Why does ethan feel a dread in Zeenas silence on this particular morning?

Zeena's complaints always took the form of intermittent science. On this particular morning Ethan was filled with dread. "Zeena's obstinate silence.....told him on certain stainless mornings, that before night there would be rain." Rain, in the metaphorical sense, meant something bad was to happen. Ethan cringed at the thought that something might happen to Mattie. Instead Zeena said she was going on one of her sickly excursions to find a new doctor.