Air carrier pilot reported a terrain warning while descending for a visual approach. The pilot continued the approach and landed.

Narrative

As pilot flying; coming into TYS for runway 23L. During the entire flight we had our radars on due to building thunderstorms which we had to dodge enroute. As we were approaching downwind for 23L we were at 4000 feet and cleared the visual approach for 23L. I dialed in 2800 to descend down because that's what the final approach fix was. As our descent began the captain turned off his radar and switched to terrain mode; at which point he saw that terrain elevation in that sector was 2900 feet. We were at 3400 feet when I leveled off which was followed by a terrain proximity warning message. We had visual of the towers; so I began my base turn for 23L at which turn ATC inquired us about the terrain and advised not to descend any further. We never received a pull up oral announcement and maintained visual with 23L and landed with no issue.I began the descent when cleared to land runway 23L before switching to terrain mode and was unaware of the towers on the down wind.I learned that while flying into mountainous terrain; and avoiding weather; the pilot monitoring or pilot flying; should have terrain mode on to avoid any terrain conflicts. I should have also checked to see if there was any terrain obstacles on the company pages to make sure that we avoided any conflicts.