bad weather vs overcast

bad weather and overcast both are nouns.

Word	Noun	Adjective	Verb	Adverb
bad weather	Yes	No	No	No
overcast	Yes	Yes	Yes	No

As nouns, overcast is a hyponym of bad weather; that is, overcast is a word with a more specific, narrower meaning than bad weather:

Other hyponyms of bad weather include raw weather, storminess, cloud cover, cloudiness, turbulence.

bad weather (noun)	overcast (noun)
weather unsuitable for outdoor activities	a cast that falls beyond the intended spot
	a long whipstitch or overhand stitch overlying an edge to prevent raveling
	the state of the sky when it is covered by clouds
	gloomy semidarkness caused by cloud cover

bad weather (adjective)	overcast (adjective)
	filled or abounding with clouds

bad weather (verb)	overcast (verb)
	sew with an overcast stitch from one section to the next
	sew over the edge of with long slanting wide stitches
	make overcast or cloudy