Getting Forecast Data from NCEP

Download the analysis and model forecast data from NCEP

The NCEP analysis and forecast data are available in grib format from NCEP's anonymous FTP site: ftp://ftp.ncep.noaa.gov. Data from a many different forecast models are available. For a complete listing, go to the pub/data/grib/ directory. This page provides guidance for viewing forecasts from three of the models: the ETA model, the Aviation model (AVN), and the Medium Range Forecast model (MRF).

Model Coverage Initialized Interval Duration Directory Filenames
ETA N. America 00Z, 06Z, 12Z, 18Z 6 hrs 48 hrs pub/data/grib/eta/ eta_yymmdd_##_**
AVN Global 00Z, 06Z, 12Z, 18Z 6 hrs 4 days pub/data/grib/avn/ avn_yymmdd_##_**
MRF Global 00Z 12 hrs 16 days pub/data/grib/mrf/ mrf_yymmdd_00_**
## Represents a wildcard for the hour of forecast initialization. Use leading zeros for 00 and 06 hour forecasts.
** Represents a wildcard for the number of hours into the forecast. Use leading zeros for 00 and 06 hour forecast periods.

The forecasts are available for downloading between 2 and 12 hours after the initialization time. Be sure to log in as anonymous and set the filetype to binary. The sizes of the individual data files range from about 3 to 13+ MB. You may want to be judicious in your selections, e.g. grabbing only every 12 hours of the AVN forecasts instead of every 6.

Other Files You Will Need

Before you can display the forecast data with GrADS you will need to get two additional files. The first is a control file, which GrADS needs to define the 4-dimensional space of the data and assign the variable names to the fields in the data. The second file is a map file, which is particular to GRIB format data. The map file allows GrADS to navigate efficiently.

1. The GrADS Control File

Below are the links to download sample control files for each of the major forecast models:

These control files may be copied and reused for any new forecast data provided you modify the tdef line which specifies the start time of the forecast. These samples also assume that the data and map files are in the same directory as the control file. If they are in different directories, you must provide the full path in the dset and index lines of the control file.

N.B.: The resolution of the MRF data decreases from 1 degree to 2.5 degrees after 7 days. Thus, 2 control files are required, one for each resolution. However, the time ranges in the tdef statements in both control files are the same, so the user must keep track of the time and invoke the right control file accordingly. Plotting a variable at a particular time from the wrong file will result in the error message, "entire grid undefined".

2. The GRIB Map File

Each time you download new forecast data, you must generate a map file before you can plot anything with GrADS. The map file is generated using the gribmap program, a separate utility that is supplied with GrADS. gribmap is executed from the command line -- it scans the header in the GRIB files and constructs a table to help GrADS navigate through the data.

For example, to make the GRIB map for the ETA data, the command would be:

By default, the output file will be called eta.map. The map file name is specified in the index line of eta.ctl). You may use the -o option to specify a different name for the map file, but you must also change the map file name in eta.ctl. gribmap uses information in eta.ctl to make the map file, so be sure to make your changes to the dates, etc. in eta.ctl before running gribmap.

You can use a similar utility called gribscan to examine the structure of a GRIB data set directly. This is very useful for customizing your own .ctl files. The .ctl files provided here do not allow access to all of the fields in the NCEP data, just the more generally significant ones. Compare the output from gribscan with one of the corresponding .ctl files provided here.

Further Information