It's so cold that if it rains it'll snow

Solution 1:

Don't change a word! Your sentence is a fine example of a Yogiism (a pithy comment or witticism that uses paradox or tautology). Here are some of my favourite quotes attributed to Yogi Berra:

It ain't over till it's over.

This is like deja vu all over again.

You can observe a lot just by watching.

If you can't imitate him, don't copy him.

Baseball is 90% mental -- the other half is physical.

It was impossible to get a conversation going; everybody was talking too much.

Nobody goes there anymore; it's too crowded.

A nickel isn't worth a dime today.

I take a two hour nap, from one o'clock to four.

When you come to a fork in the road, take it!

I didn't really say everything I said.

Solution 2:

The way it is now is perfectly correct and actually very witty and funny, so there's no need to change anything, but let me propose another idea, which should be equally good and maybe a bit simpler to understand.

It's so cold that if it wants to rain tomorrow, it will snow.

I'm inspired by the tagline of Resident Evil (1996 video game), which says:

If these walls could talk, they would scream.

It's not really the same thing, but you can see the similarities.

Solution 3:

It's so cold that if rain falls, snow will land.

Solution 4:

You could go the technical route with:

It's so cold that, if the fall speed of water droplets (or ice particles) in clouds exceeds their updraft speed, it'll snow.

Edited to include:

It's so cold that, if the conditions are right for precipitation, it'll snow.