Thank you Crandles and Oren for your comments. I didn't notice before that the thickness grows until early May.
Regarding the values that I used on Piomas Monthly Volume, I took it directly from Piomas, so they are the official monthly data.
I made a forecast, looking to the max and min change between a given month and March, for each year, on 2007-2016.
So the maximum value for September is if 2017 follows the path of 2014, which will bring a value of 4.77k km3.
The minimum value for September is if 2017 follows the path of 2010, which will bring a value of only 1.26k km3.
The average of both values is 3.01k km3, that will be a huge record, if it happens.
In both cases, max and min, the average of the year will be less than the average of 2012: 13.55k km3 (2012) versus 13.12k km3 (max) and 11.06k km3 (min).