  We describe realistic observing scenarios for early warning detection of
binary neutron star mergers with the current generation of ground-based
gravitational-wave detectors as these approach design sensitivity. Using Fisher
analysis, we estimate that Advanced LIGO and Advanced Virgo will detect one
signal before merger in their fourth observing run provided they maintain a
70\% duty cycle. 60\% of all observations and 8\% of those detectable 20
seconds before merger will be localized to $\lesssim 100 \thinspace
\mathrm{deg}^2$. If KAGRA is able to achieve a 25 Mpc horizon, these prospects
increase to $\lesssim 2$ early detections with 70\% of all BNS localized to
$\lesssim 100 \thinspace \mathrm{deg}^2$ by merger. As the AHKLV network
approaches design sensitivity over the next $\sim10$ years, we expect up to 1
(14) detections made 100 (10) seconds before merger. Although adding detectors
to the HLV network impacts the detection rate at $\lesssim 50\%$ level, it
improves localization prospects and increases the completeness of compact
binary surveys. Given uncertainties in sensitivities, participating detectors,
and duty cycles, we consider 103 future detector configurations so
electromagnetic observers can tailor preparations towards their preferred
models.
