Over a number of days the rainfall totals in the mountains can really add-up. When only mid-level moisture is present it is very difficult for thunderstorms to venture into the desert areas. These storms, while in the mountains, are likely to be stronger and produce more rain than their mid-level moisture counterparts. But once again, as these storms move towards the low desert, the air is too dry and stable and they diminish. On days such as this a thunderstorm or two will usually manage to make it to Tucson but they are weak with only a trace to half inch of rain.
Storms form over the mountains first but head towards the desert area by mid-afternoon.
Storms can move across the low desert when plenty of low-level moisture is present.