An empirical model to estimate daily leaf wetness duration (LWD), developed in the midwestern US, was modified with energy balance principles in order to estimate LWD from site-specific weather data in Costa Rica. Air temperature, RH, wind speed, rainfall, and wetness were measured hourly at five stations in northwestern Costa Rica during May to September 1999 (wet season) and December 2000 to April 2001 (dry season). Site-specific weather estimates were obtained from SkyBit Inc. for the same locations in Costa Rica during the study period. The CART/SLD empirical model was modified using vapor pressure deficit (VPD), a key variable in calculating energy balance on crop surfaces. The resulting model was termed the CART/SLD/VPD model. The CART/SLD/VPD model estimated LWD with less error and variability than either the original CART/SLD model or SkyBit’s proprietary wetness estimation model during both wet and dry seasons. Although the CART/SLD model had been trained in the midwestern US, it was possible to obtain reasonably accurate estimates of LWD in northwestern Costa Rica using site-specific weather estimates because of the model’s hierarchical decision-tree structure and the use of VPD.