Possible Clues:

• In summer, archaic city linked to The Last Mummy and The Omen
• A uniform rule defended by chap? It's a telling sign
• Ominous sign
• Divination from omens
• Pre-29-Across
• A surge up occasionally tracks forecast
• Sign of important things to come
• Sign a posh wig should be returned by end of day
• Omen produced by Pharaoh over fairly ugly old city
• Portent or harbinger