Historically, tornadoes often have been modelled in open domains in wh
ich the boundary conditions on the top of the modelled vortex are impo
sed, specified by flow-straightening, or extrapolated from upstream. I
n current three-dimensional modelling efforts, reasons of computationa
l efficiency still impose a need to model the tornado in isolation fro
m the storm using what could be called a ''one-way nested model''. The
effect of such isolation on a tornado simulation is investigated here
with a redundant one-way nested model embedded within a high-resoluti
on, axisymmetric storm model. The tornado in the nested model is compa
red with the tornado in the complete model. (The complete model by its
elf can also be thought of as containing a ''fully interactive, two-wa
y nested model''). The use of extrapolated or straightened outflow bou
ndary conditions on the nested model is shown to have a profound effec
t on the tornado solution. Maximum windspeeds in the nested model can
differ by more than a factor of two from those in the corresponding re
gion of the complete model. Only subcritical vortices are considered h
ere.