Don't beat yourself up. There is actually some predictive value to the early frames, but I find it's more useful to avoid the precip panels and start high. Take a look, for instance, at the steering flow itself, the 250mb pattern. Compare 36 on the NAM and 42 on the 18z GFS. On the NAM you can see there's a piece of energy over Chicago (etc); on the GFS, it's not there. It's always a guessing game, but what that piece of energy ends up doing is forcing the NAM south; without it there, the GFS flow is more flat, allowing the energy in the west to ride straight across country, and further north by the time it reaches the Atlantic.