I live about 20 minutes south of Haifa with my family. Radio Haifa, a local radio station, has announced it has opened a channel which will be silent unless there is an emergency such as, I guess, news of an impending missile strike by Hizbullah. Is it authorised to have this channel on over Shabbat.

It sounds like the same solution that was used during the Gulf War; if so, you can leave the radio on during Shabbat without any doubt.