The center of max vorticity in the upper levels of the Attm will not correspond to the highest precip. It lags to the west (and usually sw) from location of the developing surface low. This is because downstream upper level divergence east of the vort max fuels the surface low development. The heaviest precip is generally determined by the corresponding conveyor belts of air. A warm moist bet from the sw and a cold conveyor from the North east so you always have to displace the heaviest precip from the center of the surface low. Now what you also might be seeining is another effect. The first part of this storm is energy dropping out of canada. That's going to bring snow to the Idaho and Montana regions...but the "bulk of the precip" predicted by the models comes from the axis of the trough moving from the pacific inland into Cali and Utah. That's south of Idaho in the sunday-tuesday time frame