RE: Rain / Heat in Sept & Octb
Rain can come at any time of year but is "most likely" in winter. Last winter's storms brought exceptionally heavy rains to Death Valley and some of the roads were washed out.
The North American monsoon rarely extends as far to the north and west as Death Valley - that's one of the reasons it's so dry. Usually the monsoon has retreated back into northern Mexico by the middle of September. Monsoon related thunderstorms could occur, usually over the surrounding mountains, any time from about mid July to early September though.
Any rain in late September or October mostly likely would be due to an unusually strong early winter type storm or from the remnants of a tropical storm - but they are both rare events.
Early September will be hot on the valley floor during the afternoon but by the end of October afternoon temperatures on the valley floor will be comfortable - at least to someone from Phoenix.
See the National Park Service website for more info.
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