There's a chart viewer here.. shows you everything you need to know... dew points, temps... all at 3 hourly intervals... Knock yourself out http://www.netweather.tv/index.cgi?action=nwdc;sess=
There's probabbly a rough grid you can't see with temps and due points and because its just a rough estimate the shape of this grid is probabbly square... so it just works out if the precipitation is in a grid that has right dew points for snow/ sleet etc...