\begin{equation}
  \begin{array}{lcl}
    % tex4ht fails unless \mathbf is in a group
    \Delta_{\mathrm{upw}} & = & {\mathbf{q}}_{i} - {\mathbf{q}}_{i-1}\\
    \Delta_{\mathrm{loc}} & = & {\mathbf{q}}_{i+1} - {\mathbf{q}}_{i}\,,\\
  \end{array}
\end{equation}