  We test the ability of a general low-dimensional model for turbulence to
predict geometry-dependent dynamics of large-scale coherent structures, such as
convection rolls. The model consists of stochastic ordinary differential
equations, which are derived as a function of boundary geometry from the
Navier-Stokes equations (Brown and Ahlers 2008). We test the model using
Rayleigh-B\'enard convection experiments in a cubic container. The model
predicts a new mode in which the alignment of a convection roll switches
between diagonals. We observe this mode with a measured switching rate within
30% of the prediction.
