she wrote it off as a nightmare, right after she had it.

"She had dreamed of the Seanchan, too, of women in dresses with lightning bolts woven on their breasts, collaring a long line of women who wore Great Serpent rings, forcing them to call lightning against the White Tower. That had started her awake in a cold sweat, but that had to be just a nightmare, too."

One argument is that she is only trying to tell herself that it's a nightmare, not a Dream. But I don't buy that, because she had 3 other true Dreams of Seanchan that she hated and she didn't try to fool herself into thinking they were normal dreams.

Aes Sedai aren't even able to call lightning against the WT. Perhaps they can find a "long line of" Accepted, but I don't think we have time for that.