The Spanish word for forecast is pronóstico. It is pronounced "pro-NOS-tee-koh". See this site for confirmation: http://www.answers.com/library/Translations
You can say "La pronóstico para mañana."
El pronostico para manana..
Pronóstico de tres días
"prognostico (del tiempo)"
although tiempo is the word for weather, Weather Forecast is prevision metereologica
You say "weather forecast" in French as "prévisions météorologiques".
Prévision
πρόβλεψη (provlepsi).
Usually it will say that on the forcast.
"la programacion semanal del canal 5"
Sister in law in Spanish is cunada.
you forecast the weather by goig outside and say hi to the sky and everybody u see outside and then say what are you looking at and then u have forecasted the weather