I trucked in a cubic yard of garden mix (topsoil, sterilized mushroomcompost, compost and sand blend) and have it in piles and barrels and the bed of my truck. We get 66" of rain annually out here, which since I brought the soil means it's rained 3x, once pretty hard. I always kept the piles covered.
But is this necessary? In how much rain? I'm concerned with the good stuff leaching out of the garden mix in the wrong place, but at least none of the piles are in standing water areas. But this may all be moot as the places we buy compost/soil from keep them in 4-sided concrete bunkers open to the rain!
I would be more concerned about the rains separating out the sand and topsoil particles than "good stuff" going missing. Nutrients in that mix are most likely being held by the bacteria and fungi in the blend.
By the way, If you can find non-sterilized mushroom compost, it will be far better since all the micro biota will still be alive and thriving.
Sterilized mushroom compost has been heated in a pressure cooker (autoclave) so anything that was living in it is now dead.
All the mushroom compost I've ever tested never had any pathogens present, so it is really a waste to sterilize it except for the fact that the government requires it to be killed prior to being sold.
Thanks as ever, Bryant. I'm sure you're right about the mushroom compost and I lamented it being sterilized when the saleswoman joked, "people hope that mushrooms will come out of it but it's sterilized!" She said it's also good for having chicken manure in it but that must have been added before the autoclaving. I imagine they threw their spent straw and wood pulp to chickens for bedding.
When you can get mushroom compost, it has been used up anyway.
It is the government that requires it to be sterilized.
If you want to "recharge" the microbiota, see if you can find some manures; cow, horse, chicken, hog (not pink pigs), goat manures are super for bringing microorganisms along with them.
If there is a horse stable near you, see if you can acquire some used bedding straw from them to add to your compost in waiting. That will add lots of good bacteria.
Just letting the air hit your sterile compost for a few weeks to months will bring bacteria back into it. Don't forget that any worms that come to that pile laying on the ground will also bring in microbiota that you want.