How should I be countering storms TvP in Starcraft 2?

Spreading out your units really helps with storms. Try hard to keep them separated.

Cloak is essential for Ghost vs Templar, that additional scouting is crucial. You will have to watch for observers, but sacrificing a ghost to emp is a necessity for a Terran bio army.

A single ghost can nullify a nearly unlimited number of high templars. EMP drains all their energy and removes 100 (I believe) shield points.

EMP range is longer than the range of Feedback so they won't be able to snipe your ghost.

Just a note that storm is not nearly as strong as in StarCraft 1, and simply dancing your units around will avoid the vast majority of the damage (there is actually a delay after the graphic appears before the damage starts kicking in). Not to mention storm is not as effective against marauders as it is against marines.

If you see the protoss player going anti-bio units like high templars, you should do a tech switch to mechanical units like tanks and thors.