Is it correct to say "it is pouring down snow". Usually one says pouring down rain,conjuring an image of rain being poured out of buckets. However, this doesn't seem right to use the idiom when speaking of snow.
"It is snowing heavily"
"The snowfall is heavy today"
As you have very well pointed out, 'snowing' doesn't have many idioms attached to it, when compared to 'raining'.
"there is thick snow falling"
"it's chucking down snow" (less often)
"there is heavy snow falling"
You are right, 'pouring' is only for fluids, most often liquids. Although it is also possible to pour some gasses, and granulated solids e.g. "to pour sugar" or "to pour sand"
Snow is solid water, so does not pour down. Plus its feathery texture means that it generally drifts down slowly, no matter how much snow is falling at once.