  Evidence for shocks in nova outflows include (1) multiple velocity components
in the optical spectra; (2) keV X-ray emission weeks to months after the
outburst; (3) early radio flare on timescales of months, in excess of that
predicted from the freely expanding photo-ionized gas; and (4) ~ GeV
gamma-rays. We present a 1D model for the shock interaction between the fast
nova outflow and a dense external shell (DES) and its associated thermal X-ray,
optical, and radio emission. The forward shock is radiative initially when the
density of shocked gas is highest, at which times radio emission originates
from the dense cooling layer immediately downstream of the shock. The radio
light curve is characterized by sharper rises to maximum and later peak times
at progressively lower frequencies, with a peak brightness temperature that is
approximately independent of frequency. We apply our model to the recent
gamma-ray classical nova V1324 Sco, obtaining an adequate fit to the early
radio maximum for reasonable assumptions about the fast nova outflow and
assuming the DES possesses a velocity ~1e3 km/s and mass ~ 2e-4 M_sun; the
former is consistent with the velocities of narrow line absorption systems
observed previously in nova spectra, while the total ejecta mass of the DES and
fast outflow is consistent with that inferred independently by modeling the
late radio peak. Rapid evolution of the early radio light curves require the
DES possess a steep outer density profile, which may indicate that the onset of
mass loss from the white dwarf was rapid, providing indirect evidence that the
DES was expelled by the thermonuclear runaway event. Reprocessed X-rays from
the shock absorbed by the DES at early times may contribute significantly to
the optical/UV emission, which we speculate is responsible for the previously
unexplained `plateaus' and secondary maxima in nova optical light curves.
