CL65 CREW WAS CLRED TO USE LOW ALT AIRWAYS WHILE OPERATING ABOVE FL180.

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ZHU CLRED OUR FLT TO A FIX THAT WAS NOT ON OUR FILED RTE. WE QUERIED THE CTLR AND HE WAS SURPRISED THAT WE HAD NOT RECEIVED AN AMENDED CLRNC. HE THEN RECLRED US AFTER SJU VOR V198 PENSI DIRECT KENS. WE DID NOT HAVE OUR LOW ALT CHARTS READILY AVAILABLE BECAUSE OUR FLT WAS ABOVE FL180. SO THE FO TRIED TO INPUT THE NEW ROUTING INTO THE FMS. THE CTLR THEN GAVE US A CLRNC TO CROSS LOXLY INTXN AT 11000 FT. THERE WERE NUMEROUS LARGE CUMULUS NIMBUS CLOUDS BELOW US WHICH NECESSITATED DEVS AROUND OUR ORIGINAL ROUTING. THE FO HAD TROUBLE INPUTTING THE NEW ROUTING IN THE FMS; AND DUE TO THE WX; I ASKED FOR AND RECEIVED CLRNC TO DEV OFF COURSE. THE CTLR THEN ASKED IF WE WERE GOING TO MAKE THE XING RESTR. WE WERE NOT FLYING TOWARDS THE XING FIX; SO I ASKED HIM IF HE NEEDED TO ALTER OUR COURSE. HE INFORMED US TO BE BELOW FL190 IN LESS THAN 1 MIN OF WHICH WE COMPLIED. THE FO FOUND OUT AFTER REFING THE LOW ALT CHART THAT V198 DOES NOT BEGIN OFF OF SJI VOR BUT STARTS AFTER LOXLY INTXN. IF OUR FLT HAD RECEIVED THE AMENDED CLRNC EARLIER; I BELIEVE WE WOULD HAVE RECOGNIZED THE CTLR'S ERROR SOONER.