In the wintertime, reporting severe weather conditions is extremely important to the National Weather Service.
NWS personal need to be able to make the best forecast possible and without the timely and accurate reports from spotters on changing conditions, their jobs in forecasting the weather would be extremely tough.
They rely on not what they see on radar, but also what the spotters report.
Radar is a good tool of telling where a changeover to sleet is occuring from snow but radar along cannot do the job.
Spotter reports are very important when changeovers occur because road conditions then change as well as the forecast.
The reporting criteria is not the same with ever office, but the criteria that is listed below is a sampling that most NWS offices use.
For your local office's reporting criteria, you should get in contact with them so that they can provide you with the neccassary information.