It really depends on which way the rain is falling! The back few rows of the new stand will certainly protect spectators from rain falling vertically or from the direction of Bridgford Road, but not if is driving in from the Radcliffe Road end of the ground. Trent Bridge has always been a windy ground and I think it is more so since the erection of several new stands. Even in the Members' Area, in front of the pavilion (opposite the stand with the two pyramid towers), it is only the back row that is entirely 'safe' from the rain. Even so, this row can also be extremely draughty as there's often a cross wind since the erection of the new tall tower that houses the digital scoreboard/message board.
If you come to Trent Bridge, raise your hankie to test which way the wind is blowing before you decide where to sit & always bring a fold-up umbrella!