The character of the solar wind plasma data observed by Voyager 2 recently changed to a regime in which the speed, density and magnetic field magnitude are positively correlated. In the inner heliosphere, the density and speed are generally anti-correlated. As streams interact while propagating outward, this correlation weakens. Outside 65 AU, Voyager observed large, in-phase, fluctuations of speed, density, and magnetic field magnitude with time scales of 6--12 months. The dynamic pressure varies by a factor of ten in these fluctuations, which should produce motions in the termination shock. We use ACE data from 1 AU as input to a 1-D MHD model which includes pickup ions to model the radial evolution of the solar wind. The model reproduces the basic character (but not the details) of the observations, predicting correlated variations in speed, density, and magnetic field with time scales similar to those observed.