Originally Posted by
steve64
Leaving DEN, the plane needs to be fueled to fly and make an approach attempt at SEA, then (practically at SEA) divert to an alternate airport. And yoy can't pick any close-by airport as your alternate. The weather forecasts has to meet FAA minimums. Thus, if the "entire northwest" (probably was expecting fog/whatever, then the closest legally available alternate may have been a long ways from SEA. The distance DEN-SEA-Alternate (plus required "reserve" fuel at touchdown and/or anticipated "holding" fuel and/or extra fuel for headwinds and/or zig-zags between storms, etc) may have exceeded the range of the aircraft.
The only possible downside to all the required planning is when the "worst case" doesn't happen, and you end up with more fuel than you would have needed. Or the old saying goes, "the only time you can have too much fuel is when you're on fire".