National Weather Service (NWS) new API

NWS API has a bunch of changes coming 2017. The documentation is a little light right now, but it sounds like the old DWML/XML feeds are going away.  We will have to pull the data using the new api.weather.gov API. Here is what I have learned about this new API so far while replacing the digitalDWML FcstType feed for my Illustrated Weather tool.

If you are using a Weather Service XML feed today and need someone to update it to api.weather.gov, drop me an email.

National Weather Service API Streaming Setup

To stream anything from https://api.weather.gov you must first set the stream header to include the Accept, Version and User-Agent. If you do not set the header the stream will return empty. NWS asks that you include your website and email address in the User-Agent so the they can contact you if need be.

$UserAgent = "YOURWEBSITE (your@emailaddress.com)";

// HTTP stream options:
$opts = array(
 'http'=>array(
 'method'=>"GET",
 'header'=>"Accept: application/geo+json;version=1\r\n" .
 "User-agent: $UserAgent\r\n"
 )
);
$context = stream_context_create($opts);

After the header is set you can stream the /points/lat,lng URL into a json object and pull forecastGridData, which is the /gridpoints/office/x/y URL. If you do not need a detailed gridded forecast take a look at the /points/lat,lng/forecast/ or /points/lat,lng/forecast/hourly in the documentation.

$lat = 43.0204;
$lng = -71.6002;
$pointMetaUrl = "https://api.weather.gov/points/$lat,$lng";
$pointMetaFile = file_get_contents($pointMetaUrl, false, $context);
$pointMetaArray = json_decode($pointMetaFile, true);
$forecastGridData = $pointMetaArray["properties"]["forecastGridData"];

Now stream the detailed gridded forecast using the forecastGridData /gridpoints/office/x/y URL. Then start pulling the data you want.

$gridDataFile = file_get_contents($forecastGridData, false, $context);
$gridDataArray = json_decode($gridDataFile, true);
$temp = $gridDataArray["properties"]["temperature"]["values"];

Just about all weather elements returned, like temperature, are an array of arrays including a time and value (e.g. temperature). The time format is ISO 8601, plus a PHP  DateInterval. “PT2H”  is a 2 hour interval.  Temperatures are in Celsius.

"values": [
                {
                    "validTime": "2017-02-17T08:00:00+00:00/PT2H",
                    "value": -6.1111111111111
                },

Now all I have to do is retool my Illustrated Weather functions to deal with this new format. UGH!

Published by

Kimball

Kimball is a website designer and developer in Goffstown, NH.

11 comments on:
“National Weather Service (NWS) new API”

  1. I’m trying to do this same thing in VBA, but all I’m getting is “Method Not Allowed” responses when I try to POST or PUT:

    Dim xmlhttp As Object
    ‘Set xmlhttp = CreateObject(“WinHttp.WinHttpRequest.5.1”)
    Set xmlhttp = CreateObject(“MSXML2.serverXMLHTTP”)
    xmlhttp.Open “GET”, “https://api.weather.gov/alerts/active/zone/PZZ530”
    ‘xmlhttp.Open “PUT”, “https://api.weather.gov/products/types/CWF/locations/MTR”, False
    xmlhttp.setRequestHeader “Accept”, “application/vnd.noaa.dwml+xml;version=1”
    xmlhttp.setRequestHeader “User-Agent”, “OCSCSailing chris@ocsc.com
    xmlhttp.Send

    ‘xmlhttp.Open “GET”, “https://api.weather.gov/products/types/CWF/locations/MTR”
    ‘ Send Request.
    ‘xmlhttp.Send
    ‘And we get this response
    MsgBox xmlhttp.responseText

    1. Not sure but perhaps the issue is you are calling for a format the endpoint does not support. application/vnd.noaa.dwml is only available at /points/{point}/forecast.

      The available formats are listed on the API Reference tab.

  2. Can you explain Noaa’s point in adding php DateInterval to validTime?
    Could it mean for example PT2H – for this hour and next the value is the same?

    1. Yes, exactly. Below is a sample of temperatures pulled from https://api.weather.gov/gridpoints/GYX/36,21

      In the sample you will see:
      [validTime] => 2017-10-25T07:00:00+00:00/PT2H covers 2 hours (7AM, 8AM)
      [validTime] => 2017-10-25T09:00:00+00:00/PT3H covers 3 hours (9AM, 10AM, 11AM)
      [validTime] => 2017-10-25T12:00:00+00:00/PT1H covers 1 hour (12PM)

      "temperature": {
      "sourceUnit": "F",
      "uom": "unit:degC",
      "values": [
      {
      "validTime": "2017-10-25T07:00:00+00:00/PT2H",
      "value": 16.666666666667
      },
      {
      "validTime": "2017-10-25T09:00:00+00:00/PT3H",
      "value": 16.111111111111
      },
      {
      "validTime": "2017-10-25T12:00:00+00:00/PT1H",
      "value": 17.222222222222
      },

  3. Thanks for this. When trying to retrieve the forecast for a given lat, lon pair, I am getting this error message:

    “The requested forecast grid was issued 2018-02-20T14:04:39+00:00 and has expired”

    Any idea of what’s going on?
    Thanks

  4. I’m looking for the definition of the terms used by the NWS API to help understand the context for terms like: map, grid, current, forecast etc. related to data returned from requests.

    For example, what is the definition for a grid? How is a grid define by the NWS? What is the size of a grid? Is the grid dynamic? There are many grid systems used by maps, such as: USNG, UTC, MGRS etc.

    Is there a link to a NWS page that contains term definitions and examples of API requests? I understand the NWS is using JSON-LD, but still, there should be more documentation, yes?

    Thanks you,
    Mark

Leave a Reply

Your email address will not be published. Required fields are marked *