I doubt whether the gushing weather-wenches on TV can provide anything more accurate before Monday.
Looking at the surface pressure forecast sequence, I'd say they'll have a reasonable idea on Sunday when the particular area of low pressure that's supposed to cause the problem is predicted to form.
They'll have a reasonable idea - they have their model with all its chaos theory and the sim gets run repeatedly with slightly different initial conditions. If they all tend to converge on the same answer then they'll claim it as the forecast, if the sims all diverge then they usually just declare the weather as unsettled without predicting anything.
By Wednesday we'll have the answer.