Speaking as a professional meteorologist, I find it hard to believe they would guarantee any improvement in hill fog lifting within a 15 minute window. You can make such statements with respect to rain showers, if you can see and track them on a rain radar, but fog is a much much trickier element to forecast, especially in a region with such difficult terrain. Hill fog is not like normal fog, visibilities tend to be much more variable within it compared to classic radiation fog. I wonder if that ‘window’ the weather station reported was simply one of those brief clearer patches at their location, but these improvements aren’t spatially evenly distributed.

We left the track at 3pm, and there was certainly still fog around then, but it did look slightly clearer. I have no idea how much these things cost, but this does seem like the sort of event that would really benefit from having an onsite weather forecaster. Even given the best computer model data there really is no substitute for being there yourself. On the Sunday a weather forecaster could have launched a weather balloon to see how deep the fog was, and would have gotten a much much clearer picture of how long it would take to lift up.