I believe I discussed this with you prior, but there does seem to be a persistent tendency of low height anomalies in the SW North American region — from the Pacific off California, into Baja/NW Mexico and the SW US. I think that plays a role in interfering in the winter cold that would otherwise strike the Atlantic South.
It's not a big deal if the troughs remain in the Baja/nearby Pacific — the flow is effectively zonal from there, so things stay quite mild
But those troughs have a way of "hanging around" in Arizona, New Mexico, and other SW US regions — either as "positive tilt" artifacts or complete bowling balls. These western troughs "bleeding east" set up the situation wherein cold focuses more on Texas, Louisiana, and western Southern states instead of spreading eastward into the Atlantic South states.
As far as mountains, if the Apps are continuously ~5000+ in height (i.e. as opposed to just isolated peaks reaching that point), then that will indeed stop anything up to ~850mb emanating from the west. But, in situations with warm sectors encompassing Alabama, Mississippi, Louisiana, etc that is west of those ranges, the trough orientation definitely is important.
