This will get the weather and play it. Replace 12345 with your U.S. zip code and change *21 to the extension you want to use. This goes in extensions_custom.conf, of course...
exten => *21,1,ringing
exten => *21,2,System(/usr/bin/curl -o /tmp/12345.mp3 -s http://audio.pirateweather.com/12345.mp3)
exten => *21,3,Answer
exten => *21,4,Wait(1)
exten => *21,5,MP3Player(/tmp/12345.mp3)
exten => *21,6,System(rm /tmp/12345.mp3 -f)
exten => *21,7,Hangup
By the way this still uses a synthesized voice but it is better than festival in my opinion. Downside is a VERY short ad at the end for Feed2Podcast.com