¥Mostly because of the nonlinearity of the equations, small differences in the initial state eventually grow to completely change the solution.
¥"Does
the Flap of a Butterfly's Wings in Brazil Set
Off a Tornado in Texas? " (E. Lorenz, 1972, via P. Merilees or D. Lilly)
¥Do
multiple simulations starting from slightly different initial states: ensemble
prediction
¥Also
create ensembles by using multiple
models.