Federal officials are planning to roll out a new terror threat system in coming days, on the heels of new warnings about home-grown attacks. The new system would be the third major federal warning system in the years since Sept. 11, 2001, beginning with the original color-coded Homeland Security Advisory System, which was abandoned in 2011. The current National Terrorism Advisory System is meant to notify the public about an "imminent" or "elevated" threat to the country. However, it has never been used, in part because the bar for triggering an alert is too high, Homeland Security Secretary Jeh Johnson...