I've stored Anthracite pea coal outside uncovered for several years on black top/concrete with no ill effects to the coal. See my avatar. It was rained on, snow on and pee-ed on by any number of animals. When it was finally all removed by use or transport to my new coal shed, the only ill effect was a noticeable difference in the color of the blacktop/concrete vs. what didn't have coal on it. I attribute it to the lack of sunlight and some washing of the coal. After two years of the coal being removed the difference is not noticeable. I believe the advantage of having the coal rained on is it washes the coal dust off. For me the rain that made it thru the coal pile went into a drain. Depending on your situation in may accumulate as "fines" in a low spot.