  We present an observational reconstruction of the radial water vapor content
near the surface of the TW Hya transitional protoplanetary disk, and report the
first localization of the snow line during this phase of disk evolution. The
observations are comprised of Spitzer-IRS, Herschel-PACS, and Herschel-HIFI
archival spectra. The abundance structure is retrieved by fitting a
two-dimensional disk model to the available star+disk photometry and all
observed H2O lines, using a simple step-function parameterization of the water
vapor content near the disk surface. We find that water vapor is abundant
(~10^{-4} per H2) in a narrow ring, located at the disk transition radius some
4AU from the central star, but drops rapidly by several orders of magnitude
beyond 4.2 AU over a scale length of no more than 0.5AU. The inner disk
(0.5-4AU) is also dry, with an upper limit on the vertically averaged water
abundance of 10^{-6} per H2. The water vapor peak occurs at a radius
significantly more distant than that expected for a passive continuous disk
around a 0.6 Msun star, representing a volatile distribution in the TW Hya disk
that bears strong similarities to that of the solar system. This is
observational evidence for a snow line that moves outward with time in passive
disks, with a dry inner disk that results either from gas giant formation or
gas dissipation and a significant ice reservoir at large radii. The amount of
water present near the snow line is sufficient to potentially catalyze the
(further) formation of planetesimals and planets at distances beyond a few AU.
