According to Wikipedia:
A moonbow (also known as a lunar rainbow, lunar bow or white rainbow) is a rainbow produced by the moon rather than the sun. Moonbows are relatively faint, due to the smaller amount of light from the Moon. They are always in the opposite part of the sky from the moon.That explains our conditions perfectly. The moon was rising in the East and there was rain over the ocean to the West. The moonbow we saw was a full moonbow; we could see the full arc stretching across the far ends of the Philippine Sea.
It is difficult for the human eye to discern colors in a moonbow because the light is usually too faint to excite the cone colour receptors in human eyes. As a result, they often appear to be white. However, the colours in a moonbow do appear in long exposure photographs.
Moonbows are most easily viewed when the moon is near to full (when it is brightest). For other than those produced by waterfalls, the moon must be low in the sky (less than 42 degrees and preferably lower) and the sky must be dark. And of course there must be rain falling opposite the moon. This combination of requirements makes moonbows much more rare than rainbows produced by the sun.
Up until about two hours ago I had never even heard of something like a rainbow. A rainbow at night? Are you kidding?
I guess I can chalk this up to just another one of the cool things you get to see when you live on Saipan.