Related Clues:

• Red sky, maybe
• Herald
• Thunderclouds, perhaps
• See 29-Down
• Presage
• Spilled salt, say
• Sign of trouble
• Future indicator
• Sudden clouding-over, maybe
• Sign from above
• Thunderclouds, for sure
• Writing on the wall, e.g.
• Mysterious sign
• An eclipse, some think
• Foreshadowing