I can have a 10 degree temperature increase or decrease on my commute some days, coming in from or returning to the colder suburbs along the river valley. Cold air from the water becomes trapped and creates a frigid micro climate. The key to dressing is using multiple layers that you can either unzip while riding or remove and stuff in your bag. As others have said, start out a little underdressed, because you will warm up in 5 minutes. There's nothing wrong with stopping to add or take off a layer if you are getting too warm or cold.