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How NDFD Treats Weather

Weather in NDFD is handled by including a weather string table in the local use section of the GRIB2 message. The grid cell values in the data part of the GRIB2 message are numbers which refer to that entry in the weather string table. The entries consist of ASCII weather strings which have upto 5 weather words separated by '^'.

Each weather word has 5 pieces separated by ':'. The 5 pieces of the weather word are as follows:

The possibilities for Coverage / Probability are:

Code Meaning Code Meaning
<NoCov> No Coverage / probability Iso Isolated
Sct Scattered Num Numerous
Wide Widespread Ocnl Occasional
SChc Slight Chance Chc Chance of
Lkly Likely Def Definite
Patchy Patchy Areas Areas of
Added 8/13/2004
Pds Periods of Frq Frequent
Inter Intermittent Brf Brief

The posibilities for Weather Type are:

Code Meaning Code Meaning
<NoWx> No Weather K Smoke
BD Blowing Dust BS Blowing Snow
H Haze F Fog
L Drizzle R Rain
RW Rain Showers A Hail
Hail was removed 8/12/2004
FR Frost ZL Freezing Drizzle
ZR Freezing Rain IP Ice Pellets (sleet)
S Snow SW Snow Showers
T Thunderstorms    
Added 1/20/2004
BN Blowing Sand ZF Freezing Fog
IC Ice Crystals IF Ice Fog
VA Volcanic Ash ZY Freezing Spray
WP Water Spouts    

The posibilities for Intensity are:

Code Meaning Code Meaning
<NoInten> No Intensity -- Very Light
- Light m Moderate
+ Heavy    

The posibilities for Visibility are:

<NoVis>, 0SM, 1/4SM, 1/2SM, 3/4SM, 1SM, 11/2SM, 2SM, 21/2SM, 3SM, 4SM, 5SM, 6SM, P6SM

Hazards (or Attributes) are broken up into up to 5 pieces separated by ','s. The posibilities for Hazards (Attributes) are:

Code Meaning Code Meaning
None None FL Frequent Lightning
GW Gusty Winds HvyRn Heavy Rain
DmgW Damaging Wind SmA Small Hail
LgA Large Hail OLA Outlying Areas
OBO on Bridges and Overpasses    
Added 8/13/2004
OGA On Grassy Areas Dry dry
TOR Tornado Primary Highest Ranking
Mention Include Unconditionally OR or
MX mixture    

Handling Weather in Degrib

Handing Weather: Degrib -P Option

Degrib has several different ways of handling weather. Starting with the "degrib -P" option, it can:

  1. Output the weather string:
    degrib wx.bin -P -pnt 35,-76 -WxParse 0
  2. Convert the weather string to english:
    degrib wx.bin -P -pnt 35,-76 -WxParse 1
  3. Convert the weather string to an NDFD simple weather code:
    degrib wx.bin -P -pnt 35,-76 -WxParse 2

What is an NDFD simple weather code?

In order to create graphical images of weather, NDFD needed a way to combine the various possibilities of weather into a discrete number of colors. So they came up with NDFD simple weather codes

degrib currently (as of 12/15/2006) recognizes four iterations of simple weather codes, as follows:

  1. Simple weather code 1: 6/2003 version
  2. Simple weather code 2 & 3: 1/2004 & 2/2004 version
  3. Simple weather code 4: 11/2004 version
(Note: If you are interested in encoding schemes, you might also be interested in how we encoded the weather for ESRI Shapefiles. You may also view wxshp.php.)

To get a particular simple weather code using the -P option use:

  • degrib wx.bin -P -pnt 35,-76 -WxParse 2 -SimpleVer 2

  • Note: -SimpleVer defaults to the most recent one (currently 4).

Handling Weather: Degrib -C Option

Continuing to the "degrib -C" option, it is limited in some respects by the file formats that it writes to.

The -C -Flt option

  1. degrib wx.bin -C -Flt -Met -msg 1 => convert the message to a NxM 4 byte float file where each cell contains the number in the weather string lookup table contained in the .txt file (created with the default -Met option)
  2. degrib wx.bin -C -Flt -nMet -msg 1 -SimpleVer 2 -SimpleWx => convert the message to a NxM 4 byte float file using simple weather scheme 2 (see above).

The -C -Csv option

  1. degrib wx.bin -C -Csv -msg 1 => convert the message to an ASCII comma delimited file with a x, y, lat, lon, and weather value for each cell. In this case the weather value is the weather string.
  2. degrib wx.bin -C -Csv -msg 1 -WxParse 1 => convert the message to an ASCII comma delimited file with a x, y, lat, lon, and weather value for each cell. In this case the weather value is the weather string converted to english.
  3. degrib wx.bin -C -Csv -msg 1 -WxParse 2 => convert the message to an ASCII comma delimited file with a x, y, lat, lon, and weather value for each cell. In this case the weather value is the weather string converted to an NDFD simple weather code (see above).

The -C -Shp (and -C -Shp2) options


With shapefiles, I could get creative, so I broke up the weather into more of its component parts. The data I provide are as follows:
  • Wx: The original weather string.
  • Wx-index: The index into the original weather table that came with the message.
  • Visibility: The minimum visibility in the 5 weather words, in 1/32 of a statute mile.
  • Ndfdwxcode: The NDFD simple weather code (see above)

Next, for each of the 5 weather words, I provide the following:

  • Weather: An english translation of the weather word.
  • Wx-inten: A combination of the weather type and the intensity. (see degrib_wxshp)
  • Cover: A numeric value for coverage. (see degrib_wxshp)
  • Hazard: A numeric value for hazards. (see degrib_wxshp)