While full AP may work (probably still sucks) as support, do not try to sit in an ADC lane with full AP, it is an absolute nightmare for last hitting at the start and not reliable end game.

Manamume combined with Liandry's combined with your Runes will give you around 200 attack damage late game but more importantly before you start actually building any AP you'll have over 150 attack damage which allows you to last hit, farm, and get the gold you need for this build.

Having this hybrid damage will make it harder for people to build against you, once your Liandry's is finished you get the benefit of slowing them in the rain and then hitting them with auto attacks which do amplified damage from cheap shot so you become an all out force to be reckoned with.

It will also keep your ultimate respectable so that when you rain and ult at the same time you'll literally destroy anyone very fast besides big tanks but you can continue to use your rain to tap them down.

I can't tell you how many times I've actually had people run up to my tower with a full minion wave thinking they were going to take it out only to have rain and an ult land on them simultaneously wiping out the minion wave and if it doesn't kill them the tower does.

AP Fortune should be illegal, it is that dangerous, even though you'll lead damage in most games including losses, you'll still get blamed for not doing any damage or costing your team the game only to have them be silent when they see the stats screen.

The main problem with AP Fortune isn't damage, it's that you might not be powerful enough to stand in a duel 1v1 late game to finish off a fight so if you get on a team where another damage dealer is slacking you will get blamed for that as well. Lich Bane and some quick abilities definitely helps but if you get caught by surprise in a 1v1, most of the time you won't win that fight.

Stay with the team and keep your vision up and you will be amazed at how well this works, I've been playing AP Fortune full time since season 5 normally with over a 55% win rate and even in losses or terrible games still being at the top of the damage meters.

People are not accustomed to a mana burning playstyle so they tend to conserve too much. With this build and starting with Tear you want to be spamming your E as often as possible. You have teleport chosen so that you can simply base to refresh your mana and come back if needed.

After manamune is upgraded, you won't have any mana problems so get used to using your abilities as soon as they come up, this includes your Ultimate which can get down into the 20's for cooldown depending on game length and items built but will usually be 40s or below. No Ultimate is wasted because it will almost always be up, clear minions if you wish to push a wave, but keep your abilities used.