PA24 PLT ENCOUNTERS UNEXPECTED POOR WX AT DEST AND DIVERTS TO ALTERNATE IN SGF AIRSPACE.

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PLAN WAS TO FLY BOLIVAR; MO (M17); FOR BREAKFAST AND TO BUY FUEL; THEN PROCEED TO CLARK FIELD; MO (POK); FOR A RENTAL CAR TO CONTINUE WKEND PLANS. WX FROM WEDNESDAY THROUGH FRIDAY WAS CLR AND BRIGHT IN THE MORNINGS; AND I EXPECTED SATURDAY TO BE THE SAME. IT WASN'T. WE AWOKE TO DENSE FOG HERE. I CONSIDERED THE FOGGY MORNING NO PROB. I CALLED FSS FOR A BRIEFING AND FILED IFR TO GET OUT OF IXD ARPT. THE BRIEFER ASSURED ME THE WX WOULD BE LIFTING AT BOLIVAR WITH 1000 FT BROKEN (SPRINGFIELD IS THE CLOSEST WX RPTING STATION). CLARK FIELD; S OF SPRINGFIELD; HAD A SIMILAR FORECAST. IT SOUNDED OK TO ME; AND WITH A LIGHTHEARTED CONVERSATION WITH THE BRIEFER; CHKED NO FURTHER. WE DEPARTED IFR IN THE FOG SHORTLY THEREAFTER. WE WERE ON TOP OF THE OVCST AT 3000 FT. THERE WAS A SOLID OVCST FOR AS FAR AS I COULD SEE; BUT STILL FELT CONFIDENT IN THE ATC SYS. WHEN I GOT TO M17; SPRINGFIELD APCH LET ME DSND AS LOW AS 2900 FT. AT THAT ALT; I WAS SKIMMING THE TOPS OF A SOLID OVCST. IMMEDIATELY I GOT VECTORS FOR THE ILS APCH TO RWY 2 AT SGF; MY ALTERNATE. DURING THE VECTORING PROCESS; APCH ASKED ME TO DIAL IN THE ATIS. ATIS INFO WAS A SPECIAL RPT GIVING 100 FT OVCST AND POOR VISIBILITY. THAT'S BELOW IFR MINIMUMS ON THE SGF RWY 2 CHART. I COULD FLY THE APCH FOR A LOOK-SEE; HOWEVER; I DID FEEL A BIT SCREWED BY BELIEVING THE FORECAST. DURING VECTORING; I ASKED IF OTHER PLANES HAD MADE LNDGS OUT OF THE APCHS. WITH AN UNKNOWN QUANTITY OF FUEL LEFT; SCATTERED ABOUT IN 4 TANKS; I FELT THIS ONE HAD TO END IN A LNDG. NOBODY ELSE HAD PROBS; WHICH WAS ENCOURAGING. I FLEW THE APCH DOWN TO MINIMUMS AND BROKE OUT OVER THE BRIGHT APCH LIGHTS AND LANDED NORMALLY. WHAT NAGS ME IS THAT I WAS SO NONCHALANT ABOUT A FLT WITH A GOOD FORECAST THAT I WAS CAUGHT WITH MY PROVERBIAL PANTS DOWN. I TOOK OFF WITH ONLY 3 HRS OF FUEL ON BOARD; NOT THE USUAL 7. I FELT KNOWING THE EXACT AMOUNT WAS UNIMPORTANT BECAUSE I PLANNED TO BUY GAS AFTER A LEISURELY BREAKFAST. I REALLY TRULY EXPECTED TO BE LOOKING AT THE SUNSHINE AT AN ARPT SOMEPLACE ELSE. I TOOK ON 67 GALLONS; SO I ACTUALLY HAD SOME 20 GALLONS OF USABLE FUEL LEFT ON BOARD; GOOD FOR A MISSED APCH AND ANOTHER HR OR SO. THERE WASN'T ANOTHER ILS WITHIN MY FUEL RANGE.