There are plenty of on-line sources on information.
First and foremost, I go to www.duats.com. This is site provided to us by GTE and the FAA. You must be a licenced pilot with a current FAA medical to gain access to this site. This site provides every type of weather information a pilot can use. Currents and forecast conditions, Surface to upper level winds, temps, dew points, ect.. as well as graphic weather charts to view precipitation, frontal locations, cloud types and heights, ect..
Many of the aviation message boards, or web sites may have links to other weather site, such as the National Weather Service, (www.nsw.noaa.gov) or even the weather channel.
As a pilot, I utilize all different types of resourses when flight planning. I watch the news, read the forcasts, call flight service for a weather brief, and do some guess work on my own.
As pilots, we should have more than a basic understanding on the weather and how our weather systems work. With this understanding, we can take a look at the present conditions and make an educated guess as to what will happen.
I hope this helps answer your question.