@chgowiz@dice.camp@Irisfreundin Gotcha. If you were to mentally divide up the US into thirds going E/W, Zone 5a is in the top (northern) third at the bottom.
It starts at the border of the states of Wisconsin/Illinois and heads due west, mostly, till it hits the Rocky Mountains where it smears across that area going N/S. It then follows a rough line tracing the border between Idaho and Montana. It's not a perfect line, there are bulges and points here and there.