Bring a large pot of salted water to a boil. Cook the rotini according to the package directions until just tender, usually 8–10 minutes, then drain and rinse under cold water until cool.

Drain the pasta very thoroughly so excess water does not thin the dressing. Let it sit in the colander for a minute or two while you prepare the creamy mixture.

In a large mixing bowl, whisk together the mayonnaise, sour cream, lemon juice, dried dill, garlic powder, salt, and black pepper until smooth and evenly combined.

Add the cooled pasta, imitation crabmeat, thawed peas, celery, red bell pepper, and green onions to the dressing. Fold gently with a large spatula so the ingredients are coated without breaking up the crab too much.

Cover the bowl and refrigerate the salad for at least 1 hour before serving. Stir well just before serving, then taste and adjust the seasoning if needed.