According to Wikipedia, "In 2002, the National Weather Service contracted with Siemens Information and Communication and SpeechWorks to introduce improved, more natural voices. The Voice Improvement Plan (VIP) was implemented, involving a separate computer processor linked into CRS that fed digitized sound files to the broadcast suite.... Additional upgrades in 2003 produced an improved male voice nicknamed "Tom", which could change intonation based on the urgency of a product"

Also, here are some audio clips http://www.nws.noaa.gov/nwr/info/newvoice.html