PLT OF A BE18 TWIN BEECH FAILED TO FOLLOW TWR'S CIRCLE- TO-LAND INSTRUCTIONS WHEN PASSING OVER THE RWY INSTEAD OF MAKING A L DOWNWIND FOR CIRCLING DUE TO LOW CLOUDS IN THE CIRCLING AREA.

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LUBBOCK ATIS WAS CALLING FOR A SCATTERED LAYER AROUND 2000 FT. WHEN APPROX 15 MI OUT TWR SAID CLOUDS WERE MOVING IN FROM THE N AND AN APCH WOULD BE NECESSARY. I FLEW THE ILS RWY 26 CIRCLE TO LAND ON RWY 8. I WAS CLRED BY THE TWR TO ENTER A L DOWNWIND AND CLRED TO LAND ON RWY 8. I WAS ABLE TO MAKE VISUAL CONTACT WITH THE RWY AT THE MINIMUMS ON THE CHART. I WAS EXPECTING TO BREAK OUT MUCH HIGHER BASED ON WHAT THE TWR WAS SAYING. AFTER BECOMING VISUAL WITH THE RWY; I COULD SEE THAT IT WAS GETTING WORSE TO THE N AND CEILING WAS DROPPING FAST. INSTEAD OF ENTERING A L DOWNWIND; I FLEW A LOW APCH OVER THE RWY FOLLOWED BY A TEARDROP AND LNDG FOR FEAR OF LOSING THE ARPT BY ENTERING THE DOWNWIND. WHILE ON FINAL; I ADVISED THE TWR THAT THE WX WAS GETTING LOWER IN PLACES. IMMEDIATELY AFTER I HAD MADE THAT ADVISORY; TWR CAME ON AND SAID NEW ATIS WAS AVAILABLE STILL CALLING FOR 1500- 2000 FT SCATTERED. AGAIN WHILE TAXIING IN; I AGAIN ADVISED THEM THAT IT WAS AROUND 300 FT OVCST IN CERTAIN AREAS AROUND THE FIELD. WHILE FLYING THE LOW APCH I DSNDED ABOUT 150 FT BELOW THE CIRCLING MINIMUMS UNINTENTIONALLY BECAUSE I WAS WATCHING THE CLOUDS AND ARPT OUTSIDE. FACTORS THAT I FELT THAT CONTRIBUTED TO THE PROB WAS INADEQUATE INFO FROM THE TWR. NOT ONCE DO I RECALL BEING INFORMED ABOUT THE LOWERING CLOUD DECK.