The black hole binary (BHB) coalescence rates inferred from the Advanced LIGO detection of GW150914 imply an unexpectedly loud gravitational-wave (GW) sky at millihertz frequencies accessible to the Evolved Laser Interferometer Space Antenna (eLISA), with several outstanding consequences. First, up to thousands of BHBs will be individually resolvable by eLISA; second, millions of nonresolvable BHBs will build a confusion noise detectable with a signal-to-noise ratio of a few to hundreds; third-and perhaps most importantly-up to hundreds of BHBs individually resolvable by eLISA will coalesce in the Advanced LIGO band within 10 y. eLISA observations will tell Advanced LIGO and all electromagnetic probes weeks in advance when and where these BHB coalescences will occur, with uncertainties of <10 s and <1 deg^{2}. This will allow the prepointing of telescopes to realize coincident GW and multiwavelength electromagnetic observations of BHB mergers. Time coincidence is critical, because a prompt emission associated to a BHB merger will likely have a duration comparable to the dynamical time scale of the systems and is possible only with low-frequency GW alerts.