It's quite simply a matter of science actually: Storm created a cloudburst, that's all.
"A cloudburst is an extreme amount of precipitation, sometimes with hail and thunder, which normally lasts no longer than a few minutes but is capable of creating flood conditions. Colloquially, the term cloudburst may be used to describe any sudden heavy, brief, and usually unforecast rainfall."
Although it wasn't executed well visually (did they forget to turn on the fans to make her cape billow in the wind while she was suspended in mid-air?
), all Storm did was create a tornado--but out of water. Just as winds are pulled together into a vortex in one collected, focused place, Storm summoned the precipitation in the air (she likely increased amounts from the water surrounding Alcatraz) and psionically focused it on that crowd of mutants. Once the water hit the ground, she simitaneously created a wall of wind to drive it forward, thereby bulldozing and overwhelming the crowds. Basically, they were hit above AND from the front with the elements in a rapid, unexpected fashion as the cloudburst definition explains.
And of course, the Lightning Strikez part is easy.