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Meaning of yellow alert

yel'low alert'

Pronunciation: [key]
  1. (in military or civilian defense) the first alert given when enemy aircraft are discovered approaching a military installation, city, coastline, etc. Cf.
  2. the signal or alarm sounded for this alert.
  3. any primary stage of alert, as when toxic chemicals have escaped in a particular area and a potential hazard exists.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.