B777 FLT CREW DIVERTS TO ALT DUE TO DEST ARPT BEING CLOSED DUE TO WX. ALT WX BECOMES A FACTOR DUE TO FAST MOVING WX FRONT AND FLT CREW IS FORCED TO EXECUTE A GAR AND RECEIVES SEVERAL GPWS WARNINGS FOR WINDSHEAR DURING THE SECOND APCH INTO COS.

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SEA-DEN DEPARTED SEA ON SCHEDULE WITH ETA DEN XA08Z. AT DISPATCH TIME; THE FORECAST WX FOR DEN AND ALTERNATE COS WERE: DEN 01024G35 3SM BLDU SCT 070 DEGS PROB 30XAZ -- TSRA BROKEN 070 DEGS CIRCUIT BREAKER COS 19015G25 P6SM SCATTERED 100 BROKEN 200; FM XC00Z 35020G30 6SM BROKEN 100 JUST PRIOR TO TOD; ATC INSTRUCTED US TO REDUCE SPD DUE TO ARR TFC CONGESTION AT DEN. SHORTLY AFTER; ATC INSTRUCTED US TO HOLD AT FROGS AND ADVISED THAT DEN WAS ON A SINGLE RWY OP. AT THIS POINT; CAPT AND I DISCUSSED DIVERSION POSSIBILITY AND AGREED ON AMOUNT OF DIVERSION FUEL. WE GOT COS WX; ADVISED ATC AND DISPATCH THAT WE MIGHT HAVE TO DIVERT TO COS. SHORTLY AFTER; ATC ADVISED THAT DEN WAS CLOSED. WE IMMEDIATELY REQUESTED AND GOT CLRNC TO COS. COS WX FAVORED S LNDGS -- CLR BELOW 12000 FT; BUT ILS RWY 17L AND DME OTS. SO; WE PLANNED A VISIBILITY APCH ON RWY 17L. ON HANDOVER TO COS APCH ATC; WE WEREN'T GIVEN DSCNT CLRNC RIGHT AWAY; SO WE SLOWED DOWN; AND WHEN EVENTUALLY CLRED FOR DSCNT AND INTERCEPT OF THE RWY 17L; WE NEEDED SPOILERS AND GEAR TO GET DOWN. WE DSNDED THROUGH MODERATE TURB AND INCREASING TAILWIND. IT BECAME EVIDENT THAT WE WERE HIGH AND FAST; SO WE ELECTED TO GAR. A 'TIGHT' L-HAND VISUAL PATTERN IN MODERATE TURB WAS EXECUTED. WE OBSERVED VIRGA TO THE N; AND HEARD RPTS OF SEVERE TURB AT 8500 FT. DURING THIS APCH; WE ENCOUNTERED WINDSHEAR; AND THE 'WINDSHEAR' WARNING SOUNDED; AND AT TIMES; THE 'SINK RATE' TOO. I ADVOCATED TO GAR; BUT ON FURTHER CONSIDERATION OF THE RAPIDLY CHANGING WX (WINDS; VIRGA; TURB) AND FUEL; BOTH THE CAPT AND I DEEMED IT SAFER TO CONTINUE AND LAND RATHER THAN GO AROUND. WE TOUCHED DOWN BEYOND 3000 FT; BUT ON SPD AND STOPPED WELL WITHIN THE REMAINING LENGTH OF THE 13500 FT RWY. THIS RAPID AND EXTREME DETERIORATION OF WX CONDITIONS WASN'T ANTICIPATED AND LEFT HARDLY ANY ROOM FOR ALTERNATIVE ACTION. IT WOULD BE PRUDENT IN THE FUTURE FOR FLTS TO DEN TO BE PLANNED WITH A MORE DISTANT ALTERNATE (EG; ABQ) WHEN FORECAST WX HAS PROBABILITY OF TSTMS AND WINDSHEAR; SINCE COS IS LIKELY TO BE IN THE SAME SIT. SUPPLEMENTAL INFO FROM ACN 594107: ON DOWNWIND; WE STARTED TO ENCOUNTER THE SAME FAST MOVING COLD FRONT THAT HAD JUST MADE SUCH A MESS AT DEN. THE TURN TO BASE WAS TURB; AND WE GOT A WINDSHEAR GPWS ALERT THAT WENT AWAY QUICKLY. WE GOT ANOTHER ALERT IN TURN TO FINAL; AND FO ADVOCATED GOING AROUND AGAIN. AT THAT POINT; I WAS SURE WE WERE THROUGH THE BIG WIND SHIFT AS COS TWR STILL HAD 220 DEGS 15 DME ON THE WIND; SO I TALKED HIM INTO CONTINUING THE APCH BELOW ABOUT 800 FT; WHERE THE LAST WARNING CEASED. ALL WARNINGS CEASED; THE AIR SPD STABILIZED; AND WE LANDED ON SPD ABOUT 4000 FT DOWN RWY 17L. WE STOPPED WITHOUT EXTRA EFFORT ON 13500 FT RWY AND TURNED OFF SHORT OF THE END. THIS WAS ONE OF THE MOST DIFFICULT APCHS I HAVE EVER HAD TO DEAL WITH. THE FO FLEW FINE; AND WE ONLY HAD ONE SMALL MOMENT OF CONFUSION ABOUT WHETHER TO GO AROUND AGAIN. WE; HOWEVER; BOTH AGREED TO CONTINUE AS SOON AS THE SIT STABILIZED. OTHER THAN THE ALERTS; WE WERE NOT AWARE OF ANY FLAP OVERSPDS; ETC; HOWEVER; WE DID USE SPD BRAKES DOWN TO ABOUT 500 FT.