also: あめもよう
noun
signs of rain; threatening weather
Describes the look of the sky or weather when it seems about to rain. Often used in weather forecasts or casual conversation.
朝から雨模様だから、傘を持って行ったほうがいいよ。
It's been looking like rain since morning, so you'd better take an umbrella.
天気予報によると、
According to the weather forecast, it will be threatening rain tomorrow.
曇り空 simply means a cloudy sky, while 雨模様 specifically implies that rain seems imminent.
Compound of 雨 (rain) and 模様 (pattern, appearance). The reading あまもよう is a common rendaku form.