As a good pessimist, I frequently wish to humorously convey extremely low probabilities. I'll often use the phrase "a snowball's chance in hell," or a variation of it, to express this:
There's a snowball's chance in hell that my candidate gets elected
There's a snowball's chance in the Sahara that my efforts accomplish anything
And so forth. Are there any common Latin expressions used for such situations? I'm particularly interested in phrases attested in Classical Latin.
I checked Wikipedia's list of Latin phrases but didn't find anything suitable.