I think precipitation amounts (as well as sky cover, wind, temperature) are just populated from a grid and there probably isn't much human manipulation in those point-and-click forecasts. I wish I could remember all this better because oceanstatewx has explained all this in great detail several times how this process works . I think this is why you'll often see forecasts saying "mostly sunny" when it ends up being filtered sun behind high clouds...I know at least NBM does better with this but I believe MOS won't report or forecast clouds above like 10,000 feet (or 12,000 feet)?
But for QPF amounts with convection, I would assume in the grid its just averaging whatever is falling within that grid and that's how it determines the ranges? But I also think oceanstate has said that the wording used in the forecasts is based on QPF totals and visibility? So showers versus heavy rain wording will be tied into the QPF range and with snow light versus say heavy is tied into rate/visibility.