Precipitable water can be obtained from zenith travel-time delays from Global Positioning System (GPS) signals if the pressure and temperature at the GPS site is known. A data set of precipitable water dating back to 2003 for over 500 sites in California and Nevada is used to study the North American Monsoon in the desert regions of California and Nevada. Precipitable water is found to follow a lognormal distribution at all sites in the region, with some stations showing a small bimodal characteristic due to the influence of the North American Monsoon. An index is proposed that measures the bimodality and hence the “monsooniness” of a site. A normalized precipitable water index is employed to indicate when the monsoon starts and stops and to investigate spatial variability. The GPS data set has the advantage of having much higher spatial resolution than surface-based methods, such as dewpoint criteria and rainfall. This data set reveals the northwestward propagation of the monsoon onset both synoptically and seasonally, and provides a high resolution description of the variation in monsoon duration.