By the time it stopped snowing the snowing inches should not increase nor decrease, then Morgan should represent this period of time in his graph as an horizontal line (with slope equals 0), by looking at the graph we notice that this happened between the 12 and the 14 hours, at this point the snow on the ground had a height of 12 inches.
Then, there were 12 inches of snow when it stopped snowing for the first time.