I've also noticed the "hourly" forcasted mins and maxes often are off by a few degrees when compared to the "summary" max and min forecasts. Not only on WU, but Accuweather.com, and the US NWS. My guess is the "summary" is for whatever nearby Asos weatherstation, usually an airport. The hourly forecast may be extrapolated from the summary, based on your exact coordinates/elevation. This is just my guess why there is a discrepancy. Anyone have any better ideas?