Addon

Is there any addon that shows me when I have reached 5 stacks of weapon of maelstorm? I'd like to know when my I can cast instant heals

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Nug combo bar seems to be the default choice, but i hate the looks. Almost any combo bar addon for a rogue/druid has been updated to work for shamans too.

I use power auras classic ^^ for it :D

There are a few that show it, including some, if not most, unit frame addons, though that is hardly ideal.

My vote would also go for a Power Auras setup, since you can make any sort of visual demonstration you want with it. In my case, I have a totem graphic pop up for each stack of Maelstrom Weapon I have, and at five stacks they disappear and are replaced with a large lightning icon thingy, along with playing a little ding noise to ensure I don't miss it. Of course it can be configured any way you like, it's got plenty of customization to it.

Power Auras is awesome for that (also useful for plenty of other things)

Maelstrom maelstro, also does a sound when you reach 5 stacks

Try the add on Tell Me When