npm run webpack -- -w not watching

If I install webpack globally, then I can run

webpack -w

and it works fine, and continues running, watching my files. If I try to run it locally with

package.json

"scripts": {
    "webpack": "webpack"
}

cmd

npm run webpack -- -w

Then it runs, but it does not watch the files. It completes and return to input on cmd in windows.

via tic