  The quasar 3C454.3 underwent a uniquely-structured multi-frequency outburst
in June 2016. The blazar was observed in the optical $R$ band by several
ground-based telescopes in photometric and polarimetric modes, at $\gamma$-ray
frequencies by the \emph{Fermi}\ Large Area Telescope, and at 43 GHz with the
Very Long Baseline Array. The maximum flux density was observed on 2016 June 24
at both optical and $\gamma$-ray frequencies, reaching
$S^\mathrm{max}_\mathrm{opt}=18.91\pm0.08$ mJy and $S_\gamma^\mathrm{max}
=22.20\pm0.18\times10^{-6}$ ph cm$^{-2}$ s$^{-1}$, respectively. The June 2016
outburst possessed a precipitous decay at both $\gamma$-ray and optical
frequencies, with the source decreasing in flux density by a factor of 4 over a
24-hour period in $R$ band. Intraday variability was observed throughout the
outburst, with flux density changes between 1 and 5 mJy over the course of a
night. The precipitous decay featured statistically significant quasi-periodic
micro-variability oscillations with an amplitude of $\sim 2$-$3\%$ about the
mean trend and a characteristic period of 36 minutes. The optical degree of
polarization jumped from $\sim3\%$ to nearly 20\% during the outburst, while
the position angle varied by $\sim120\degr$. A knot was ejected from the 43 GHz
core on 2016 Feb 25, moving at an apparent speed $v_\mathrm{app}=20.3c\pm0.8c$.
From the observed minimum timescale of variability
$\tau_\mathrm{opt}^\mathrm{min}\approx2$ hr and derived Doppler factor
$\delta=22.6$, we find a size of the emission region
$r\lesssim2.6\times10^{15}$ cm. If the quasi-periodic micro-variability
oscillations are caused by periodic variations of the Doppler factor of
emission from a turbulent vortex, we derive a rotational speed of the vortex
$\sim0.2c$.
