That's what I call it when one day you're looking at the leafless trees of early Spring, and then...suddenly...without warning...FOOMP! Every leaf in the Northern Hemisphere says "HERE I AM!", and the trees are magically transformed to greeny goodness.
It's unpredictable. No one knows exactly when it'll happen, but everyone does the "when-the-hell-did-that-happen?" double take when it does. It usually happens when you least expect it, and definitely when you aren't looking... hence the WTHDTH look.
For us, it's happened coming home from the annual Spring trip to my brother-in-law's in Boston, or on the way home from a Lions Club state convention. I think one time it happened on our way home from South Carolina... which was pretty cool given the whole distance and climate change.
Well, I welcome the FOOMP!, as it's the sure way to know that Spring is just teasing us.