Minimal gear for dry winter riding in the city
For such light weight I suggest a handlebar bag: 

Advantages:


They don't get dirty from spray from the wheels unlike panniers (which transfer dirt to your clothes)
Your phone is handy if you want to check e.g. a map (I mean stop but don't get off).
They've typically got a shoulder strap and quick release mounting so they're easy to take indoors.
They've got slightly more capacity than you say you need -- you'll soon use it.
Unlike a backpack they won't make you sweaty
You can always add a backpack for shopping


Disadvantages:


They're quite small
Cheaper ones soon start to let water in (true of any bag though)
(This is the big one for me) they get in the way of handlebar mounted lights.  This may or may not be an issue for you, but don't ignore it.


Even a wedge bag would hold all your stuff except the helmet.

Whatever you go for either clip the helmet on the outside or get a mesh helmet bag - helmets can do with some fresh air and a bag that will hold a helmet is quite bulky.

I've got an Altura dryline -- not the cheapest option but it has a mapcase to go over the top and can keep a netbook and change of clothes dry.  It would take a helmet for a small head, but not mine. I recently switched back to using it because of a baby seat.

There are much cheaper options (example) out there as well, and most come with a shoulder strap.