What you see on twitter - the colourful screenshot shared above - is a raw model output rather than a forecast. Model runs happen frequently, with lots of different models (ensembles). There isn’t just a single output from their supercomputer that tells you what will happen. The forecasters use their experience to know what outputs are more or less likely (how much confidence there is).
Sometimes five or six days out there is quite a bit of certainty. For example a storm moving down the east coast in mid winter.
Having said all of that the forecasting community seems worried about this one, and we have saturated catchments, so in the flood community we’re worried too.