I had this reply ready yesterday but the forums weren't working for a while and I had left the acronyms on my work computer, so now Tony's beaten me to it.
Impossible, you say? No acronym is impossible (except for IMPOSSIBLE*)
In the sense 'Living or lasting only for a day':
Earth's Pirouette Has Especially Morbid Effect, Restricting A Life.
There's a 'rule' around here saying that if you capitalise a word, you must acronymise it (note there's a disclaimer on the first 'what people are looking for' thread)... since you're new, I'll let you off doing EPHEMERAL, but now I must do the impossible.
*IMPOSSIBLE: I Mustn't Pass On Something So Irresistible... 'Bad' Letters Expanded!
Insomniac Mrs. Pleads, "Oh, Stop Snoring!" Instead Bringing Louder Explosions. (Some people won't stop snoring no matter what you do!)
If Not Proven, One Shouldn't Say 'Impossible' But 'Little Expected'
(I know it breaks your rule, Tony... but you break your own rules sometimes!)