There can be, as erm I saw demonstrated recently, very large thunderstorm occurring in between observations, and that thunderstorm cannot be seen by any one of the surface observations, but it is there, radar can pick it up, for instance, or a satellite can see it, but the man on the ground can't see it necessarily, and so that could slip through and it may be the only thunderstorm in a vast area, but it is there, but the observations don't show it.