As the Webstorm IDE has its own terminal you can run grunt from within your IDE. Unfortunately it isn't that easy when you work on a Mac. If you open the terminal in Webstorm and run
grunt you will get an error that the program grunt couldn't be found. The reason is that GUI applications under MacOSX dont have the same environment variables as the terminal. This form post describe how to set your environmental variables. After following the description and rebooting your machine it will work as expected.
Now you can go to the preferences and search for
external tools. Create a new tool. Give it a name for example
grunt watch, in the program field enter
grunt and in the parameter field
watch. Set the working directory to your project using
$ProjectFileDir$. After closing the settings view you should have new entry under
Main Navigation -> Tools to run your grunt task from with WebStorm. You can also add short cut for the task in
Preferences -> Keymap.