I'm not the best person to understand office wear, but Land's End sells a range of "squall" outerwear that to my eye looks acceptable and would meet y6our temperature requirements and not weigh too much. The one I guess might suit you is the stadium squall parka, which is knee-length or a bit longer. These items are basically shell with fleece lining; they work well in all sorts of weather and you can manage the sizing so you can fit other clothes underneath.

wg