ragtop,
Follow the 558 line (for ex.) as it comes onshore in So. Cal. Arcs up over southern Canada and exits off Maine. Essentially everything below it is just one big southern ridge of air. None of that (underneath it) is very cold at all really.
Now find the 540 line and trace it. THAT'S the one we need running at or very near our state, but it's hell & gone from the US once it exits the state of Washington.