C560XL EXPERIENCES SEVERE TURBULENCE AT FL390. NO WEATHER WAS DETECTED BY THE WEATHER RADAR.

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WHILE IN CRUISE FLT AT FL390; APPROX 350 NM SSW OF SBY VOR (E OF AR3; N OF SCOBY INTXN) MY ACFT ENCOUNTERED A VERY LOCALIZED AREA OF SEVERE TURB. WX RADAR WAS ON AND SHOWED NO THREATS. AT APPROX 2-3 MI CREW OBSERVED A VERT CLOUD FORMATION IN OUR PATH OF FLT. THE CLOUD APPEARED TO BE BTWN 1/4 AND 1/2 MI IN DIAMETER AND LACKED DEFINITION. I DISCONNECTED THE AUTOPLT AND ENTERED A 15 DEG L BANK TO AVOID FLYING THROUGH THE CTR OF THE COLUMN. AS THE R WING PASSED THE OUTER EDGES OF THE CLOUD WE EXPERIENCED A ROLL UPSET OF APPROX 45 DEGS L THEN R; IMMEDIATELY AFTERWARDS WE ENCOUNTERED A SEVERE DOWNDRAFT RESULTING IN A LOSS OF APPROX 1000 FT IN APPROX 10 SECONDS. WITH LITTLE PLT INPUT THE ACFT RETURNED TO FL390 ALMOST IMMEDIATELY. WE ESTIMATE THE WHOLE EVENT TO HAVE LASTED LESS THAN 45 SECONDS. DURING THE DSNDING PORTION OF THE ENCOUNTER I WAS ABLE TO BRIEFLY RPT TO CTR (MIAMI; I THINK) THAT WE WERE UNABLE TO MAINTAIN ALT DUE TO SEVERE TURB; ATC ACKNOWLEDGED OUR RPT. UPON RETURNING TO FL390 WE ENTERED AN AREA OF VMC AND SMOOTH CONDITIONS. I CAN ONLY CONCLUDE THAT AS WE PASSED A FRONTAL BOUNDARY THAT WE ENCOUNTERED THE FINAL STAGE OF AN IMBEDDED TSTM THAT DID NOT CONTAIN ENOUGH MOISTURE TO REGISTER ON OUR RADAR.