A network of more than 70 continuously operating Global Positioning System (cGPS) stations is used to observe a strong diurnal variation in total column precipitable water vapor (PW) in Taiwan. A data record of more than two years has been created that captures both its spatial and temporal variation. The peak signal generally appears in the late afternoon, but the timing varies both spatially and seasonally. The largest variation, exceeding more than 15 mm/day in PW, can be found in the Lanyang Valley. An exceptionally coherent signal is also observed in the southwest plains, extending into the central mountains from July through September. This variation in total column water vapor is well correlated with previously identified sea-breeze and precipitation cycles. This indicates that frequent and timely PW estimates may be valuable in short-term forecasts of rain that are not forced by larger scale weather systems.