Blip, I don't want to get into a raging debate; a 20% chance is a 20% chance, not 100%... If it remains a 20% chance then it will not happen (it's reviewed constantly)... The forecast in a TAF is a living event; if the chance goes up; ie it becomes more likely that a storm will actually hit within the 5NM of the aerodrome reference point then the chance goes up and the TAF is amended to include the prob/inter period; mentioning a PROB 30% has the same effect as saying 100% possibility; but a 20% chance has the same effect as saying 0%; you have to draw the line somewhere; I believe it is in the right place.
You will never see a TS hit an aerodrome that is not forecast in a TAF; fact. It might be forecast only moments before, but it will happen.
I do see what you are saying; I just disagree with your point. I see no value in having a 10% chance being considered for fuel/alternate planning; that is 90% chance of not happening; the industry standards currently revolve around 30%; it seems to work, so what is the problem exactly? You seem to believe that we should be considering the chance, but why stop at 10% under your logic, why not 2%?
Having done some met stuff in my time; it's a science, not a science... It's a guess more often than not, educated yes, but still a guess; the change of a 20% chance, to a 30% chance maybe be just nothing more than a gut feeling of an individual based on previous experience, computer modelling be damned.
I guess my final point is, is it broken? if not, then don't mess with it.