A geocast alerting system employs wireless geocast transmissions to deliver customized alerting messages to particular geographic areas. Geocast alerting regions and sub-regions can be defined within the system. Upon notice of the occurrence of an emergent situation, one or more geographic areas that may potentially be affected by the emergent situation are determined. The potentially effected geographic regions are compared with geocast alerting regions and sub-regions corresponding to particular geographic areas. Target alerting regions that should receive alerting messages are identified based on the comparison. The content of alerting messages can be tailored for each target alerting region. The content of alerting messages can contain distinct portions of content, each portion tailored based upon one or more conditions related to communications devices within the target alerting region.