That chestnut? More
sucker-bait, Cored? A bit passé; Planet Oil Barrel is the new black.
Firstly, "horizontal" weather balloons don't exist, they all rise by intent, and the actual wind speed is calculated from that. But, even in some idealized case of a perfect flow, the balloon will not reach the speed of the medium.
If that were possible, the balloon would never move in the first place.
http://www.physicsforums.com/showthread.php?t=210611
http://www.fkm.utm.my/~ummi/SME1313/Chapter 1.pdf
http://www.ias.ac.in/resonance/Apr2004/pdf/Apr2004p50-60.pdf
http://en.wikipedia.org/wiki/No-slip_condition
Another way to see it, is that the boundary layer adds mass. That can never be zero, so the force to accelerate it can never be zero, and that means the balloon will always be below WS.
Yet another is to see that the boundary layer means the path the air must take to get from one side to the other, is longer that the diameter of the balloon, meaning that the speed of that air would at need to be faster than the free-stream as the balloon approaches the speed of the free stream. It must stop short of that speed.
Vortex shedding, energy lost to angular momentum ( rotation of the air), will absolutely guarantee that the balloon will not get to WS.
Arguments from momentum transfer, work of friction etc etc....
I posted an equation to SZ, when he asked about the speed of dust. That too. It's based on added mass.