  The Event Horizon Telescope (EHT) is a very long baseline interferometer
built to image supermassive black holes on event-horizon scales. In this paper,
we investigate candidate sites for an expanded EHT array with improved imaging
capabilities. We use historical meteorology and radiative transfer analysis to
evaluate site performance. Most of the existing sites in the EHT array have
median zenith opacity less than 0.2 at 230 GHz during the March/April observing
season. Seven of the existing EHT sites have 345 GHz opacity less than 0.5
during observing months. Out of more than forty candidate new locations
analyzed, approximately half have 230 GHz opacity comparable to the existing
EHT sites, and at least seventeen of the candidate sites would be comparably
good for 345 GHz observing. A group of new sites with favorable transmittance
and geographic placement leads to greatly enhanced imaging and science on
horizon scales.
