If there is a storm in Edinburgh at 12pm, and it is moving south at 60mph, then we can forecast that it will rain in London around 5pm. Generally, this provides quite a good forecast for the short term - this is known as "now-casting".
However, there are variations in the wind speed and direction which are too small to measure. These variations can add up over the course of the storm's journey, and it might finish up in Cornwall at 5pm.
