How do you say "freezing rain" as if it's actually happening? Freeze raining? Freezing raining? Neither really make sense?

Past tense - There was freezing rain.

Present tense - There is freezing rain.

Future tense - There will be freezing rain.

FREEZING is the adjective to describe the noun RAIN. Neither one is a verb, so there isn't a question of past or present. The rain freezes when it hits the ground. The sentence could be past or present or future:

There was freezing rain last night.

There will be freezing rain tomorrow.

It is raining but it is freezing rain. Or, freezing rain is coming down.

No expert here, but I'd avoid the problem by treating "freezing rain" as a noun and add a verb to absorb the tense issues.

"freezing rain fell"

"freezing rain is falling"

"freezing rain will fall"

this would be congruent with

"snow fell"

"snow is falling"

"snow will fall"