  Environmental decoherence of oscillating neutrinos of strength $\Gamma = (2.3
\pm 1.1) \times 10^{-23}$ GeV can explain how maximal $\theta_{23}$ mixing
observed at 295 km by T2K appears to be non-maximal at longer baselines. As
shown recently by R. Oliveira, the MSW matter effect for neutrinos is altered
by decoherence: In normal (inverted) mass hierarchy, a resonant enhancement of
$\nu_{\mu} (\bar{\nu}_{\mu}) \rightarrow \nu_{e} (\bar{\nu}_{e})$ occurs for $6
< E_{\nu} < 20$ GeV. Thus decoherence at the rated strength may be detectable
as an excess of charged-current $\nu_{e}$ events in the full $\nu_{\mu}$
exposures of MINOS+ and OPERA.
