In recent years several authors have developed strategies that estimate the Integrated Water Vapor (IWV), proportional to the Zenith Tropospheric Delay (ZTD), in near real time --i.e. with a delay of one hour or more--. The success of these estimations depends on the accuracy of the GPS orbits, so these strategies require that data from a regional GPS network be processed in near real-time, using predicted IGS orbits in combination with partial orbit relaxation. Ambiguity resolution can help determine in real-time such quantities as the ZTD in a regional network. This approach allows any user in the area of the network to get real-time precise ZTD estimates, which can be converted to IWV using measurements of atmospheric pressure. As shown recently, in Wide Area Differential GPS (WADGPS) networks several hundred Km across, double-differenced carrier phase ambiguities can be computed on-the-fly, using a real-time tomographic model of the ionosphere obtained from the same GPS data. The comparison between the real-time strategy presented in this work and the postprocessed approach shows a general agreement of about 1 cm in the ZTD in WADGPS scenarios. This is 30-40% better than without fixing the carrier phase ambiguities. These results can be achieved not only in quiet ionospheric conditions but also during ionospheric disturbances and Solar Maximum conditions. This is also of importance for ionospheric modeling, which is a key element of the technique.