If one were to use a more substantial helium lifting device other than a kids 'party balloon'...
and one were to tie several of the 2 cubic foot helium filled balloons together in a series some 10 yards apart with something like 50lb fishing
line...
and crimp on fishing clips to secure several light weight, 2 AA battery electronic-flares. (seen here)
www.familydefense.com...
i bet the observed 'Formations' could be seen in an 8pm sky.
people would not notice the lit up electronic flares until they were high enough above the city skyline to be prominant...
at low altitude the lights-on-balloons would be caught in the confusion of
streeet lights & high rise buildings and the 'haze' of the Phoenix city.
Only when the drifting lights were 2-3000 ft altitude would they become noticeable to the valley dwellers.
a good
out of the way launch site (also being East) of Deer Valley and Scottsdale airports might be the Granite Reef Dam parking area for the
Salt River tubing enthusiests...
or perhaps Fountain Hills.