The weather forecast sounds very similar to what it was in 2015. That was a cool day. I wore a hat, gloves, arm sleeves, singlet and shorts. I usually would throw off the hat, gloves and arm sleeves once I felt I was warm enough but in 2015 I kept them on throughout the race. I ran 2:39 that year. Honestly, with the strong headwind and cool temps, I did start to get a bit hypothermic by the end despite keeping all my gear on. My face was getting really cold and I was almost having trouble keeping my eyes open over the last few miles. Once I finished, I had to walk apprx. 1mi to the gear check in the pouring rain with only the heat blanket. After I got my gear, I was shivering so bad in the portapotty trying to get my warm clothes on. Then I had to walk another 3/4 mile to my hotel. I got to my room, turned on the hot shower and sat in it for about 40 minutes.
It probably would've been helpful to have a compression top under my singlet instead of just arm sleeves. And instead of the split shorts, probably compression tights would be better. Also, I come from the midwest, so I'm used to the cold. To someone coming from the south, the weather probably felt much colder.
I'm taking this Boston off, but if I was dressing for it, I'd probably go compression shorts, hat, gloves, arm sleeves, and instead of a singlet go with a t-shirt. That said, we're still almost 6 days out. The weather forecast for Boston is so unpredictable that I absolutely would not bank on the forecast remaining the same.
One thing is for certain though ---- make sure to completely overdress and bring plenty of extra clothes for the athlete village before the start. In 2010, I made the mistake of not dressing warm enough and sat there shivering for apprx. 2 hours, wasting tons of energy trying to stay warm. Not a good thing just prior to running a marathon!!