Due to showers on the ILS runway, the crew elected to circle and land runway 30. MDA was 540 ft, adjusted VREF was 130 kts. At the 90 degree position to the runway, IAS was 131 kts. Altitude was 528 ft. During the descending base to final turn, the power went to flight idle. The airspeed went below adjusted VREF, and the radar altitude went down to 176 feet. The radar altitude was 56 feet, and the airspeed was 112 knots when the torque started to increase. Lowest radar altitude was 16 feet, and lowest airspeed was 100 knots. Airplane impacted tops of trees, approximately 2,200 feet from approach end of runway. PIC said that during turn from base to final, F/O lost sight of runway in a rainshower and a go-around was initiated. PIC of airplane, waiting at hold line for weather to improve before takeoff, saw the airplane disappear behind trees, during heavy rain. His F/O called it a wall of water.