Dog clothes and costumes almost always provide some benefit or health protection for your pet. Raincoats and parkas for dogs are no exception.
Things to consider: Keeping her head and ears dry, easy fitting, and nighttime visibility.
No Water Allowed - Raincoats and parkas with hoods and flaps for the ears are the best type of dog clothes and costumes to protect your dog fully from the rain. An excess of moisture in the ear is the most common cause of wax buildup, which, if unattended, can lead to infection. Some flaps over the ears are all it takes to keep the rain from causing you (and your dog) from having to deal with ear problems later.
Proper Fitting - Not all doggie raincoats and parkas are made alike. Look for Velcro straps, a hood to keep her little head dry (with ties to keep it on), and a pocket or pouch that's either sealable or that she at least can't get into herself where you can store dog treats or dog toys for when you are out and about.
Shining in the Storm - Rainy days and nights are often dark days and nights. For added protection for your pooch while she is out in the elements, consider dressing her in reflective and lighted dog raincoats and parkas.