I never believed in the accuracy of Heat Indexes. Since it is a "feels like" index, it is all based on qualitative and therefore shaky data, if it is even based on any data. Maybe 90 F with 50% and 95 F with 40% can relate to each other through some higher numbers on a "feels like" index, but I keep hearing about indexes of 110 in city X on the east coast and I think "110 in the Canyon was nice, what are they complaining about?". Sure, 110 in 10% humidity is pretty nice, but it won't actually be 110 in these cities and 99 with 50% humidity won't feel anything like it did in the canyon. Water condensing on the skin feels nothing like water vaporizing from skin, and that is and will always be the problem with an attempt to arrive at what humidity feels like in a thermo sense. The sensation of the affects of water on the skin is not the same as an actual temperature, so in essence, it's comparing apples to oranges. What they need, what most of the horribly humid world needs (and Arizona on a monsoon day) is a Disgusting Index. An index that incorporates temperature and humidity to arrive at a non-temperature based number, perhaps on a 1 to 10 scale, or maybe even color coded, that gives people something to report on that doesn't cheapen or pervert an actual temperature of 110.
In Medford, NJ today, the forecast high is 101, and humidity will be about 46%, so the disgusting index can be a 7 or 8 out of 10. It is currently 99 with 48%, dew point 76 degrees, but the claimed heat index is 114. I guarantee you that it is nothing like 114 in Phoenix.
I remember being in 60% humidity on a 88 degree day, and that is about as nasty as it gets for me. It would be a 9 or 10/10. White skies, puffy clouds with no blue visible at all, so humid that you're drenched in sweat just sitting in the shade..
I think Monsoon season will begin around June 20, plus or minus 5 days, not by the calendar according to the NWS, but when dew points rise dramatically, and it begins to rain over the Sacramento Mountains. It will start about 10 days later in Arizona.