Narrative:

Airborne approximately 40 mins established in level cruise flight over ZZZZ in south america our WX radar failed. We noted isolated thunderstorms building around our departure station and heavy WX was building at the time further up north along our route of flight. We would not be able to stay clear of thunderstorms without a WX radar. I declared an emergency with all ATC agencies so we may return to slvr visually clear of clouds and isolated thunderstorms. New york radio via HF passed along our information to company and ATC passed along our information back to slvr air carrier operations. WX was building rapidly around the station so I elected to land overweight. I logged the overweight landing.

Related reports:

Google
 

Original NASA ASRS Text

Title: A B757 PILOT RETURNED TO HIS SOUTH AMERICAN DEP ARPT FOR AN OVERWT LNDG FOLLOWING RADAR FAILURE IN TSTMS AND WITH HEAVY WX BUILDING ALONG HIS RTE.

Narrative: AIRBORNE APPROX 40 MINS ESTABLISHED IN LEVEL CRUISE FLT OVER ZZZZ IN SOUTH AMERICA OUR WX RADAR FAILED. WE NOTED ISOLATED TSTMS BUILDING AROUND OUR DEP STATION AND HVY WX WAS BUILDING AT THE TIME FURTHER UP N ALONG OUR RTE OF FLT. WE WOULD NOT BE ABLE TO STAY CLR OF TSTMS WITHOUT A WX RADAR. I DECLARED AN EMER WITH ALL ATC AGENCIES SO WE MAY RETURN TO SLVR VISUALLY CLR OF CLOUDS AND ISOLATED TSTMS. NEW YORK RADIO VIA HF PASSED ALONG OUR INFO TO COMPANY AND ATC PASSED ALONG OUR INFO BACK TO SLVR ACR OPS. WX WAS BUILDING RAPIDLY AROUND THE STATION SO I ELECTED TO LAND OVERWT. I LOGGED THE OVERWT LNDG.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.