A VFR C182 PLT DEPARTING OSH ENTERED IMC WHILE ATTEMPTING TO GET VMC. RPTR LOST SITUATIONAL AWARENESS AND ENTERED CLASS D AIRSPACE.

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UPON DEPARTING OSH VFR; I INADVERTENTLY FLEW INTO IMC AND STRAYED UNANNOUNCED INTO THE CLASS D AIRSPACE SURROUNDING ATW. I RECEIVED A BRIEFING FROM THE FAA MOBILE FSS STATION AT OSH 30 MINS PRIOR TO TKOF; WHICH INDICATED MARGINAL VFR OVER OSH BUT CLEAR SKIES WITHIN 10 MILES ALONG MY RTE OF FLT TO THE NW. I DEPARTED FROM RWY 9 AT OSH; AND PER THE NOTAM; FLEW 040 DEGS AT 1300 FT MSL AND TURNED TO THE NW AS SOON AS I WAS CLEAR OF THE OSH CLASS D (5 MILES). APPROX 2 MILES LATER; THE CEILINGS ABRUPTLY DROPPED TO 1000 FT MSL AND I FOUND MYSELF IN THE CLOUDS AND LOSING SIGHT OF THE GND. I IMMEDIATELY TURNED BACK TO THE E WHERE I KNEW THE CEILINGS WERE HIGHER (AND THERE WERE NO TOWERS) AND QUICKLY REGAINED VMC. WHILE FLYING TO THE E AND LOOKING FOR A WAY AROUND THE IMC; I LOST SITUATIONAL AWARENESS AND STRAYED INTO THE CLASS D AIRSPACE SURROUNDING ATW. AT THIS TIME THERE WAS AN OPENING IN THE CLOUD LAYER ABOVE MY POSITION AND I WAS ABLE TO CLB ON TOP OF THE LAYER AND PROCEED VFR ON TOP UNTIL CLEAR OF THE CLOUD LAYER BELOW ME (APPROX 10 MILES TO THE NW). BECAUSE OF THE FSS BRIEFING STATING THAT THE CLOUDS EXTENDED ONLY 10 MILES ALONG MY RTE OF FLT COMBINED WITH THE FACT THAT EVERYONE WAS DEPARTING OSH VFR; I ASSUMED THAT I WOULD BE CLEAR TO FLY TO THE NW UNTIL CLEAR OF THE CLOUDS. I DID NOT CONSIDER THAT THE CEILINGS COULD BE BELOW MINIMUMS FOR VFR FLT; AND I DID NOT HAVE AN ALTERNATIVE PLAN IN PLACE IN THE EVENT THE CLOUDS WERE LOWER. THUS I WAS SURPRISED WHEN THE CEILINGS DSNDED AND I WAS FORCED TO MAKE IMMEDIATE ACTION TO REMAIN IN VMC. DURING THIS CONFUSION I LOST SITUATIONAL AWARENESS WITH REGARDS TO MY POSITION (AND OTHER TFC TOO) AND FLEW INTO CLASS D AIRSPACE.