B717 FLT CREW EXPERIENCES WINDSHEAR ON VISUAL APPROACH AND USES EMERGENCY POWER TO ESCAPE; WITH DIVERSION TO ALTERNATE.

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ARRIVING AT ZZZ; THE FIELD WAS RPTING VFR WITH STRONG TSTMS RPTED AND NOTED BY FLT CREW 7 MI N OF FIELD MOVING SLOWLY SSE. AN INITIAL SOUTHERLY RWY ASSIGNMENT WAS GIVEN AND AS THE ACFT APCHED THE FIELD; THE CREW REQUEST A NORTHERLY RWY BECAUSE THEY CONSIDERED THE TSTM TOO CLOSE TO THE ARPT FOR A SAFE BASE TO FINAL TURN. THE ACFT WAS GIVEN A HDG FOR A VISUAL APCH TO RWY XXL AND EVENTUALLY CLRED FOR THE VISUAL APCH. THE ACFT WAS CONFIGURED FOR LNDG USING FLAPS 40 DEGS; INSIDE THE FAF WHEN THE CAPT NOTED VERBALLY AN INCREASE IN AIRSPD; A DECREASE IN PWR (AUTOTHROTTLES ON); AND THE ACFT PITCHING UP AND GAINING ALT. THE CAPT DECIDED TO EXECUTE A GAR AND TURN R TO A HDG OF 090 DEGS (AS BRIEFED BEFORE THE APCH) TO AVOID THE WX AT THEIR 12 O'CLOCK POS. AS THE PLANE WAS STARTING THE TURN; THE 'HEADWIND SHEAR' ALERT WAS ANNOUNCED OVER THE CAWS SYS. A FEW SECONDS LATER; AS AIRSPD BLED OFF QUICKLY; THE SYS CALLED OUT 'TAILWIND SHEAR.' WITH THE ACFT BELOW 1000 FT AGL; EMER PWR WAS SELECTED AND A STANDARD WINDSHEAR RECOVERY WAS PERFORMED MAINTAINING ALT WITH AIRSPD HOVERING AROUND THE 140-145 KT RANGE. EMER PWR WAS USED FOR A PERIOD OF 5-15 SECONDS UNTIL IT WAS DETERMINED THAT THE ACFT WAS CLR OF THE EVENT AND NORMAL CLB PWR WAS RESTORED. THE FADEC SWITCHES WERE RESET TO ALLOW FOR USE OF THE AUTOTHROTTLES AND THE ACFT PROCEEDED TO ZZZ1 FOR A LNDG. THE EVENT WAS CAUSED BY THE CONVECTIVE TSTM LOCATED N OF THE ARPT. THE ACFT WAS VMC; AND THE TWR RPTED STEADY STATE WINDS UNTIL JUST BEFORE THE INCIDENT OCCURRED.