Define 'weather'. Thats just one of the difficulties.
Easy. Weather is "something I'm not going to fly through".
Pretty much all the hazards that an aircraft is going to want to avoid are related to precipitation, though there are obviously difficulties in that the picture that the aircraft sees is not necessarily the same as what a ground based system would see.
That said, I remain stunned that UK ATC doesn't have some sort of precip picture superimposable. It might not be perfect but it would surely offer some clue that deviations around particular areas are very likely to be requested.