For more and a first-guess forecast, go below the fold. And if anybody here wants to disagree, please feel free to!
The strong heating of the valley and the desert plateaus that surround it is strongest near the surface during the day, but temperatures several hundred meters above the surface are the highest at night.
Fun Fact #1: If temperatures rise as you gain altitude, a "temperature inversion" is said to exist. The air below the inversion is very stable and, absent strong winds, any pollution emitted under the inversion will stay there.
Fun Fact #2: Meteorology instructors and models like to track 850 mb temperatures since they do not vary with diurnal heating. If surface temperatures in early June over Des Moines are 56 F at 7 am and 78 F at 7 pm, chances are that the weather over Des Moines did not change. Chances are also good that the temperature at 850 mb remained around 54 F all day. Given the same temperatures at the surface and a 5 F rise in 850 mb temperatures, it's safe to assume that interesting weather took place over Des Moines (the nature of which must be determined from other sources like satellite photos and damage reports from points west...)
Anyway, Dr. Teresa Bals-Elsholz at Valparaiso University made a drawing that I've modified and reproduced below on the right. On the left is an idealized map of the lower Colorado Valley, with Lake Mead/Las Vegas to the north, the Imperial Valley to the west and the Gila Valley to the east.
If you heat a column of air, it expands. Thus the distance between the 925 mb pressure level and (say) the 700 mb pressure level increases. (This evenings's NAM model would probably draw a 500 mb isobar where I drew a 700 mb isobar). The drop of the 925 mb isobar indicates where a thermal low can be located. Like any low in the Northern Hemisphere, air around it circulates counterclockwise.
Next month, throw in some mid-level moisture carried over from the Gulf of Mexico as the Bermuda-Azores high punches westward over the Deep South (maybe). Perhaps add outflowing moisture from tropical systems farther south next month and moisture from a warming Gulf of California (likely!). Fold in some flow around the thermal low, and the monsoon hits Arizona.
And now, an amateur forecast for yKos
Thursday: Low 79, High 104
Friday: Low 81, High 106
Saturday: Low 78, High 103
Sunday: Low 78, High 101
To get this forecast, I looked at the GFS model 850 mb temperatures, added about 11 C for the high (seemed to work this weekend) and subtracted 2 C for the low (past behavior suggests a 13 C difference between max and min in Las Vegas when relatively hot in June -- I don't know if a real forecaster would take that into consideration).
The NWS predicts brisk upper 90s for Friday and Saturday, 101 on Thursday and 100 on Sunday. Of course, 4-7 days out the models may be spectacularly wrong, but 90s or 100s in Vegas in early June are near certainties.
Not that I'm bitter, but next weekend looks like another round of sunshine and seventies near Chicago.