Red Flag alerts are issued when low humidity raises the fire danger to a critical level. Many of my early experiences in ham radio public service were associated with "red flag" patrols. I literally had a route to drive through the back country to check for any hotspots. Amateur radio provided a means of directly alerting the forestry and fire officials to jump on a problem while it was small.
Thanks for the info. Myrddin....Well at least I can say I learned something new today! I usually assoc. red flag weather warnings to high waves, or high winds on the coast.