A primitive equation, non-hydrostatic mesoscale model is used to study the sensitivity of changes in land surface characteristics on surface dynamic and thermodynamic fields over the city of San Juan, Puerto Rico, and surrounding rural areas. Two model configurations, each depicting a different land use scenario, are used in this work. A control (CTL) run is made using the urban land use scenario as depicted by the standard vegetation files from the model. The urban area for this experiment encloses an area of ~110km2. The second experiment, the extended urban (EU) area run, uses an urban scenario of ~330km2. Results show an increase in the 2-m height maximum temperatures over central parts of the city when extending the urban landscape from the CTL to the EU urban scenario, whereas a decrease in maximum temperatures occurs in areas closer to the coast. An enhanced onshore flow is hypothesized to cool the temperatures near the coastal zones, but analysis remains inconclusive as model output does not strongly support this idea. On the contrary, a slight decrease in the minimum temperatures is seen over most of the urban San Juan area, except for the immediate coastline sectors. A drier and calmer surface atmosphere, seen in the EU run, over the coast-protected zones of the city, is the likely cause for this decrease. Wind speed generally decreases over most of the urban area when examining the EU case, a sign of increased frictional effects caused by the extended urban path of surface roughness. The divergence pattern shows negative values (i.e. convergence) over most of the urbanized domain, with values greater over the central areas of the city. CTL run values, however, show slightly more convergence over the central parts of San Juan than the EU run. However, convergence is greater in the eastern part of the extended urban area, as compared to the CTL scenario. Results offer an interesting view of the effects of increasing the urban mass over San Juan, as it seems urban-related effects do not include the rising of temperatures in the city, but a decrease on certain areas, both at daylight and nighttime, with only a slight increase over the inner parts of the city at daylight.