We consider the dynamical evolution of an interstellar cloud that is initially in thermal equilibrium in the warm phase and is then subjected to a sudden increase in the pressure of its surroundings. We find that if the initial plasma beta of the cloud is of order unity, then there is a considerable period during which the material in the cloud both has a small beta and is in the thermally unstable temperature range. These conditions are not only consistent with observations of star forming regions, but are also ideally suited to the production of density inhomogeneities by magnetohydrodynamic waves. The end result should be a cloud whose size and average density are typical of Giant Molecular Clouds (GMCs) and which contains denser regions whose densities are in the range inferred for the translucent clumps in GMCs.

9 pages, 3 figures, accepted for publication in ApJ Letters