Ice Storm Warning
From The weather Vane WikiNWS, once the area given has reached criteria. The criteria is based on the given location. Often times an ice storm warning is issued when at least one quarter of an inch of freezing rain is expected.
Criteria based on location
Different areas has different charistics even though areas in the North areas are more prepared for it these areas has a different Criteria verses the Southern regions which are not as prepared for it.