After a sandstorm: paying a bit more to avoid paying much more
Shamal winds and summer dust events change the economics for a few days. Demand spikes, so slots vanish and some vendors add AED 10–30 to usual rates or stop discounting. Booking the evening the storm ends, rather than joining the next-morning rush, is the cheapest timing you will get.
Skipping the post-storm wash entirely is the false economy: grit left in door shuts, wiper channels and window seals grinds every time you use them, and one thorough wash after each major storm is cheap insurance against paint correction at AED 150–400 per stage.