ALTDEV ALT EXCURSION WHILE IN PROX TO TSTM ACTIVITY IN A NIGHT OP.

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PICKED UP PAX IN NATCHEZ; MS; FOR A LEG TO OXFORD; MS. HAD TO OVERFLY SAME SIGMET AREA. CALLED FSS FOR RADAR UPDATE; LITTLE ROCK RADAR OTS; NO HELP. PLANNED TO FLY SAME RTE I JUST FLEW. AFTER ESTABLISHING CONTACT WITH ARTCC ASKED FOR HIGHEST ALT AVAILABLE; EITHER FL330 OR FL350. FINALLY GIVEN FL290 AND TOLD IT WOULD BE THE FINAL. CTLRS SAID FL290 RPTED BEST RIDE; MOST ACRS WERE GOING AROUND RATHER THAN OVER. STORMS HAD INTENSIFIED; HOWEVER; THERE WAS AN AREA I FELT I COULD CIRCUMNAV AND GO OVER JUST AS I HAD 30 MINS EARLIER. NO RADAR RETURNS AHEAD AND SPLITTING 2 SMALL WX AREAS 12-15 NM EITHER SIDE OF THE ACFT. ALL OF A SUDDEN; ALT HOLD WASN'T HOLDING AND WE BEGAN TO CLB. AT 300 FT ABOVE ASSIGNED ALT I CANCELLED ALT HOLD; INFORMED THE ARTCC WE WERE IN THE TOP OF ONE (POOR PHRASEOLOGY) AND CLBING. CTLR IMMEDIATELY ASSIGNED FL310. CLBED SLOWED AT FL303 AND WE CONTINUED TO FL310. WITHIN 5 MINS; SAME OCCURRED. I RPTED CLBING AND WAS IMMEDIATELY ASSIGNED BLOCK ALT FL310 THROUGH FL330. OUT OF FL320 NORMAL CLB ESTABLISHED AND HAD A SMOOTH RIDE AT FL330 OVER THE REST OF THE AREA NNEBOUND. AFTER LEVELING AT FL330 CTLR ASKED IF I HAD CLBED ON MY OWN TO AVOID WX. I SAID NO (NEGATIVE). I HAD NO CTL OF THE SIT. RADAR INDICATED I WAS OVER THE TOPS; OBVIOUSLY NOT HIGH ENOUGH AND I GET CAUGHT IN UPDRAFTS ABOVE THE BUILD-UPS. ATC ISSUED FL290 AS FINAL. ALTHOUGH I WAS TOPPING THE STORM; I SHOULD HAVE INSISTED ON HIGHER SINCE I HAD JUST FLOWN OVER THE AREA. ATC IMMEDIATELY CLRED HIGHER IN BOTH INSTANCES WITHOUT INTERFERING WITH OTHER ACFT. QUESTION: WHY WASN'T I GIVEN FL330? WHY IS ATC SO QUICK TO ASSIGN ANOTHER ALT IN UNUSUAL FLT CONDITIONS? IF I HAD BEEN STILL CLBING AT FL310 I WOULD NOT HAVE ATTEMPTED A LEVEL OFF. NOT RECOMMENDED PROC. ATC CONTINUAL RESISTANCE TO ASSIGN ALTS ABOVE FL290 TO TURBOPROP ACFT IN ANY FLT/WX CONDITIONS.