Despite its small quantity, the upper tropospheric water vapor plays an important role in determining the Earth's radiation budget. In mid-latitudes, the mechanism of moistening/drying the upper troposphere is still unclear. In this paper, we will investigate a moistening mechanism by analyzing the observation of upper tropospheric water vapor from the Microwave Limb Sounder (MLS) on the Upper Atmosphere Research Satellite (UARS), in conjunction with other space-based measurements and model simulation products. During the migration of subtropical storms to the US West Coast, large enhancements in upper tropospheric water vapor were observed by UARS MLS along warm advection regions. The moisture sometimes produced heavy rains in the Pacific Northwest or California, depending on the position of the subtropical jet stream in Pacific. During warm episode ENSO events, more water vapor was found in mid-latitude upper troposphere, in association with more frequent and more vigorous subtropical storms mentioned above