From the National Weather Service:

Statement as of 4:15 PM PDT on June 1, 2006

… Las Vegas records third warmest may on record…

The average temperature in may 2006 was 81.0 degrees. This was the
third warmest may on record. The warmest may on record was 82.2
degrees set in 2002 followed by 81.6 degrees set in 1997.

There were 10 record high minimum temperatures set and one record
maximum temperature set.

I’m fairly confident that most of the record high lows are due to urban heat island effects as Vegas grows.