Well, I do know some people put blanks in the freezer to dry them since a freezer and refrigerator are dehydrators. I would probably cover them during the winter. Should be fine in the summer as well. Most air dried wood that is left outside gets down to about 15% MC. You bring lumber back into the drier shop for 6 or so months before you make furniture out of it. I would expect that 15% to be fine for twice turned bowls.