The sections that follow list the contents of the GRIB2 bulletin, octet by octet. When contents are described as "Missing," all bits are set to "1" in accordance with regulation 92.1.4. When contents are described by a number, that number will be stored in the appropriate octet.

Section 0--Indicator Section

Section 1--Identification Section

In Section 1, octets 13-19 contain the reference time for the GRIB bulletin. The NDFD is currently produced once an hour. The reference time in Section 1 represents the day/hour of the current NDFD production cycle.

Section 2--Local Use Section

Certain weather elements have provided particular challenges in encoding them in GRIB2. Using Section 2 to encode various forms of metadata provides one solution. For the NDFD elements Weather ("Wx"), and Hazards, MDL GRIB2 encoding software uses the Local Use section for packing metadata unique to each grid. At this time, for parameters other than Wx and Hazards, the Local Use section is omitted.

Grids of these 2 elements are stored as two distinct entities. The first entity is a grid of small integer values that will generally store very compactly in Section 7. The second entity is a series of null-terminated strings of ASCII characters called keys (either Wx or Hazard keys). The integers on the weather grid serve as indices, each pointing to one of the unique keys in the series. Each key is comprised of one or more subkeys, separated by a caret "^".

The metadata packed in the Local Use Section for the Wx and Hazard elements is a lookup table. The ASCII characters are loaded, one per octet, and then compressed using the Simple Packing Scheme for integers defined for Section 5. The Simple Packing Scheme treats the set of keys as an integer array. Each integer in the array is a decimal representation that corresponds to an ASCII character. For instance, the integer 65 translates to the ASCII character "A". The null character is represented by the integer 0, so that whenever a 0 is encountered in the array, it marks the ending of a unique key. The tables that follow show the specific contents of the Weather keys/subkeys and Hazard keys/subkeys.

Weather Keys and Subkeys

A weather subkey has can have up to five parts, each part separated by colons ":". The parts are:

• one weather type code. See Weather Type Codes, below.
• one coverage/probability code. See Coverage/probability Codes, below.
• one intensity code. See Intensity Codes, below.
• one visibility code. See Visibility Codes, below.
• zero or more attribute codes, separated by commas ",". See Attribute Codes, below.

Coverage/probability Codes

Intensity Codes

Visibility Codes

The following visibility codes may appear in WFO-generated GRIB2 bulletins.  The units are statute miles:  <NoVis>, 0SM, 1/4SM, 1/2SM, 3/4SM, 1SM, 11/2SM, 2SM, 21/2SM, 3SM, 4SM, 5SM, 6SM, P6SM.

Attribute Codes

Size Limitations for NDFD National Grids 

There is no (practical) limit to the number of weather keys, subkeys, etc.  For NDFD grids that are national mosaics of WFO-generated grids, the following limitations will be applied:

Weather Key Examples

Hazard Keys and Subkeys

A hazard key has two parts, separated by a  "."
The parts are:

• a 2 letter phenomenon code.  This could be something like "HW" (for High Wind), or "SV" (for Severe Thunderstorm).
• a 1 letter signifucance code.  This could be either "Y" (Advisory), "S" (Statement), "A" (Watch), or "W" (Warning).

A full list of Hazard keys found on NDFD grids can be found here .

Section 3--Grid Definition Section

Section 3 will vary depending upon the map projection the grid is defined.  A table listing more detailed information for each NDFD geographic region (CONUS, CONUS subsectors, Puerto Rico, Hawaii, Guam, Alaska) can be found here.  Latitude and longitude values are encoded in 10 ^-6 degrees.  All longitudes are east.

Lambert Conformal (includes CONUS and CONUS subsectors)

Polar Stereographic (Alaska)

Mercator (includes Puerto Rico, Hawaii, Guam, Oceanic and Micronesia domains)

Section 4--Product Definition Section

Section 4 will vary from one weather element to the next.  The formats for the various elements are detailed in the following sections:

Maximum Temperature (MaxT), Minimum Temperature (MinT), Maximum Relative Humidity (MaxRH), Minimum Relative Humidity (MinRH)

Twelve-hour Probability of Precipitation (PoP12)

Temperature, Dew Point Temperature, Sky, Wind Speed, Wind Direction, Wave Height, Weather, Apparent Temperature, Relative Humidity, Wind Gust, Hazards, Snow Level, Wet Bulb Globe Temperature, Ceiling, Visibility, Low Level Wind Shear (Speed, Direction, Height)

Parameter Categories and Parameter Numbers for Temperature, Dew Point Temperature, Sky, Wind Speed, Wind Direction, Wave Height, Weather, Apparent Temperature, Relative Humidity, Wind Gust, Hazards, PPI, Snow Level, Wet Bulb Globe Temperature, Ceiling, Visibility, Low Level Wind Shear (Speed, Direction, Height)

Note 1: Wave Height is defined as an Oceanographic Product in Section 0 (Indicator Section, octet 7 = 10).

QPF, Snow Amount, Snow Liquid Ratio, Ice Accumulation, Daily FRET, Daily FRET Departure and Total Weekly FRET

Tropical Wind Speed Probabilities (Incremental and Cumulative) greater than 34, 50, and 64 knots - from Tropical Prediction Center

Hazard Probabilities - from Storm Prediction Center (Includes Probabilities of Tornado, Hail, and Damaging Winds, Probabilities of Extreme Tornado, Hail, and Damaging Winds, Total Probability of Severe, and Extreme Severe Thunderstorms

Parameter Categories and Parameter Numbers for Hazard Probabilities

Categorical Outlooks:

1) Storm Prediction Center (includes Convective Hazard Outlook, Fire Wx Critical and Extremely Critical Areas, and Fire Wx Critical Areas for Dry Thunderstorms)

2) Tropical Cyclone Categorical Threats from WFOs (include Tropical Cyclone Storm Surge Threat, Tropical Cyclone Flooding Rain Threat, Tropical Cyclone Tornado Threat, and Tropical Cyclone Wind Threat)

3) Heat Risk

click here to see explanation of Tropical Cyclone Categorical Threat and Heat Risk values

Parameter Categories and Parameter Numbers for Categorical Outlooks and Tropical Cyclone Threats

Climate Outlook Probability Elements - from Climate Prediction Center (Average Temperature and Total Precip Above and Below Normal for 8-14 Day Outlook, 1 Month Outlook, and Seasonal Outlook)

Section 5--Data Representation Section

The GRIB2 standard supports eight forms of data packing.  Grids transmitted as part of the NDFD will use complex packing (Data Representation Template 5.2), and optionally, second order spatial differencing (Data Representation Template 5.3).

NDFD grids will generally contain a significant number of missing values.  National mosaic grids are defined to be rectangles, and they generally extend into areas where NWS WFOs do not have forecast responsibility.

Section 6--Bit-map Section

Bit-maps will not be used in NDFD GRIB bulletins.  Missing values will appear in those grid locations where forecasts are not valid.
 

Section 7--Data Section

Section 8--End Section