No, it's not about full load. You get this incomplete page, and everything just stops. You can let it sit for minutes after that, and a complete page isn't going to happen. WU 10-day has just decided that it's done. Take that! You reload, and the same thing happens. But as I say, this incomplete page business happens about half the time. Never seen any correlation with page load success with anything else.
I never used to see this, and this incomplete page business has been showing up for at least the last month or two.
Actually, the log in/out business seems to have some effect. When I'm logged in, and get an incomplete 10-day page, and then log out, BOOM! the 10-day is displayed just fine. Some evidence that in order to confidently get a 10-day forecast, I shouldn't be logged in.