A Q400 at 8200' descending into its destination encountered a windshear which caused the aircraft to exceed Vmo for about five seconds. After the crew responded the aircraft slowed to 220 kts as it exited the shear.

Narrative

On descent into our destination; passing through about 8200'; we encountered windshear. We were stabilized at about a 1000-1100 FPM descent; at approximately 230 KIAS. While putting away my STAR chart I heard the overspeed warning. I acknowledged the overspeed condition; pulled power to flight idle and changed VSI on the Flight Guidance Controller to stop the descent; ultimately setting +1800 FPM climb. I noted airspeed to be about 256 KIAS. It took about 5 seconds; it seemed; for the airspeed to correct and decrease. Momentarily after the speed decreased below Vmo; we encountered a decreasing windshear that reduced speed to approximately 220 KIAS and the deceleration arrow head was full scale off the bottom of the speed tape. We were experiencing light to moderate turbulence at the time also. Upon stabilized conditions; after both shears; I pushed the event marker. The flight continued to a landing on Runway 24 without further problem. We exceeded Vmo; an operating limitation of the aircraft; but these shears were unexpected and beyond my control I feel.