This didn't turn out the way I envisioned it, but I think next time I'll get closer.
The first error was I chose the wrong paper. Actually, I did four drawings--all subjects requiring wet, flowing washes-- on paper I pulled from a Kilimanjaro sample pack, assuming it was 140 lb, but once the paper got wet (and floppy and wouldn't hold more pigment), I began to suspect it was 90 lb.
My storm clouds started off too intensely blue violet. (Can't lay off the smalt!) Adding burnt umber helped.