兆し and 兆候: signs of weather, recovery, and a possible cold
Read storm easing, sales recovery, and a possible cold as the targets of signs, preserving possibility and slight beginnings rather than a good-versus-bad rule.
兆し in weather and sales recovery
空が明るくなり、嵐が収まる兆しが見えた。 reports the brightening sky showing signs that the storm was easing. 兆し, read きざし, identifies an indication of change rather than a supplied result that the storm stopped completely.
売上に回復の兆しが現れ、店主は少し安心した。 reports signs of sales recovery giving the shop owner some relief. The signs and the owner's actual reaction with 少し安心した are stated; full sales recovery is not.
Possible and emerging 兆候
喉の痛みは風邪の兆候かもしれない。 says that a sore throat may be a sign of a cold. Read 兆候, pronounced ちょうこう, within the possibility expressed by かもしれない, rather than as a definite diagnosis.
売り上げの数字に回復の兆候が少し見えてきた。 describes sales figures beginning to show slight signs of recovery. 少し and 見えてきた limit the degree and the emergence of those signs.
Signs and their stated targets
Both 兆し and 兆候 occur with sales recovery. An absolute division assigning only favorable changes to 兆し and only unfavorable changes to 兆候 would misread the recovery example.
Identify storm easing, sales recovery, and the possible cold as the respective targets. Keep the indications, the owner's reaction, and the possibility separate from an unstated final outcome.
Do the examples allow 兆候 to be defined as a sign of unfavorable change only?
Show the answer and why
No. 兆候 also describes signs of sales recovery. The sore-throat sentence states a possibility, and the sales example supplies only slight signs beginning to appear.