This paper introduces a method named “hybrid-downscaling” to estimate the future extreme hourly precipitation intensity based on observational evidence of the 99th percentile precipitation intensity against air temperature in Sapporo and Tokyo, Japan. The future projected air temperature under the Special Report on Emissions Scenarios (SRES) A1B was used after dynamical downscaling using 3 different regional atmospheric models (RAMs) with lateral boundary conditions from 3 different general circulation models (GCMs). We analyzed the 99th percentile of hourly precipitation intensity against daily mean air temperature in Sapporo and Tokyo. The 99th percentile precipitation intensity tended to have an approximate equation in which the rate of increment was similar to the Clausius-Clapeyron rate of change in the saturated water vapor. This study also showed that the precipitable water vapor and the convective instability followed the Clausius-Clapeyron-like rate of change during the hours with the 99th percentile precipitation intensity.