NDFD stands for National Digital Forecast Database and is a database of forecast data maintained by the National Weather Service. The database contains a suite of gridded forecasts for standard meteorological and oceanic weather elements such as wind speed and direction, air temperature, and wave height.

NDFD is a suite of products generated by the National Weather Service (NWS) using data from regional NWS Weather Forecast Offices (WFOs) and the National Centers for Environmental Prediction (NCEP). The database contains forecasts produced at WFOs, as well as NCEP gridded model data enhanced by WFO staff. NWS field offices forecasts working in collaboration with the National Centers for Environmental Prediction (NCEP) are combined in the NDFD to create a seamless mosaic of digital forecasts. Most of the data in the database is produced by running a group of models called the National Blend of Models (NBM). A significant aspect of NDFD is that the NBM is not a single model. The NBM consists of a large group of models that are run at different intervals and generate a wide variety of data. The NBM blends multiple models including NAM (North American Mesoscale Forecast System - 12km), RAP (Rapid Refresh), Gridded LAMP, HiResWindow ARW NCEP, HiResWindow NMMB NCEP, SREF (Short Range Ensemble Forecast), and many others.

The NDFD database is continuously updated as it receives forecast data from the various WFOs throughout the day. As aforementioned, these forecasts are combined into a "mosaic" of digital forecasts. Interestingly, the WFOs send their data in grib2 format to a central server. The data is decoded by the central server every 5 minutes and stored in the database. Ten minutes before the hour, a job is kicked off that starts generating a mosaic. The mosaic is encoded into GRIB2 format and posted to the NWS Telecommunication Gateway (MWSTG), which are then available through HTTP and FTP download (usually 15 to 20 minutes after the hour).

A mosaic in this context is a seamless composite of grid forecasts. Each WFO provides forecasts for one or more grids. A mosaic is composed of all of the WFO grids and of additional grids that are not the responsibility of any WFO (e.g., ocean grids). Creating the mosaic is basically a geometric assembly process that involves the following steps:

1. Aligning the individual WFO grids with a standard grid structure and projection
2. Assembling the individual WFO grids into a seamless national rectangular grid.
3. Identifying boundary issues such as discontinuity (missing data) and conflicts.
4. Resolving boundary issues. Resolution for these issues is typically done by NWS staff working with the surrounding WFOs and with local forecast tools.

_Note that a lot of this information is based on a 2003 slide deck presentation, so not the most recent information.

The mosaics described above are extracted from the database and encoded into GRIB2 formatted files. These files typically contain detailed information about the predictions in addition to the predictions themselves. For example, they typically contain reference times (aka prediction times) and lead times, data that is not available through the various API that return NDFD data. The files are available via http and ftp and organized based on several characteristics: geographical sectors, lead times, and variables.

NDFD GRIB2 files are organized by geographical sectors. For the sectors relevant to South Texas see below (for up-to-date sector info see NDFD geographical sectors. Each geographical sector has a dedicated folder (and thus URL) for the GRIB2 files.

• Southern plains includes all of Texa (AR.splains) - URL segment : 'DF.gr2/DC.ndfd/AR.splains/'
• Full CONUS (AR.conus) - URL segment: 'DF.gr2/DC.ndfd/AR.conus/'

Each NDFD GRIB2 file contains the predictions for a single weather element for a given geographical sector. For example, the wind speed predictions are contained in a file named 'ds.wspd.bin'. NDFD GRIB2 files are further organized in folder based on lead times (and possibly other things? need to research more)(*1). Most of the regions and weather elements, including the Southern Plains and basic weather elements such as air temp and wind data follow the below organization:

• VP.001-003: contains files with lead times up to 54 or 72 hours (seem to vary depending on time of day and/or weather element)
• VP.004-007: contains files with lead times past what the previous folder contains and typically up to 156 hours
• VP.008-450: contains files with long range forecast typically 12 days and/or more.

Files are named base on the weather element they provide predictions for and have a '.bin' extension. For example wind speed predictions are in a file named 'ds.wspd.bin'.

Note that not all regions support all 3 folders. The Southern Plains regions does support all three. For a description and additional information of all weather elements, NDFD files and their contents and grids and sectors, see NDFDSpreadsheet.xls. To download the latest spreadsheet go to NDFD Information Links and scroll down to the NDFD Products and status section to see the link to download the spreadsheet.

NDFD GRIB2 files are available through both https and ftp at the following base URL (for operational weather elements)(*2):

• https://tgftp.nws.noaa.gov/SL.us008001/ST.opnl/
• ftp://tgftp.nws.noaa.gov/SL.us008001/ST.opnl/

To obtain a file, we then construct the URL following this template:

<base url><geagraphical sector><folder name for targeted lead times><weather element file name>

For example, to download the wind speed direction for the southern plains for lead times from 1 hour up to 54 or 72 hours, use the following link

ftp://tgftp.nws.noaa.gov/SL.us008001/ST.opnl/DF.gr2/DC.ndfd/AR.splains/VP.001-003/ds.wspd.bin

Note that the server is typically locked down when files are being uploaded so it is a good idea to schedule download enough minutes away from the top and bottom of the hour.

Files are updated on the FTP server regularly but the schedule seem to vary by variable and lead time. Here's what we have witnessed/experienced so far.

1. For predictions from 1 to 55 hours (provides predictions at 3 hours interval, so lead times are 1h, 4h, 7h, etc.): file is uploaded every hour, about 10 minutes before the hour.
2. For predictions from 66 to 156 hours (provides predictions at 6 hours interval, so lead times are 66h, 72h, 78h, etc.): file is uploaded every 6 hours (more specifically, they are issued at 00, 06, 12, 18, and 22 UTC), 5 minutes passed the bottom of the hour.

Getting data out of the files with wgrib2. Because of the way the files are organized on the ndfd server, each file contains predictions for only one series (e.g., air temp, wind dir, etc.). So it's not necessary to ask for the variable or restrict the search to that variable but it is good practice to do so. So getting the data out of an air temp prediction file would look like this:

wgrib2 ds.temp2.bin -match ':TMP' -T -ftime -var -lon -97.31286 27.46767

-T gets you the prediction time and -ftime gets you the lead time (the example is for the sbib location - or close to it). The above command would get you results like the below: Note that D provides the prediction time.

(*1) As far as we are able to tell, NDFD generates forecasts with different offsets during the first half of the hour compared to during the second half of the hour:

• first half of the hour:
  • reference time is at the top of the hour (e.g, 1500)
  • time offset unit is hours, starts at 1 hour and is in 1 hour increments. so 1 hour, 2 hours, 3 hours, etc.
• second half of the hour:
  • reference time is at the bottom of the hour (e.g, 1530)
  • time offset unit is minutes, starts at 30 min (so the forecast is for the top of the hour) and is in 60 minutes increments. so 30 min, 90 min, 150 min, etc.

_(*2) There is also an experimental server that is not included in this documentation

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The web service can be accessed via SOAP or by REST. It can be somewhat frustrating to find information about these (links on the various government websites tend to point to each other in a seemingly endless circles) so here are a few useful links.

1. SOAP client: https://graphical.weather.gov/xml/SOAP_server/ndfdXML.htm. This soap client is somewhat useful to see the kind of results is returned by the service but note that it does not call the service directly so it is not useful to see the actual call to the service. It provides a UI to select the parameters of the call and then post the parameters back to itself and makes the calls server-side not client-side.

2. XML-Based REST Service information: https://digital.weather.gov/xml/rest.php

3. JSON-Base REST Service: https://www.weather.gov/documentation/services-web-api (https://api.weather.gov/)