whizofoz
and bubbers 44
whiz...your own statement says: almost always...not always.
It is entirely possible that you may observe meteorological clues that might lead you to believe windshear/microburst is possible...but not assured...so, do you sit on the ground for a couple of hours or do you add speed to Vr to help out?
IF windshear/microburst was totally avoidable all the time by a simple metric of temps, wind and so forth, we would do it. And doppler radar isn't available everywhere.
I was in CLT when a DC9 crashed during approach/go around and it was blamed on microburst among other things. I watched the smoke come up...I knew I wasn't going to takeoff in that crap.
I observed the weather personally as I was doing my walkaround. the sky was green, the storm had EVIL in it 10 miutes before the crash...but there is no requirement for ATC to advise you that if you continue you will die.
windshear is still in the infancy of predicting and avoiding, so Bubbers 44 is right...you never know and 99.44percent is not good enough