A B737-300 WAS DISPATCHED WITH AN MEL LIMIT OF 25000 FT AND CLR OF ICING CONDITIONS. THE FLT CREW ENCOUNTERED WX ENRTE THAT FORCED A CLB TO 29000 FT.

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PRIOR TO DEPARTING MCI WE RECEIVED THE RELEASE AND WX AND SAW THAT WE HAD AN MEL WHICH RESTRICTS US TO FL250. I CALLED DISPATCH BECAUSE I WAS CONCERNED ABOUT A LINE OF WX OVER THE BHM AREA SHOWING TOPS OF 280-360 IN THAT AREA. DISPATCH ASSURED ME THAT THE TOPS WERE ONLY FL250 AND ALSO INFORMED ME EARLIER THIS PLANE WAS PLANNED TO GO TO MDW FOR MAINT (SHOULD HAVE STAYED WITH THAT PLAN). ABOUT 100 NM FROM BHM WE ENCOUNTERED A HIGH CIRRUS LAYER OF CLOUDS. WE WERE JUST IN THE TOPS AND THE OAT -30 DEGS SAT. THE FO AND I DISCUSSED OUR SITUATION AND WE DECIDED TO CONTINUE HOPING THE CLOUDS WOULD DISSIPATE WHICH THEY DID. HOWEVER ONCE IN THE CLR WE COULD SEE A LINE OF MORE CLOUDS IN FRONT OF US. BEING NIGHT WE COULD NOT TELL EXACTLY HOW HIGH THE CLOUDS WERE. SO WE DECIDED TO DEVIATE TO THE S TO TRY AND GET AROUND THE LINE OF WX. AS WE PROCEEDED S WE COULD SEE THERE WAS NO COMPLETE BREAK IN THE LINE. WE HAD HEARD RPTS THAT OTHER PLANES WERE IN THE CLR AT FL300; BUT DUE TO OUR MEL WE COULD NOT CLB. IT WAS COMPLETELY OVCST BELOW; I WOULD GUESS AT 15000 FT. THE FO SUGGESTED A DIVERT INTO BHM; BUT I FELT THAT WE WOULD POSSIBLY SPEND MORE TIME IN ICING CONDITIONS IN THAT SITUATION. WE DISCUSSED THE LIMITATIONS OF THE MEL AND I DECIDED THAT IF WE HAD TO WE COULD CLB TO FL290 WITHOUT ENDANGERING THE SAFETY OF THE FLT. I DECIDED OUR BEST PLAN OF ACTION WAS TO FIND A THIN SPOT IN THE LINE AND HOPE FOR THE BEST. AS WE STARTED INTO THE LINE EVERYTHING WAS FINE; BUT SHORTLY WE BEGAN TO PICK UP MODERATE ICING. I CONTACTED CTR AND WE CLBED TO FL290. AT FL290 WE WERE IN THE CLR. AFTER 10 MINS WE WERE CLR OF THE WX AND DSNDED BACK TO FL250. WE PROCEEDED TO TPA AND LANDED UNEVENTFULLY. THE BEST THING THAT COULD HAVE BEEN DONE WAS TO HAVE THE PLANE GO TO MDW TO FIX THE MEL. THIS WOULD HAVE PREVENTED PUTTING ME IN A SITUATION THAT I WAS PUT IN. DISPATCH FORCED MY HAND IN THIS SITUATION KNOWING THAT THERE WAS WX ENRTE TO TPA. YOU JUST CANNOT FLY A JET ACROSS THE COUNTRY WITHOUT ENCOUNTERING SOME SORT OF WX. IN THE FUTURE I WILL REFUSE TO TAKE AN ACFT WITH THIS TYPE OF LIMITATION IF THERE IS ANY POSSIBILITY OF ENCOUNTERING WX. SUPPLEMENTAL INFO FROM ACN 715970: WE WERE GIVEN AN ACFT WITH AN MEL WHICH RESTR US TO FL250 AND PROHIBITED FROM ICING CONDITIONS. THERE WAS A LINE OF WX FROM THE GULF OF MEXICO TO THE CAROLINAS. ACCORDING TO THE WX DATA WE RECEIVED; THE TOPS WERE IN THE 30'S. I TOLD THE CAPT; I HAD CONCERNS WITH ACCEPTING THIS ACFT. APPROX 150 MI NW OF BHM; WE ENCOUNTER CLOUDS AND ENRTE ICING. I SUGGESTED WE EITHER LAND IN BHM OR BNA OR RETURN TO ZZZ. I SUGGESTED NUMEROUS TIMES TO THE CAPT THAT WE SHOULD DIVERT OR RETURN TO ZZZ BECAUSE THE CLOUDS WERE CLRLY HIGHER THAN US. HE CHOSE TO CONTINUE THE FLT. HE SAID ONCE WE GOT IN THE CLOUDS WE WOULD TURN BACK. AT THIS POINT IT WAS APPARENT HE WAS NOT GOING TO DIVERT OR RETURN THEREFORE I QUIT SUGGESTING THAT WE DIVERT AND ASSISTED THE CAPT IN HIS DECISION TO CONTINUE.