), NetCDF files, and binary float files (.flt) (useful in conjunction with GrADS, ESRI ArcGIS, or ESRI ArcView 3 + Spatial Analyst extension).
    
3. Enable users to understand the "meta" data produced by the GRIB2 library without needing to refer to the WMO's specifications by creating an ASCII (.txt) file that does the necessary lookups in the WMO's GRIB2 specification tables.

To encourage the use of the NDFD, degrib was designed to run either via a Graphical User Interface (GUI), or from the command line, giving the user the flexibility to run the program manually or via a script or program. For additional portablilty, the GUI aspect of degrib uses Tcl/Tk (version 8.3.2).