Forecaster

Forecaster is a gem wrapping wgrib2 to fetch and read weather data directly from the Global Forecast System.

It comes with a library and a CLI allowing you to type commands like forecast for this afternoon in paris in your terminal to get the latest weather forecast.

Installation

gem install forecaster

Alternatively you can build the gem from its repository:

git clone git://github.com/vinc/forecaster.git
cd forecaster
gem build forecaster.gemspec
gem install forecaster-1.0.0.gem

In both cases you need to make sure that you have wgrib2 present in your system.

To install the wgrib2 from source:

wget https://ftp.cpc.ncep.noaa.gov/wd51we/wgrib2/wgrib2.tgz
tar -xzvf wgrib2.tgz
cd grib2
export CC=gcc
export FC=gfortran
make
sudo cp wgrib2/wgrib2 /usr/local/bin/

Usage

require"forecaster"

To configure the gem:

Forecaster.configuredo |config|
config.wgrib2_path="/usr/local/bin/wgrib2"config.cache_dir="/tmp/forecaster"config.records={temperature: ":TMP:2 m above ground:",humidity: ":RH:2 m above ground:",pressure: ":PRES:surface:"}end

Forecaster saves large files containing the data of GFS runs from the NOAA servers in the cache directory, but only the parts of the files containing the records defined in the configuration will be downloaded.

You can find the list of available records online or by reading any .idx files distributed along with the GFS files.

A record is identified by a variable and a layer separated by colon characters. In the case of the temperature for example, those attributes are TMP and 2 m above ground. See the documentation of wgrib2 for more information.

To fetch a forecast:

t=Time.now.utc# All the dates should be expressed in UTCy=t.year# year of GFS runm=t.month# month of GFS rund=t.day# day of GFS runc=0# hour of GFS run (must be a multiple of 6)h=12# hour of forecast (must be a multiple of 3)forecast=Forecaster.fetch(y,m,d,c,h)# Forecaster::Forecast

To read the record of a forecast:

res=forecast.read(:temperature,longitude: 48.1147,latitude: -1.6794)# String in Kelvinval=res.to_f - 273.15# Float in degree Celsius

Command line

Forecaster has a command line tool that try to be smart:

$ forecast for tomorrow afternoon in auckland
GFS Weather Forecast
Date: 2016-05-13
Time: 12:00:00
Zone: +1200
Latitude: -36.8 °
Longitude: 174.8 °
Pressure: 1013.8 hPa
Temperature: 21.7 °C
Nebulosity: 0.0 %
Humidity: 65.1 %
Precipitation: 0.0 mm
Wind: 8.0 m/s (NE)

But you can use it in a more verbose way:

$ TZ=America/Los_Angeles forecast --time "2016-05-12 09:00:00" \
--latitude "37.7749295" \
--longitude "-122.4194155" \
--debug
Requested time: 2016-05-12 09:00:00 -0700
GFS Run time: 2016-05-11 23:00:00 -0700
Forecast time: 2016-05-12 08:00:00 -0700
Downloading: 'http://www.ftp.ncep.noaa.gov/data/nccf/com/gfs/prod/gfs.20160
51200/gfs.t00z.pgrb2.0p25.f015'
Reading index file...
Length: 4992281 (4.76M)
100% [===========================================>] 696 KB/s Time: 00:00:07
GFS Weather Forecast
Date: 2016-05-12
Time: 08:00:00
Zone: -0700
Latitude: 37.8 °
Longitude: -122.4 °
Pressure: 1013.5 hPa
Temperature: 13.4 °C
Nebulosity: 0.0 %
Humidity: 89.7 %
Precipitation: 0.0 mm
Wind: 1.0 m/s (NE)

To use automatically the timezone of a location you will need to create a free GeoNames account and export your username in an environment variable:

export GEONAMES_USERNAME=<username>

And while you're doing that, you can also export your favorite location to avoid typing it every time:

export FORECAST_LATITUDE=<latitude>
export FORECAST_LONGITUDE=<longitude>

License

Copyright (c) 2015-2018 Vincent Ollivier. Released under MIT.