If those downstream levels drift below -15 dBmV (the specified minimum for the modem), and if Comcast doesn't readily fix it, you can remedy it, at least temporarily, by putting a line-powered drop amp near or at the tap. See the lengthy and spirited discussion of this at »[Connectivity] Long battle with bad connectivity solved
in which I recount a situation similar to yours. Suitable drop amps are made for outdoor locations, are not expensive, and the power supply is remotely located within your dwelling unit. My drop amp was co-located with the tap in the slam box with the express consent of Comcast employees who had the documented authority to allow it, and there was never the slightest curfuffle. Luckily, your SNR numbers are very good as of this reading.