#! /bin/sh
set -efu

prefix=/tmp/CWAT
grib_path=$prefix.grib
json_path=$prefix.json
pgm_path=$prefix.pgm
png_path=$prefix.png

date=20191210
# Possible values for hour: 00 06 12 18
hour=12

url="https://nomads.ncep.noaa.gov/cgi-bin/filter_gfs_0p25.pl?file=gfs.t12z.pgrb2.0p25.anl&lev_entire_atmosphere_%5C%28considered_as_a_single_layer%5C%29=on&var_CWAT=on&leftlon=0&rightlon=360&toplat=90&bottomlat=-90&dir=%2Fgfs.$date%2F$hour"

curl -fsS "$url" > "$grib_path"

grib2json --data "$grib_path" > "$json_path"

width=$(jq '.[0].header.nx' < "$json_path")
height=$(jq '.[0].header.ny' < "$json_path")

# The maximum gray value.  Must be bigger than 0 and less than 65536.
maxval=256

# pgm - Netpbm grayscale image format
# http://netpbm.sourceforge.net/doc/pgm.html
{
  echo P2
  echo "$width $height"
  echo "$maxval"
  cat "$json_path" |
  jq --argjson maxval "$maxval" -c '
    .[0].data[] * $maxval | round
  ' |
  xargs -n "$width"
} > "$pgm_path"

convert "$pgm_path" "$png_path"