Did you get system wet or run it hot not getting enough cooling?
Black pixilation snow often tends to be a corrupted RAM buffer the Intel Iris iGPU is using which it a segment of your main memory.
Double check by connecting to an external display if the problem is not present on it then the displays T-CON logic within the display has failed requiring a new display assembly [guide|45135]
If you have access to a hot air rework station you could try touching up the RAM chips solder joints by carefully lifting the shield as well as inspect the CPU module after lifting the heat sink off and the thermal paste. Here the I/O chip may be physically damaged (cracked)
[image|3519487]
Sadly, either one will require a new logic board [guide|45134]