This is unfortunately a statistic that the AGW deniers would use their standard “but UHI” argument on, and ignore the fact that these 3 day streaks of lows in the 70s are 10 times as common as they used to be and that Boise is a relatively small city where UHI wouldn’t be nearly as pronounced as, say, PHX or MSP.
One other fact that should be included: I’m not familiar with Boise, nor have I ever been within 250 miles of it, but it would appear from looking at a map that the airport sits literally right at the edge of the developed area, so that would also mitigate UHI to some extent.