What it really means in terms of exact timing is that an AWOS (weather station) will record measurable precip between the 1000 report and the 1100 report. Some

What it really means in terms of exact timing is that an AWOS (weather station) will record measurable precip between the 1000 report and the 1100 report. Sometimes percent means forecast confidence, sometimes it means that they can't predict what area will get hit with rain. Like synoptic conditions will say "this region will definitely get a few thunderstorms", but when you drop down to mesoscale then you can't tell which city will actually get the rain. In longer-term forecasts, 10% usually means "I have no idea what's going to happen, so I guess precip is possible then." At mid range, like 3-5 days, it usually means "we haven't nailed down the timing of this thing yet". If you actually want to know what the logic is, go read the forecast discussions.