Halliday & Matthiessen (1999: 553-4):
What we are saying is that there is a global pattern of probabilities in English, including a probability profile of the tense system whereby the probability of future is (say) 0.1. The register of weather forecasting, however, sets up a local pattern in which the probability of future is (say) 0.5. This changes the meaning of the system of primary tense, because it reverses the marking and hence sets up a new relationship among the different terms. It construes a realm of experience in which the future becomes the familiar dimension of time, the point of reference by which both present and past are defined.