A forecast is just a guess of what the weather will be like in the near future based on current data. When I fly full scale, I don't even get a forecast until fewer than 6 hrs. before the flight. The TAF (Terminal Aerodrome Forecast) is usually pretty accurate. TAF's are renewed every six hours (at the 12's and 6's). Valid times are GMT.

Link to TAF's:
http://adds.aviationweather.gov/tafs/

Type in nearest airport identifier and click "Get TAFs and METARs". If anyone would like to know how to decode TAFs and METARs, ask and yee shall receive.