Lyric discussion by hothothot! 

Cover art for It Always Rains on a Picnic lyrics by Modest Mouse

To me it's just saying that even the good things in life (here the picnic) always have bad patches i.e. the rain.

No matter what we do in life there are always going to be bad spells, and the "Don't let go quite yet" is the plea for not giving up on life because at least there are the good times, whether they are marred (by rain...) or not.

The repetition of "downpour" to me represents the shit days in life, and how some people dwell on them (the way he sings downpourrrr is kinda sarcastic, almost of if he's mocking those people who are constantly thinking of half empty glasses instead of half full.

wortel - I think the picnic is just a metaphor but it could always be a physical thing that the protaganist has gone to great pains to organise.

This is almost certainly my favourite Modest Mouse song, simply yet haunting. It's a really positive image as well, wrapped up in all the sadness (downpour) that at least we get the chance to have picnics (the good times) so we should all float on (to use a MM song) with life.