“The forecast calls for rain.”
You and your roommates are planning what to do this weekend. Someone suggests going fishing, but you watched the news this morning and heard that the weather is going to be bad this weekend.
The forecast calls for rain.
A "forecast" is a prediction of what's going to happen in the future.
The most common example of a forecast is a weather forecast. Weather reporters give predictions of what the weather is going to be like for the next few days.
Another common type of forecast is a "financial forecast", which is a prediction of how much money a company is going to make and spend for the next quarter, year, etc.
When you're recounting what you heard or read in a weather forecast, use the phrase "calls for ___":
The weather forecast calls for snow.
The forecast calls for thunderstorms.
The news calls for rain.
It means "the forecast says that there will be ___."
This phrase is slightly formal.