Remember the old Civil Defense concept?
Any "mass alert/communication" process should include the general public as a vital component. With the advent of the semantic web, data is broken up into smaller units which then may be reformed into metadata sets and other "use specific" tasks. It allows the web to become, in essence, one big relational database. (Click on image for a larger view).
Traditional organizations and government of course will play the primary role in any distribution of "official" data. But with more and more data becoming available and the ability to use this data becoming more flexible, it is probable that over the near term, the "general public" will use this new found freedom to create its own sophisticated information gathering and "alert" systems outside the traditional sources.