Having read the book three times in a row, I now firmly believe that the storm, which conveniently occurred just on time for Violet to cross the parapet, was her mother’s doing. We know that Lilith is a storm wielder. We also know that the storm started just before Violet’s turn to cross the parapet.
On chapter 3, page 29 – Violet’s remark about the storm
“The rain eases into a drizzle, as if it had only come to make the hardest test of my life even harder…but I did it.”