Abstract Recently it has been proposed that the phase delay associated with the radio signals propagating from GPS satellites to a ground-based GPS receiving station can be used to infer the vertically integrated water vapor (precipitable water—PW) with a high degree of accuracy. Since a ground-based GPS receiving station is relatively inexpensive, a specially designed, dense GPS network can provide PW measurements with unprecedented coverage. Such a data set can potentially have a significant impact on operational numerical weather prediction. In this paper, a series of numerical experiments were conducted using a variational (4DVAR) data assimilation system based on The Pennsylvania State University –National Center for Atmospheric Research mesoscale model MM5 and its adjoint. The special soundings collected in SESAME (Severe Environmental Storms and Mesoscale Experiment) 1979 wore used in two sets of experiments. In the first set, a 1-h assimilation window and an analysis of the observed PW data were u...