  We use dynamical systems theory to construct the normal form of the
Navier--Stokes equations for the flow of a thin layer of fluid upon a solid
substrate. The normal form equations illuminate the fluid dynamics by
decoupling the long-term flow of interest from the rapid viscous decay of the
transient shear modes. The normal form clearly shows the centre manifold of the
lubrication model and shows the result that the initial condition for the fluid
thickness of the lubrication model is \emph{not} the initial physical fluid
thickness, but instead is modified by the initial lateral shear flow. With
these initial conditions, better forecasts will be made using the lubrication
model. This dynamical systems approach will also enable similar illumination of
other complicated models of dynamics.
