I beg to differ, as I am an NWS certified spotter. I took the class through NWS LZK (North Little Rock, Arkansas) and you are most definitely certified, if you take the course from an NWS representative. I do know that some Ham clubs offer the Skywarn materials through local training nets, but these are usually accompanied by the recommendation to take the class from the NWS to become officially certified.
To become NWS certified, you have to take a brief multiple choice exam over the course materials, and after passing it you will recieve a certificate of training from the local NWS Office, signed by the local Warning Coordination Meteorologist.
I personally don't use ham radio to report severe weather, since I have a cell phone, but I do listen to the excellent ham spotter networks on my scanner. The use of ham radio is optional, but usually the NWS will take reports from trained spotters in any form they can get them. The best way to find out about SKYWARN training and certification is by contacting your local NWS office.
Damon Poole,
NWS Trained and Certified Skywarn Spotter.