  We show that the black hole binary (BHB) coalescence rates inferred from the
advanced LIGO (aLIGO) detection of GW150914 imply an unexpectedly loud GW sky
at milli-Hz frequencies accessible to the evolving Laser Interferometer Space
Antenna (eLISA), with several outstanding consequences. First, up to thousands
of BHB will be individually resolvable by eLISA; second, millions of non
resolvable BHBs will build a confusion noise detectable with signal-to-noise
ratio of few to hundreds; third -- and perhaps most importantly -- up to
hundreds of BHBs individually resolvable by eLISA will coalesce in the aLIGO
band within ten years. eLISA observations will tell aLIGO and all
electromagnetic probes weeks in advance when and where these BHB coalescences
are going to occur, with uncertainties of <10s and <1deg^2. This will allow the
pre-pointing of telescopes to realize coincident GW and multi-wavelength
electromagnetic observations of BHB mergers. Time coincidence is critical
because prompt emission associated to a BHB merger will likely have a duration
comparable to the dynamical time-scale of the systems, and is only possible
with low frequency GW alerts.
