ALTDEV AND LOSS OF ACFT CTL WHEN A CE560XL FLT CREW ENCOUNTERS SEVERE TURB WHEN OVERFLYING THE TOP OF A TSTM AT FL430 IN THE AREA NEAR OMN.

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I WAS PLTING ACFT AT FL430 AND NAVING TO OMN VORTAC FROM THE N. WE SUDDENLY ENCOUNTERED SEVERE TURB AS WE PASSED CLOSE TO AND NEARLY OVER A STORM CELL. WE WERE PAINTING THE CELL WITH THE AIRBORNE WX RADAR BUT COULD NOT SEE IT VISUALLY EVEN THOUGH WE WERE IN PREDOMINANTLY VMC WITH BLUE SKIES SEEN CLRLY ABOVE. ADDITIONALLY; ZJX WAS AWARE OF THE STORM CELL AND HAD PREVIOUSLY GRANTED US PERMISSION TO DEVIATE E OF THE OMN VORTAC AS NEEDED. DUE TO NBOUND TFC FLOW AND VOLUME; NO DEVS WERE BEING ALLOWED TO THE W OF THE OMN VORTAC. OBVIOUSLY WE FAILED TO DEVIATE E OF THE STORM CELL ENOUGH. AT THE ONSET OF OUR TURB ENCOUNTER; WE INITIALLY GAINED APPROX 900 FT AND THEN ABRUPTLY AND SOMEWHAT DRAMATICALLY STARTED DSNDING AT 2000-3000 FPM; ALL THE WHILE EXPERIENCING WINDSHEAR AND SUDDEN JOLTS OF SEVERE TURB. WE MADE NO ATTEMPT TO MAINTAIN ALT BUT RATHER 'RODE' THE UPDRAFTS/DOWNDRAFTS TO AVOID; AS MUCH AS POSSIBLE; ANY ADDITIONAL STRESS ON THE ACFT. TOTAL DURATION OF THE TURB WAS ABOUT 25-30 SECONDS WITH CLR AIR AND SMOOTH CONDITIONS FOLLOWING SHORTLY THEREAFTER. WE TOLD ZJX THAT WE WERE EXPERIENCING SEVERE TURB AND THAT WE REQUIRED A DSCNT (UNABLE TO MAINTAIN ALT ANYWAY) AND A TURN MORE TO THE E. ZJX ACKNOWLEDGED OUR SIT AND PROMPTLY ISSUED US CLRNC FOR DSCNT AND FURTHER DEV TO THE E. ADDITIONALLY; A PIREP WAS INITIATED BY ZJX. THE PRIMARY PURPOSE OF THIS RPT IS NOT TO FAULT ATC. WE CAN'T; BUT RATHER TO IDENT A SIGNIFICANT CONTRIBUTING FACTOR RELATIVE TO OUR ABILITY TO ADEQUATELY CIRCUMNAV KNOWN STORM CELL AREAS WHILE ALSO ATTEMPTING TO REMAIN WITHIN THE LATERAL LIMITS OF UNITED STATES AIRSPACE. OUR ACFT HAD ONBOARD A VALID 'TEMPORARY REGISTRATION.' OUR COMPANY POLICY AS DICTATED BY FAA OR TSA RULE (UNSURE WHICH AGENCY) IS THAT WHEN OPERATING AN ACFT WITH A 'TEMPORARY REGISTRATION' WE AS AIR CREW ARE TO REMAIN AT ALL TIMES WITHIN THE CONFINES OF UNITED STATES AIRSPACE; IE; NO MORE THAN 12 NM FROM ANY COASTLINE! IT APPEARS THAT CONTINUOUS POSITIVE RADAR CTL; CONTINUOUS 2-WAY COM WITH UNITED STATES ATC; AND NAV ALONG DEFINED AIRWAYS; ALONE AND TOGETHER; DO NOT SATISFY THE INTENT OF THE AFOREMENTIONED FAA OR TSA RULE. FURTHER; IT'S NO MATTER THAT POINT OF DEP AND ARR ARE BOTH UNITED STATES CITIES! IN OUR CASE; BEVERLY; MA (BVY) AND FT LAUDERDALE; FL (FXE). IT IS MY HUMBLE; BUT PROFESSIONAL VIEW; THAT THE FAA OR TSA RULE RESTRICTING ACFT TO UNITED STATES AIRSPACE; WHEN IN POSSESSION OF A 'TEMPORARY REGISTRATION;' IS QUESTIONABLE; AND WHEN COUPLED WITH THE 'RIGHT' DYNAMICS CAN PROVE A HINDRANCE TO NORMAL SAFETY OF FLT PRACTICES WHEN OPERATING IN THE VICINITY OF STORM CELLS AND THE LATERAL LIMITS OF UNITED STATES AIRSPACE. I STRONGLY FEEL THAT THIS 'RULE' WARRANTS SERIOUS AND TIMELY REVIEW!