Degrib: NDFD and Weather

• How does NDFD treat weather?
• What are the parts of a weather word?
• How does degrib handle weather?
• "degrib -C -Flt" and weather
• "degrib -C -Csv" and weather
• "degrib -C -Shp" and 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:

1. Coverage / Probability
2. Weather Type
3. Intensity
4. Visibility
5. Hazards (Attributes)

The posibilities 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 upto 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

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

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).

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

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).

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).

1. Wx: The original weather string.
2. Wx-index: The index into the original weather table that came with the message.
3. Visibility: The minimum visibility in the 5 weather words, in 1/32 of a statute mile.
4. Ndfdwxcode: The NDFD simple weather code (see above)


Next, for each of the 5 weather words, I provide the following:
5. Weather: An english translation of the weather word.
6. Wx-inten: A combination of the weather type and the intensity. (see wxshp.php)
7. Cover: A numeric value for coverage. (see wxshp.php)
8. Hazard: A numeric value for hazards. (see wxshp.php)