Weather cancellations don't always happen because of the weather at your origin airport, ie say the plane that was supposed to operate your flight was coming from Chicago, and the fog was to thick to take off and it was canceled. The plane is now stuck in Chicago for example and is not available for your flight. So even though it could be clear in JFK, your flight was canceled because the plane was stuck in ORD because of weather.
Weather cancellations don't