Place cubed potatoes in a large pot of cold salted water and bring to a boil over high heat on the stovetop. Boil for ten minutes, or until potatoes are tender and easily pierced with a fork. Then drain from the water and rinse in a strainer under cold water to cool the potatoes.
The the potatoes cook, add mayonnaise, sour cream, vinegar, mustard, sugar, green onions, celery seed, garlic powder, salt, and black pepper to a large mixing bowl. Whisk to combine.
Add the cooled potatoes to the bowl with the dressing and fold them together with a rubber spatula until the potatoes are evenly coated. For a creamier texture, lightly mash some of the potatoes as everything is combined.
You can serve immediately, but the flavors will continue to meld if stored in the refrigerator overnight.