Theoretically simple but the devil is in the details. You start with weather maps from the past few days and track the movement (speed and direction) of the highs, lows, fronts, etc. Then you assume that these trends will continue for the next seven days. You have to account for the effects of the mountains, bodies of water, and such. I can do it fairly accurately for a few days but don't have the software to do it any further ahead than that.
BTW, that sounds like what I have been hearing in the TV forecasts here in Southside VA, although I guess that the systems involved cover a great deal more area than that.