They are considered a prediction, a hypothesis, an educated guess. Weather is not completely predictable, but it is neither a fact or an opinion. They look at many factors and then give you their best guess through use of scientific analysis. Including, but not limited to weather patterns and Doppler readings, and barometric pressures.
It's a prediction, hence "weather prediction". Although the word "prediction" doesn't mean "Knowing the future for sure", but rather just "Believing to know the future". Therefor, a prediction isn't necessarily certain.