  We present multiple-epoch photometric monitoring in the $J$, $H$, and $K_s$
bands of the T1.5 dwarf 2MASS J21392676+0220226 (2M2139), revealing persistent,
periodic ($P=7.721\pm$0.005 hr) variability with a peak-to-peak amplitude as
high as 26% in the $J$-band. The light curve shape varies on a timescale of
days, suggesting that evolving atmospheric cloud features are responsible.
Using interpolations between model atmospheres with differing cloud thicknesses
to represent a heterogeneous surface, we find that the multi-wavelength
variations and the near-infrared spectrum of 2M2139 can be reproduced by either
(1)cool, thick cloud features sitting above a thinner cloud layer, or (2)warm
regions of low condensate opacity in an otherwise cloudy atmosphere, possibly
indicating the presence of holes or breaks in the cloud layer. We find that
temperature contrasts between thick and thin cloud patches must be greater than
175 K and as high as 425 K. We also consider whether the observed variability
could arise from an interacting binary system, but this scenario is ruled out.
2M2139 joins the T2.5 dwarf SIMP0136 discovered by Artigau and coworkers as the
second L/T transition brown dwarf to display large-amplitude variability on
rotational timescales, suggesting that the fragmentation of dust clouds at the
L/T transition may contribute to the abrupt decline in condensate opacity and
$J$-band brightening observed to occur over this regime.
