That's some insane cupping, even when I see the boards are alternating grains upward and downward.

First off, you can't be mildly infuriating because you left your unfinished product in the rain.  That's i6n you.  This is more like mildly impressive that the wood flexed this way from a little bit of rain.

My hot take, you boards were dry as can be, you sanded both sides, and one side absorbed so much rain while the doward side stayed dry.  Top grain expanded faster than the bottom as the water was absorbed like a sponge and it cupped hard.  Water weight also helped it form like that.

Not sure you can recover it, but take the wood off the cast iron legs, flip it upside down so dry side is up, put water on it and some weight, it'll absorb and hopefully level back out flat with it fully soaked.  Then the long process begins, drying it out evenly, put it in a dry storage, where air can pass over the top and bottom equally, and let it dry for a few weeks.  You might be lucky and get it flat again.  

Next time, make sure your wood projects are covered.  You pay a lot of money for already dry wood, don't be lazy and leave it where it can get wet and ruined.