  Near-infrared bandpasses on spaceborne observatories diverge from their
ground-based counterparts as they are free of atmospheric telluric absorption.
Available transformations between respective filter systems in the literature
rely on theoretical stellar atmospheres, which are known to have difficulties
reproducing observed spectral energy distributions of cool giants. We present
new transformations between the 2MASS $JHK_S$ and HST WFC3/IR F110W, F125W, &
F160W photometric systems based on synthetic photometry of empirical stellar
spectra from four spectral libraries. This sample comprises over 1000
individual stars, which together span nearly the full HR diagram and sample
stellar populations from the solar neighborhood out to the Magellanic Clouds,
covering a broad range of ages, metallicities, and other relevant stellar
properties. In addition to global color-dependent transformations, we examine
band-to-band differences for cool, luminous giant stars in particular,
including multiple types of primary distance indicators.
