Wind chill = the amount of heat you perceive your body to loose because of the chilling of the wind.
The ambient temperature may be 36 C for example but the wind will reduce the temperature you fell to be less the 10 centigrade.
Wind chill I would say is a noun phrase.
Sub zero I would say is an adverb.
a word or phrase that modifies or qualifies an adjective, verb, or other adverb or a word group, expressing a relation of place, time, circumstance, manner, cause, degree, etc. (e.g., gently, quite, then, there).
Impacted is a past particle it means for you context the citizens were and have been impacted since the bad weather hit.
Impact is both a noun and a verb.
I think their may be disagreements as to what it is in your context. I Would say a verb it had a strong impact on the citizens of Madison.