Abstract: In this study, we use observations from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument onboard the Thermosphere–Ionosphere–Mesosphere Energetics and Dynamics (TIMED) satellite to develop and apply a new local-time binning method to investigate the long-term evolution of mesospheric water vapor at high latitudes. The proposed method accounts for the gradual local-time drift of the SABER orbit by aligning seasonal observation windows and selecting samples observed at similar local times. This approach minimizes tidal aliasing and ensures more consistent sampling, yielding more reliable estimates of long-term water vapor trends at high latitudes. The results show that drying signals primarily appear in the polar regions. However, in the southern hemisphere, a drying trend is observed only in autumn, whereas winter and summer mainly show moistening trends. In contrast, the northern hemisphere exhibits drying signals in the polar regions during all seasons, showing a clear seasonal asymmetry. Additionally, the water vapor trend in the northern hemisphere is particularly pronounced in February (late winter), with moistening reaching up to +2.0 ppmv. The winter in the southern hemisphere (July–August) also shows moistening, but the trend is still weaker than in the northern hemisphere. These differences highlight the strong moistening trend in the northern hemisphere during winter and underscore the significant asymmetry in seasonal water vapor changes between the two hemispheres. These findings emphasize the limitations of water vapor trend estimates across different seasons and latitudes. Moreover, they provide new insights into the spatiotemporal variability associated with tidal structures, underscoring the importance of optimizing local-time sampling strategies for reliable long-term trend detection.