Weather Underground API - Forcast Help - Javascript

Question

I am trying to get a 3 day forecast from the Weather Underground API using this:

var Forcast = parsed_json['forecast']['txt_forecast']['period'][0]['fcttext_metric'];

Its not working and i have no experience with json so i don't have a clue what i need to do in-order for this to work. Bellow is the part of the json file i want:

"forecast": {
"txt_forecast": {
"date": "2:00 PM PDT",
"forecastday": [{
"period": 0,
"icon": "partlycloudy",
"icon_url": "http://icons-ak.wxug.com/i/c/k/partlycloudy.gif",
"title": "Tuesday",
"fcttext": "Partly cloudy in the morning, then clear. High of 68F. Breezy. Winds from the West at 10 to 25 mph.",
"fcttext_metric": "Partly cloudy in the morning, then clear. High of 20C. Windy. Winds from the West at 20 to 35 km/h.",
"pop": "0"
}, {

Could Someone please show me how to get the information i need.

Answer 1

I have managed to work it out with some help from one of the users on the official Wunderground forum. Here is the new code that i am using with a few more added bits.

jQuery('#GetWeather').click(function() {
var PostCode=" ";
$.ajax({ url : "http://api.wunderground.com/api/2508132ae0c7601a/geolookup/forecast/q/UK/"+PostCode +".json",
dataType : "jsonp",
success : function(parsed_json) {

var Forcast = parsed_json['forecast']['simpleforecast']['forecastday'];

for (index in Forcast) {
var imageurl = "http://.....";
/* Weather Box 0 Day 1, 1st period */
$('.Wicon').css('background-image',"url("+imageurl+Forcast[0]['icon']+".svg)");
$('#GetWeatherDay').html(Forcast[0]['date']['weekday']);
$('#GetWeatherState').html(Forcast[0]['conditions']);
$('#GetWeatherTempHigh').html('High of '+Forcast[0]['high']['celsius']+'&#8451');
$('#GetWeatherTempLow').html('Low of '+Forcast[0]['low']['celsius']+'&#8451');
$('#GetWeatherMaxWind').html('Max Wind '+Forcast[0]['maxwind']['mph']+' Mph');
$('#GetWeatherAveWind').html('Average Wind '+Forcast[0]['avewind']['mph']+' Mph');
$('#GetWeatherWindDir').html('Wind Direction '+Forcast[0]['avewind']['dir']);
$('#GetWeatherHumidityA').html('Average '+Forcast[0]['avehumidity']+'%');