Bring a large pot of water to a boil; add salt. Add macaroni; cook until al dente. Drain. Rinse with cold water, and drain. Transfer to a large bowl. Cover with plastic wrap; refrigerate until cool.
Thaw peas under cold running water. Drain. Toss with celery, scallions, and ham in a bowl. Add to macaroni; toss.
Whisk mayonnaise, sour cream, vinegar, sugar, nutmeg, and 1/4 teaspoon salt; fold into macaroni mixture. Season with salt and pepper. Salad can be refrigerated in an airtight container, up to 3 hours.