Stall warner (JAA regs) should be regulated to go off at 5kts or 5% above the stall, whichever is the greater. I think this is a minimum requirement, but it's been a while since I sat my ATPL air law.
As the previuos posts have correctly stated. If I could remember which book it is clearly written in, I would tell you. But it's to do with vectors of wind direction and speed. If you go forward at 60kts and get up updraft of 30kts you get a resultant of x kts.
We had a similar situation here on the south coast. The r/w was changed to 20 due to a sea breeze. Although, the sea breeze was only 30-50' thick.
So every had a stonking tailwind down final until that height, then go a nice piece of windshear just before touch down, with an extra pinch of speed . People going around for about half an hour until ATC realised.
Oh how i laughed!! I wanted to send a bunch of students on a first solo. That would put hairs on their chest.
Cruel to be kind and all that.