Evan Michaels mentions that the Tempest's healing abilities will be increased when they use some of their offensive spells, through buff effects. For example, one particular ability will make it so that using an offensive spell will place a stackable buff on the caster that will make their next heal more potent. Tempest's will have the largest AoE arsenal of any class in the game, and we are informed that even though many of their spells are "elemental"-based, there will be no environmental limits imposed (for example, a storm type spell not being usable indoors).
When asked if he could give some information on a class defining max-level spell for the Tempest of Set, Michaels brought up an ability called "Eye of the Storm". This spell will conjure up a big cyclone that protects the caster in a number of ways -- it will increase their defense, make them immune to knockbacks, and people attacking them will have a chance to get blown away. On top of this, it will also greatly increase their AoE damage. It sounds like one weather pattern that you'll definitely want to avoid on the battlefield.
These damage capabilities do not mean that the Tempest will not be a viable healer, and Michaels stated that they will be able to solo-heal a group. They will also have abilities that augment the mana regeneration of a group, another thing that they will be the best at in the game. All of these skills sound fairly powerful when put together, and we're keen to see how the class will end up for the final release build. You can view the full interview at Ten Ton Hammer.