  In turbulent Rayleigh-Benard convection, a large-scale circulation (LSC)
develops in a nearly vertical plane, and is maintained by rising and falling
plumes detaching from the unstable thermal boundary layers. Rare but large
fluctuations in the LSC amplitude can lead to extinction of the LSC (a
cessation event), followed by the re-emergence of another LSC with a different
(random) azimuthal orientation. We extend previous models of the LSC dynamics
to include momentum and thermal diffusion in the azimuthal plane, and calculate
the tails of the probability distributions of both the amplitude and azimuthal
angle. Our analytical results are in very good agreement with experimental
data.
