I’ll start.
“It’s so hot that I saw a dog chasing a cat and they were both walking.”
Not exactly the same as your phrase, but I’ve always liked, “it’s hot as balls”
A snowball just moved its family to Hell.
It’s so hot today, the air smells like ironing.
…people are sticking to the roads.
I saw a roasted turkey fly by.
OK, who’s going to be first with the expected line? 
The one I always heard was that it was hot enough to fry an egg on the pavement (= sidewalk).
At the other extreme: It’s cold enough to freeze the balls off a brass monkey.
HOW HOT IS IT?
You’re welcome.
I’ve always enjoyed the colloquialism, which in no way fits to this thread’s motif, but fuck it, is …
Chrissakes, it’s hot enough in here to breed sheep.
I don’t even really know what it means, but I like it.
It’s so hot that I saw a squirrel blowing on his nuts
Here’s another:
“It’s so hot today I saw two trees fighting over a dog.”
…I set the house on fire just to cool it off.
… I saw a fire hydrant chasing a dog.
A related one:
“It’s been so long since it rained that we had to ship the fish to Seattle so they wouldn’t forget how to swim.”
…I am very uncomfortable.
…I am finally warm!
Not exactly the same phrase… It’s hotter than 3 hells!
For some reason I saw the thread title and my brain immediately came up with
“It’s so hot that my boobs are melting.”
Not having fake boobs, I can only imagine that would SUCK.
I tried doing this while camping. Apparantly, 115 is not quite hot enough to cook an egg on a rock, darnit.
It’s hotter than a whore’s ass on dollar night.