"sky" data missing overnight from 10 day hourly forecast?

I run a query a quarter to the top of each hour (so xx:45), 24 hours a day, looking at the cloud cover ("sky") for the upcoming hours. However, every night I encounter errors around 9:45 PM (mountain time) due to this node missing. Is there any reason this data would go missing entirely at a consistent time nearly every night?

Here is an example query:

http://api.wunderground.com/api/my_key/hourly10day/q/CO/KFNL.xml