In honor of D-Day this has always bothered me. Seems rather obvious that as the allied troops are landing that they could use smoke grenades/shells launched from ships to obscure the landings?
They tried smoke in several exercises in the years before D-day, ruling its widespread use out due to several factors-
The smoke was too difficult to control, it tended to blow off the beach.
It limited the effectiveness of observation for naval gunfire support and air support, reducing the effectiveness of the preliminary bombardment.
Landing craft Coxswains and attacking forces were confused and disorientated by it, slowing the landings which would cause casualties.
Consequently it was discarded.
There were at least some attempts - the first Free French casualties of D-Day were three men of 342 "Lorraine" Squadron RAF, shot down by flak while laying smoke at sea level to cover the bombarding fleet and the first wave of approaching troops.
http://www.bbc.co.uk/history/ww2peopleswar/stories/11/a2688311.shtml (a personal account by a British pilot also laying smoke) says that:
No. 342 Squadron were to cover the yank invasion at the far end of the beachhead landing area.
which to me suggests Utah.