  We present the most complete multiwavelength coverage of any dwarf nova
outburst: simultaneous optical, Extreme Ultraviolet Explorer, and Rossi X-ray
Timing Explorer observations of SS Cygni throughout a narrow asymmetric
outburst. Our data show that the high-energy outburst begins in the X-ray
waveband 0.9--1.4d after the beginning of the optical rise and 0.6d before the
extreme-ultraviolet rise. The X-ray flux drops suddenly, immediately before the
extreme-ultraviolet flux rise, supporting the view that both components arise
in the boundary layer between the accretion disc and white dwarf surface. The
early rise of the X-ray flux shows the propagation time of the outburst heating
wave may have been previously overestimated.
  The transitions between X-ray and extreme-ultraviolet dominated emission are
accompanied by intense variability in the X-ray flux, with timescales of
minutes. As detailed by Mauche & Robinson, dwarf nova oscillations are detected
throughout the extreme-ultraviolet outburst, but we find they are absent from
the X-ray lightcurve.
  X-ray and extreme-ultraviolet luminosities imply accretion rates of 3e15g/s
in quiescence, 1e16g/s when the boundary layer becomes optically thick, and
\~1e18g/s at the peak of the outburst. The quiescent accretion rate is two and
a half orders of magnitude higher than predicted by the standard disc
instability model, and we suggest this may be because the inner accretion disc
in SS Cyg is in a permanent outburst state.
