The National Weather Service offers RSS/XML feeds for weather alerts for every county. https://alerts.weather.gov/#TechNotes
There’s not currently a limitation that I could find on the number of API calls per day. This would be useful for US users as you can get faster updates than you can get with the Weather Underground alerts.
There’s also an XMPP based alerts stream http://www.nws.noaa.gov/nwws/news.html but I wasn’t able to find any details on how it works. This method would allow for even faster updates without constantly polling an RSS feed.