Although the cathedral figure is beautifully defined on this board, it's neither centered in the board nor straight with the edges.  Use a chalk line to get the geometry right.  Center the hook at one end, hold the other end to center, and snap the line. Measure to the nearest edge of the board, then measure the same distance on the other side of the line at both ends. You will decide which end will give the best figure for the length and width of the part you need.  If the harvested board is not wide enough, search for some straight-grained, riftsawn, color-matched material and glue a piece on each side. The result is what would have been there if all trees were made perfect!

The first and last boards that come from a plainsawn log normally have cathedral figure - it's the grain that looks like the undulating ripples of a pebble tossed in a pond. Boards with cathedral grain are the most prone to cupping. They will always cup away from the heart of the tree. That's because the tissue on the top half of the board is more tangentially oriented than the tissue on the bottom half. So shrinkage is greater on the top half, hence the cupping.

If the cupped board has a moisture content of 8% and you can flatten it by jointing and planing to get the thickness you want, it's not likely to cup again. So use it for its beauty and don't worry about the likelihood of distortion.

If it's cupped more than can be removed by a jointer/planer, the solution is to saw it down the center of the cup, flatten the two boards, and reassemble with a butt joint. You don't have to flatten the two pieces individually. They can be reassembled in alignment, then flattened - same difference. You will hardly be able to detect the material missing from the center, especially if you saw with a thin-kerf blade.

In a badly cupped board that's worth saving, saw it into three pieces to reduce the effect: the narrower the boards you saw, the more you reduce the distortion.

Look at the edge of this cathedral-grained board. The severe angle of the grain to the face of the board is called short grain. A rail made from this, if stressed, would be prone to breaking along the short grain. Consider this firewood, not furniture wood.