Maskout Different Domain Resolutions

[Neil Davis]

I have two model simulations, a 12km simulation of the Eastern US, and a
4km simulation over a small portion of the 12km domain.  I am trying to
make plots to compare the two, however because of the map projection, the
4km simulation does not have results over the whole plot window, while the
12km does.  I am hoping to mask out the 12km domain so that the plotted
area matches that of the  4km simulation.  Does anyone know of a way to do
this.

Thanks,
Neil Davis