The USHCN V2 docs say that they did not change the TOBS adjustment from V1.
Given these verifications, the Karl et al. (1986) TOB adjustment procedure was used in HCN version 2 without modification
The V1 docs show less than 0.4F TOBS increase since 1920, going flat after 1990.
The V2 docs show more than 0.3C exponential increase since 1920, with no leveling after 1990. Same adjustment specs, same data set, widely divergent numbers. Obviously an error.
The next graph overlays the V1 TOBS (black dashed line) on the V2 TOBS at the same scale. They are supposed to be identical and match closely until 1950, but diverge after 1950. The V2 version does not go flat after 1990, and by 2007 is double what it is supposed to be. The times of observation from 1995 obviously did not change after 1999. NOAA appears to be pulling a fast one.
This doesn’t even consider the fact that TOBS assumes that all observers are idiots. Almost anyone operating a min-max thermometer will recognize in a matter of days that they have to reset the thermometer at night. I figured this out when I was about seven years old. TOBS is not a subtle problem, and I suspect that anyone with an IQ higher than a turnip will quickly figure out how to avoid it.